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Molecular orbital studies of metal-metal bonding in bimetallic organoactinide compounds
Novo-Gradac, Kevin Joseph, Ph.D. The Ohio State University, 1989
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106 MOLECULAR ORBITAL STUDIES OF METAL-METAL BONDING
IN BIMETALLIC ORGANOACTINIDE COMPOUNDS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of the Ohio State University
By
Kevin J. Novo-Gradac, B.S., M.S.
•k k * * k
The Ohio State University
1989
Dissertation Committee: Approved by
Dr. Bruce E. Bursten
Dr. Daniel L. Leussing Bruce E. Bursten Dr. Andrew A. Wojcicki Department of Chemistry ACKNOWLEDGMENTS
I would like to thank Dr. Bruce E. Bursten for his insight and guidance in this research project. Thanks also go to the other members of my dissertation committee, Dr. Andrew Wojcicki and Dr.
Daniel L. Leussing. Thanks go to Steve McKee who provided many useful suggestions in the preparation of this document. Thanks also go to my wife Anne-Marie who assisted in the preparation of this dissertation.
I would also like to acknowledge that this research was partially funded by the Petroleum Research Fund, the U. S. Department of Energy, and the Ohio Supercomputer Center.
ii To My Parents
iii VITA
September 20, 1958 Born - Kansas City, Kansas
1980 B.S., New Mexico Institute of Mining and Technology, Socorro, New Mexico
1983 M.S., New Mexico Institute of Mining and Technology, Socorro, New Mexico
PUBLICATIONS
C. J. Popp, D. W. Love, J. W. Hawley, and K. J. Novo-Gradac "Radionuclide and heavy metal distribution in 20th century sediments of major streams in the eastern part of the Grants Uranium Region, New Mexico" in Selected papers on water quality and pollution in New Mexico, Hydrologic Report 7, New Mexico Bureau of Mines and Mineral Resources, 1984, pp 34-48.
B. E. Bursten and K. J. Novo-Gradac "Metal-Metal Bonds Involving the f-Elements. 2. Nature of Bonding in (t75-C5H5)2(I)M-Ru(»j5- C5H5)(CO)2 (M - Zr, Th) Complexes", J. Am. Chem. Soc., 1987, 109, 904- 905.
B. E. Bursten, R. H. Cayton, and K. J. Novo-Gradac "Molecular Orbital Studies of Bimetallic Organoactinide Complexes: The Effect of f Orbital Participation in Metal-Ligand and Metal-Metal Interaction in U(V) Dinuclear Systems", in preparation.
B. E. Bursten and K. J. Novo-Gradac "Xa-SW Studies of Metal- Metal Bonding in the Hypothetical Compound U2Me6", in preparation.
B. E. Bursten and K. J. Novo-Gradac "Comparison of Metal-Metal Interactions of a Series of Transition Metal and Actinide Compounds of Formulation M2Me6 (M — Zr, Mo, Ru, Th, U, or Pu)", in preparation.
B. E. Bursten and K. J. Novo-Gradac "Theoretical Studies of the Effect of jr-Donor Ligands on Actinide-Actinide Interactions in M2L6 Compounds" in preparation.
iv FIELDS OF STUDY
Major Field: Inorganic Chemistry
Theoretical Chemistry. The Ohio State University; Professor Bruce E. Bursten, Advisor.
Environmental Chemistry. New Mexico Institute of Mining and Technology; Professor Carl J. Popp, Advisor.
v TABLE OF CONTENTS
ACKNOWLEDGMENTS ...... ii
DEDICATION ...... iii
VITA ...... iv
LIST OF TABLES ...... ix
LIST OF FI G U R E S ...... xiii
CHAPTER PAGE
I. INTRODUCTION...... 1
Introduction ...... 1 Physical Properties of the Actinides ...... 2 Chemistry of the Lanthanides and Actinides ...... 6 The +3 Oxidation State ...... 7 The +4 Oxidation State ...... 9 Comparison of Transition Metal and Actinide C h e m i s t r y ...... 13 Metal-Metal Bonding ...... 14 Molecular Orbital Method ...... 18
II. COMPUTATIONAL D E T A I L S ...... 22
Computational Details ...... 22
III. METAL-METAL BONDING IN HETEROBIMETALLIC SY S T E M S ...... 27
Introduction ...... 27 Comparison of Cp2M(I)RuCp(C0)2 (M - Zr, Th) ...... 32 Cp2Th (I) RuCp ( CO ) 2 38 Cp3ThRuCp(CO)2 ...... 46 Cp3URuCp(C0)2 ...... 51 Conclusions ...... 54
IV. HOMOBIMETALLIC SYSTEMS WITH o-ONLY LIGANDS ...... 60
Introduction ...... 60 Computational Details of the U2Me„ System ...... 68 Mo2Me6 ...... 70 U2Me6 (U-U - 2.4 A, U-U-C - 105°) ...... 77
vi a27r452 Configuration of U2Me6, U-U - 2 . 4 A ...... 86 Comparison of U2Me6 Where U-U - 2.4 and 2 . 6 A ...... 90 Comparison of U2Me6 Where U-U-C — 90 and 105° .... 94 Comparison of U2Me6 Where U-U-C - 105 and 135° .... 98 Calculations on U2Me4 ...... 100 I. D2d U-U-C - 9 0 ° ...... 101 II. D2d U-U-C - 1 3 5 ° ...... 106 III. Da, U-U-C - 135° 107 U2Mea Calculations ...... 108 Conclusions ...... 115
V. STUDIES OF THE HYPOTHETICAL SERIES M2Me6 ...... 117 Introduction ...... 117 Studies of M2Me6 (M - Zr, Mo, Ru) ...... 119 Zr2Me6 ...... 122 Mo2Me6 ...... 126 Ru2Me6 ...... 126 Calculations on An2Me6, An - Th, U, P u ...... 134 Th2Me6 ...... 135 U2Me0 ...... 142 Pu2Me6 ...... 143 Comparison of Th2Me6, U2Me6, and Pu2Me6 ...... 152 Conclusions ...... 154
VI. THEORETICAL STUDIES OF An2Cp6 COMPOUNDS ...... 157 Introduction ...... 157 Cp3M Compounds: Symmetry Considerations ...... 159 Cp6U2 Dimer U-U - 2.84A 162 Cp6U2 Dimer U-U - 2.6 A ...... 167 Cp6Th2 D i m e r ...... 176 Comparison of the Cp66- Fragments ...... 182 Conclusions ...... 182
VII. MOLECULAR ORBITAL STUDIES OF BIMETALLIC ACTINIDE(V) COMPOUNDS ...... 190
Introduction ...... 190 U2(OR)10 C o m p o u n d s ...... 191 Structural Symmetry Considerations ...... 192 U2H10 196 U Z(OH)1 0 ...... 204 Conclusions ...... 217
VIII. C O N C L U S I O N S ...... 222
vii APPENDICES ...... 230
A. Data Relevant to Chapter III ...... 230
B. Data Relevant to Chapter IV ...... 236
C. Data Relevant to Chapter V ...... 247
D. Data Relevant to Chapter VI ...... 254
E. Data Relevant to Chapter VII ...... 258
LIST OF REFERENCES ...... 265
viii LIST OF TABLES
TABLE PAGE
1. Energies and decompositions of the valence levels of Cp2Zr(I)RuCp(C0)2...... 37
2. Energies and decompositions of the valence levels of Cp2Th(I)RuCp(CO)2...... 40
3. Energies and decompositions of the valence levels of the Cp2Th(I) fragment of Cp2Th (I) RuCp ( CO ) 2...... 43
4. Energies and decompositions of the valence levels of the RuCp(C0)2 fragment of Cp2Th (I) RuCp (CO) 2...... 44
5. Energies and decompositions of the valence levels of Cp3ThRuCp(C0)2...... 49
6. Energies and decompositions of the valence levels of Cp3URuCp(C0)2...... 53
7. Summary of the decompositions of the metal-metal interactions of Cp2Zr(I)RuCp(C0)2, Cp2Th(I)RuCp(CO)2, Cp3ThRuCp(CO)2, and Cp3URuCp(CO)2...... 56
8. Energies and decompositions of the valence levels of U2 (U-U - 2.6 A ) ...... 66
9. Geometries of the hypothetical U2Men molecules studied...... 71
10. Energies and decompositions of the valence levels of Mo2Me6...... 73
11. Energies and decompositions of the valence levels of the MoMe3 fragment...... 76
12. Energies and decompositions of the valence levels of U2Me6, (U-U - 2.4 A, U-U-C - 105°)...... 79
13. Energies and decompositions of the valence levels of the fragment UMe3...... 80
ix 14. Energies and decompositions of the valence levels of U2Me6 (U-U - 2.4 A, U-U-C - 105°) in a ct2tt*52 metal-metal configuration...... 87
15. Energies and decompositions of the valence levels of U2Me6, (U-U - 2.6 A, U-U-C - 105°). .... 92
16. Energies and decompositions of the valence levels of U2Me6 (U-U - 2.6 A, U-U-C - 90°)...... 96
17. Energies and decompositions of the valence levels of U2Me6 (U-U - 2.6 A, U-U-C - 135°). . . . 97
18. Energies and decompositions of the valence levels of U2MeA (D2d, U-U - 2.6 A, U-U-C - 90°) .... 103
19. Energies and decompositions of the valence levels of U2MeA (D2d, U-U - 2.6 A, U-U-C - 135°) . . . 104
20. Energies and decompositions of the valence levels of U2MeA (D2h, U-U - 2.6 A, U-U-C - 135°) . . . 105
21. Energies and decompositions of the valence levels of U2Me8 (U-U - 2.4 A, U-U-C - 105°) . . . . 110
22. Energies and decompositions of the valence levels of U2Me8 (U-U - 2.6 A, U-U-C - 105°). . . . Ill
23. Energies and decompositions of the valence levels of Zr2Me6...... 124
24. Energies and decompositions of the ZrMe3 fragment...... 125
25. Energies and decompositions of the valence levels of Mo2Me6...... 128
26. Energies and decompositions of the valence levels of the MoMe3 fragment...... 129
27. Energies and decompositions of the valence levels of RujMeg...... 131
28. Energies and decompositions of the valence levels of the fragment.RuMe3...... 132
29. Energies and decompositions of the valence levels of Th2Me6...... 138
30. Energies and decompositions of the valence levels of the fragment ThMe3...... 139
x 31. Energies and decompositions of the valence levels of U2Me6 (U-U - 2.6 A, U-U-C - 105°). . . . 145
32. Energies and decompositions of the valence levels of the UMe3 fragment...... 146
33. Energies and decompositions of the valence levels of Pu2Me6...... 149
34. Energies and decompositions of the valence levels of the PuMe3 fragment...... 150
35. Energies and decompositions of the valence levels of U2Cp6 (U-U - 2.84 A) ...... 164
36. Energies and decompositions of the valence levels of the UCp3 fragment of U2Cp6 (U-U - 2.84A)...... 169
37. Energies and decompositions of the valence levels of the U2Cp6 (U-U - 2.60 A ) ...... 172
38. Energies and decompositions of the valence levels of the UCp3 fragment of U 2Cp6 (U-U - 2.60 A) ...... 174
39. Energies and decompositions of the valence levels of Th2Cp6...... 178
40. Energies and decompositions of the valence levels of ThCp3 fragment...... 180
41. Energies and decompositions of the valence levels of the Cp6 fragment of Th2Cp6...... 184
42. Energies and decompositions of the valence levels of the Cp6 fragment of U2Cp6 (U-U - 2.84 A) ...... 185
43. Energies and decompositions of the valence levels of the Cp6 fragment of U2Cp6 (U-U - 2.60 A) ...... 186
44. Energies and decompositions of the valence levels of U2H10 (D4h, U-U - 2.90 A). The 4alg is the HOMO...... 198
45. Energies and decompositions of the valence levels of U2H10 (D2h, U-U - 2.90 A). The 4ag is the HOMO...... 201
xi 46. Energies and decompositions of the valence levels of U2H10 (Da,, U-U - 3.79 A). The 4ag is the HOMO...... 203
47. Energies and decompositions of the metal based levels of U2(OH)10 (DAh, U-U - 2.90 A). The HOMO is the 2b2g...... 207
48. Energies and decompositions of the U-0 j t levels of U2(OH)10 (D4h, U-U - 2.90 A) ...... 208
49. Energies and decompositions of the metal based levels of U2(OH)10 (D2h, U-U - 2.90 A). The HOMO is the 7b3u...... 213
50. Energies and decompositions of the U-0 j t levels of U2(OH)10 (D2h, U-U - 2.90 A)...... 214
51. Energies and decompositions of the metal based levels of U2(OH)10 (D2h, U-U - 3.79 A). The HOMO is the..... 7blu...... 219
52. Energies and decompositions of the U-0 j t levels of U2(OH)10 (D2h, U-U - 3.79 A)...... 220
xii LIST OF FIGURES
FIGURES PAGE
1. Molecular orbital diagram of the valence levels of Cp2Zr(I)RuCp (CO)2 ...... 34
2. Molecular orbital diagram of the upper valence levels of Cp2Zr(I)RuCp(CO)2 ...... 36
3. Molecular orbital diagram of the valence levels of Cp2Zr(I)RuCp(C0)2 compared with Cp2Th(I)RuCp(C0)2 ...... 39 i
4. Molecular orbital diagram of the upper valence levels of Cp2Th(I)RuCp(C0)2 and its fragments, Cp2Th(I) and RuCp(C0)2 ...... 45
5. Molecular orbital diagram of the upper valence levels of Cp2Th(I)RuCp(C0)2 compared with Cp3ThRuCp(CO)2 ...... 48
6. Molecular orbital diagram of the upper valence levels of Cp3ThRuCp(CO)2 compared with Cp3URuCp (C0)2 52
7. Contour diagrams of the metal-metal interactions of Cp2Zr(I)RuCp(CO)2, Cp2Th(I)RuCp(C0)2, Cp3ThRuCp(CO)2, and Cp3URuCp(CO)2 in xz (a, c, e, g) and yz (b, d, f, h) p l a n e s ...... 57
8. Metal-metal interactions of the f orbitals ...... 63
9. Molecular orbital diagram of the valence levels of U2 (U-U - 2.6 A) ...... 65
10. Molecular orbital diagram of the valence levels of the MoMe3 fragment and Mo2Me6 ...... 72
11. Contour diagrams of the a and j t metal-metal interactions of Mo2Me6 and their fragment orbitals ...... 75
12. Molecular orbital diagram of the upper valence levels of the UMe3 fragment and U 2Me6 (U-U - 2.4A) 78
xiii 13. Contour diagrams of the U2Me6 metal-metal a , tt , and S Interactions as formed from the fragment orbitals ...... 82
14. Molecular orbital diagram of the upper valence levels of U2Me6 (U-U - 2.4 A) in a27rA£° and o27r*52 configurations ...... 88
15. Molecular orbital diagram of the upper valence levels of U2Me6 (U-U - 2.4 A and 2.6 A) ...... 91
16. Molecular orbital diagram of the upper valence levels of U2Me6 (U-U - 2.6 A) with U-U-C - 90°, 105°, and 1 3 5 ° ...... 95
17. Molecular orbital diagram of the upper valence levels of the three U2Me4 molecules 1) D2d, U-U - 90°, 2) D2d, U-U - 135°, and 3) D2h, U-U - 1 3 5 ° ...... 102
18. Contour diagrams of the 5b3u, 4b3u, and 4bZg orbitals of the case for U2Me4 ...... 109
19. Molecular orbital diagram of the upper valence levels of U2Me8 with U-U - 2.4 A and U-U - 2.6 A . . . 112
20. General bonding scheme for transition metal M2L6 systems where L is a terminal a-only ligand ...... 120
21. Molecular orbital diagram of the valence levels of Zr2Me6 as constructed from ZrMe3 fragments .... 123
22. Molecular orbital diagram of the valence levels of Mo2Me6 as constructed from MoMe3 fragments ...... 127
23. Molecular orbital diagram of the valence levels of Ru2Me6 as constructed from RuMe3 fragments .... 130
24. Molecular orbital diagram of the valence levels of Th2Me6 as constructed from ThMe3 fragments .... 136
25. Contour diagrams of the Th-Th a and n interactions in Th2Me6 ...... 137
26. Molecular orbital diagram of the valence levels of U2Me6 as constructed from UMe3 fragments ...... 144
27. Molecular orbital diagram of the valence levels of Pu2Me6 as constructed from PuMe3 fragments .... 148
xiv 28. The electronic structure of An2Me6 (An - Th, U, Pu) ...... 153
29. Splitting of the n orbitals of Cp’, Cp33", and Cp66- 160
30. Molecular orbital diagram of the valence levels of U2Cp6 (U-U - 2.84 A) as constructed from UCp3 f r a g m e n t s ...... 168
31. Molecular orbital diagram of the valence levels of U2Cp6 for U-U - 2.84 A and 2.60 A ...... 171
32. Molecular orbital diagram of the valence levels of U2Cp6 (U-U - 2.60 A) as constructed from UCp3 f r a g m e n t s ...... 175
33. A comparison of the electronic structures of Th2Cp6 and U2Cp6 (U-U - 2.84 A) ...... 177
34. Molecular orbital diagram of the upper valence levels of Th2Cp6 as constructed from ThCp3 f r a g m e n t s ...... 179
35. Molecular orbital diagram of the valence levels of Cp6 fragments of Th2Cp6, U2Cp6 (U-U - 2.84 A), and U2Cp6 (U-U - 2.60 A) ...... 183
36. The f-orbitals under 0h symmetry ...... 193
37. Rotation of an ML5 fragment from D4h to D2h symmetry ...... 195
38. Molecular orbital diagram displaying the valence orbitals of U2H10 under D4h (left) and D2h symmetry (right) (U-U - 2.84 A) ...... 197
39. Molecular orbital diagram displaying the valence orbitals of U2H10 under D2h symmetry where U-U - 2.84 A (left) and U-U - 3.79 A (right) ...... 202
40. Molecular orbital diagram displaying the valence orbitals of U2(OH)10 under DAh (left) and D2h symmetry (right) (U-U - 2.84 A) ...... 205
xv 41. Molecular orbital diagram displaying the valence orbitals of U2H10 (left) compared with U2(OH)10 (right) under DAh (U-U - 2.84 A) ...... 209
42. Molecular orbital diagram displaying the valence orbitals of U2H10 (left) and U2(OH)10 (right) under D2h symmetry (right) (U-U - 2.84 A) ...... 216
43. Molecular orbital diagram displaying the valence orbitals of U2(OH)10 under D 2i, symmetry with U-U - 2.84 A (left) and U-U - 3.79 A (right) ...... 218
xvi CHAPTER I
INTRODUCTION
Introduction
The chemical and physical properties of the actinides have been of particular interest since the days of the Manhattan project. In the early years a heavy emphasis was placed finding compounds which were volatile enough to allow isotopic separations. Out of this work grew an understanding that the actinides displayed some unusual characteristics and behavior. In this dissertation, some of these points will be addressed. The emphasis of this dissertation is placed on metal-metal bonding involving the actinide elements. The discussion will begin with the examination of known heterobimetallic compounds. It will then be expanded to hypothetical low-valent homobimetallic compounds, and then finally a high-valent homobimetallic system will be explored. In these examinations the following questions will be addressed: 1) Why do the actinide elements not form actinide-actinide bonds? 2) What is the role of 5f orbitals? and 3) What comparisons can be made between the actinides and transition metals? In order to facilitate this study it is appropriate to first understand the known physical and chemical properties of the actinides.
1 Physical Properties of the Actinides
The actinide elements provide an interesting progression of electronic structures and magnetic properties that are, as yet, not thoroughly understood.1 The last part of the series (Cm-No) behaves according to the norm portrayed by the lanthanides, but the first part of the series (Th-Am) deviates from the standard lanthanide prescription in a variety of ways.1,2,3,4 Perhaps the chemist's best evidence of the anomalous behavior patterns of the early actinides lies in the common oxidation states of the elements.4'5 For thorium, the only common state in compounds is Th(IV), while Th(III) complexes are rare. Uranium and plutonium have a broad range of states running from +3 to +6 for uranium and +7 for plutonium.4,5 The wide range of oxidation states of these actinides resembles the behavior exhibited by transition metals. For the lanthanides and later actinides the variability in the oxidation state of the metals is greatly reduced to include only +2, +3, and +4 states. The most common oxidation state for these elements is +3.4,5
Evidence that the early actinides should be considered as a new transition series comes from a variety of solid state studies.
Experimental and theoretical studies along these lines have recently been summarized.4 The experimental evidence that has been cited as an indicator of 5f itinerant versus localized behavior in actinide metals includes 1) atomic volume studies, 2) crystal structures, 3) cohesive properties, 4) magnetic properties, and 5) spectroscopic properties.4
The metal radii of the lanthanides decrease in a monotonic fashion with increasing atomic number, with the exception of three anomalies (Ce, Eu, and Yb). The relatively small decrease in the
atomic radii (lanthanide contraction) is attributed to the filling of
the localized 4f orbitals. On the other hand, the atomic volume
studies of the actinides demonstrate at first a large transition-
metal-like decrease in the Wigner-Seitz radius for thorium through
plutonium, after which a much smaller lanthanide-like contraction is
observed.4 In addition, an anomalous transition in the metallic radii
is seen between plutonium and americium.4 This increase in size has
been explained by 5f itinerancy in the early actinides with much
greater localization from americium on.4
The appearance of low symmetry crystal structures for the early
actinide metals has been intimately linked by theory to the itinerancy
of 5f electrons.6 This view is further corroborated by high pressure
crystallographic studies, where more compact forms can be prepared for
the late actinides by now forcing 5f incorporation into the metal- metal bonding. The heavier actinides generally crystallize in a
double hexagonal close packed (dhcp) structure that undergoes a phase
transition on increasing external pressure. Americium, for example, undergoes three phase transitions below 16 GPa. The first of these
occurs at -6 GPa and is a transition from dhcp to a cubic close-packed
structure. This structural change is similar to that seen in the
lanthanides and is not observed in the lighter actinides.6 In fact, the lighter actinides require much higher pressures to induce structural changes. These changes are also accompanied by other physical properties, strengthening further the argument for 5f involvement. The magnetic behavior of the 5f shell lies between two limiting
cases: 1) complete localization of the 5f orbitals, which results in
Van Vleck or Curie-Weiss magnetism and 2) complete itinerancy, which
results in metallic conduction and Pauli type paramagnetism. The magnetic properties of the early actinide metals are found to be
different (Pauli magnets) from the later actinides, which follow normal Curie-Weiss behavior. Between plutonium and americium, a cross-over from 5f itinerancy (bandlike) to localized (atomic) behavior is observed. In other words, a true Mott transition is .
observed/’7
The spectroscopic studies are often more directly related to the
Xa-SW calculations that will be the subject of subsequent chapters
since they yield information about electronic structure. A variety of photoelectron spectroscopy (PES) techniques have been employed to probe actinide solids. These methods are 1) X-ray Induced
Photoemission Spectroscopy (XPS or ESCA), used to probe core or valence electrons; 2) Ultraviolet Induced Photoemission Spectroscopy
(UPS), useful for valence band studies; and 3) Inverse Photoemission
Spectroscopy (IPS or BIS), which is useful in determining the nature of unoccupied levels and is accomplished by bombarding the surface with monoenergetic electrons. Studies on thorium metal using a combination of XPS and BIS show four bands at -1.8, -0.6, 3.15, and
4.5 eV.8,9,10 The first two of these are attributed to 6d states and the last two to the 5f states.11 These data support the view that thorium atoms in the metal have an szd2 ground state. In a-U, the picture is not as clear as in thorium. This metal
shows a strong emission in a narrow band just below the Fermi edge in
the XPS spectrum.9'12'13'1* This emission is considerably enhanced over
that found in the thorium spectra and is regarded as a direct
identification of 5f emission. The features in the XPS spectrum have
been attributed to hybridization of 5f, 6d, and 7s states.11 The
experimental data have not been well modelled by theory since neither
extreme of 5f incorporation handles this case well. It is evident
that both the 6d and 5f orbitals are important to the filled levels11
and the strong emissions just below and above the Fermi edge are
attributed to itinerant 5f states.11 In the early actinides, the 7s,
7p, 6d, and 5f orbitals all form band-like features. The 5f band is
considered as "pinned" to the Fermi energy, as exemplified in the
electronic structure of uranium.11 In the heavier actinides, the 5f
levels are much more localized and lie below the Fermi energy.15
Based on these data, it seems likely that the early actinides
represent a new type of transition series while the transplutonium
elements are more lanthanide like.15 If this is true, then the
chemistries of the early actinides should be different from the
lanthanides and may be similar to the transition metals. It must be understood that this series would be an f-element transition series and not a d-element series. This may have a dramatic effect on the chemistry of the actinides. Chemistry of the Lanthanides and Actinides
Since the actinides, especially the heavier ones, behave somewhat
like the lanthanides, it is instructive to examine the similarities
and differences between these f-element series. This chemistry has
been the subject of numerous review articles.16,17,18’19,20*21,22 The
majority of the chemistry of the actinides is confined to the early
actinides due to the limited availability and the instability of the
heavier elements.17 As such, the majority of the reported chemistry
of the actinides is expected to differ somewhat from that for
lanthanides. Both the lanthanides and the high valent actinides are
considered to be "hard" Lewis acids.5 As such, they tend to form
ionic bonds with negatively charged and nonpolarizable ligands.17
There are notable exceptions where neutral ligands, e.g. benzene, bind
to the actinides.23 The chemistry of the actinides is in general
similar to, but more extensive than that found for lanthanides.5 This
is in part due to the vast number of oxidation states for the early
actinides. In this research, the more reduced forms are of greatest
interest, and for this reason, the organometallic chemistry of these
two series will be emphasized.
The organometallic chemistry of both f-element series are
dominated by »75-C5H5 (Cp), »j8-C8Ha (cot), and ct-bonded aryl or alkyl
ligands.5,17 The greatest parallels are found between the lanthanides
and actinides in the +3 state.17 Since the actinides and lanthanides
are both "hard" Lewis acids, little w acid type chemistry is known for
these elements.17 Some of the higher carbonyl compounds have been prepared by matrix isolation techniques, but these decompose at elevated temperatures.17,24 Spectroscopic studies of these compounds indicate bonding similar to that present in transition metal carbonyl compounds. Recently, the first actinide carbonyl compound, Cp'3UCO
(Cp' -* CsHASiMe3) was reported in the literature.25 The view that this is not an isocarbonyl compound has been supported by molecular orbital calculations.26
The +3 Oxidation State
While the chemistry of Cp3Ln is well known, Cp3An chemistry is not as well developed. Part of the problem arises from the relative instability of the transplutonium elements. Early actinides have an additional problem, in that they are commonly found in higher oxidation states than +3. Nevertheless, preparations of Cp3An compounds have been reported for the actinides from Th to Cf.27,28 The actinide and lanthanide compounds can be synthesized by similar methods. The early actinide Cp3U compounds have been prepared by a
benzene UC13 + 3 KCp ------> Cp3U + 3KC1 (1) reflux
benzene U + 3 CpAU ------> 4 Cp3U (2) reflux
benzene Cp3UCl + NaH ------> Cp3U + 3 NaCl + 1/2 H2 (3) reflux
benzene U + 3 C5H6 > Cp3U + 3/2 H2 (4) reflux variety of methods, as shown above,17,27,29 while the transplutonium
compounds are typically prepared as shown below.30,31,32,33,34,35,36,37
65°C 2 AnCl3 + 3 BeCp2 ------> 2 Cp3An + 2 BeCl2 (5)
An - Pu, Am, Cm, Bk, Cf
Studies of Cp3Cm and Cp3Am by UV-excited emission have led to the
conclusion that the An-Cp interactions in the late actinides are
slightly more covalent than the corresponding Ln-Cp interactions.38
X-ray powder diffraction data has previously shown that the Cp3Ln and
Cp3An molecules are isostructural.17 Recently, evidence suggesting
some structural differences in these molecules has appeared. The
Cp3Ln compounds have been shown to have non-zero dipole moments,33
suggesting a pyramidal arrangement. The evidence from the X-ray
patterns of Cp"3Th (Cp" - CsH3(SiMe3)2) compounds tends to support a
nearly planar structure, however.39 A number of other AnR3 compounds
are known which demonstrate a pyramidal structure.40
A number of Cp3ML type compounds are known for the lanthanides
and the actinides.17 Since both metals are "hard" Lewis acids, it is
reasonable to expect similar base adducts to form for both sets of
elements. These adducts possess similar structural patterns, as
evidenced by comparing Cp3Pr(CNCy) with Cp3UL structures.41 There are
some striking differences for the two groups, however. For example,
[Cp2MX]2 compounds are common for the lanthanide elements, but the number of actinide compounds of this type is more limited.17 The
lanthanides tend to form compounds of this type that are dimeric, but dissociate into monomeric units when dissolved in coordinating
solvents.42 Until recently, berkelium was one of the few actinides which was known to form such a compound.43 Other compounds of this
type include reports of Cp2ThCl44 and Cp2UCN.28 Cp*2UCl (Cp* -
C5(CH3)5) forms a trimeric structure with bridging chloro ligands.45,46
More recently, crystal structures have been reported for [Cp"2U(/i-
Cl)]2 and [Cp"2U(/i-Me) ]2.47,48 Both of these structures show a striking similarity to the analogous lanthanide compounds.
Compounds of the CpMX2 type are rare for the lanthanides (eq. 6).
The few compounds of this type that are known are extremely air sensitive and do not sublime.49 The structure of these molecules is
THF LnCl3 + NaCp > CpLnCl2(THF)3 + NaCl (6)
similar to that for CpUCl3(THF)2 (eq. 7), one of the THF ligands being replaced by a chloride.50*17
THF UC1A + NaCp ■> CpUCl3(THF)2 + NaCl (7)
The +4 Oxidation State
The M(cot)2 type compounds are known for both the lanthanides51'52'56 and actinides.53,54,55 The cot2- ligand also forms 10
compounds with the transition metals, but the M(cot)2 formulation is unknown. These compounds illustrate that the actinides have a much higher coordination number than transition metal compounds.5,17 This behavior has been explained by the presence of 5f orbital
participation in the actinides, a point supported by molecular orbital
calculations.5,57,58,59 Studies by LCAO and Xa-SW molecular orbital methods on Th(cot)2, U(cot)2, and Ce(cot)2" have shown that the f
orbitals are more important to metal-ligand bonding by about a factor of 7 in uranocene as compared to the Ce(cot)2" molecule.58,60 This comparison is based on the degree of f character in the metal-ligand
interaction of S symmetry (ca. 22% U 5f, ca. 3% Ce 4f) shown below.
t
t
An important observation is that the Th-C interactions are more ionic than the U-C interactions in these compounds. The PE spectra reported for the Th(cot)2 and U(cot)2 molecules indicate that the 5f electrons have a higher cross section in He(II) than He(I) spectra.61
There are indications that the majority of the metal-ligand stabilization is due to 6d interaction while significant 5f covalency is also demonstrated.62 An increase in the intensity and the splitting of the highest lying U-cot orbitals is taken as an 11
Indication of greater covalency in the U(cot)2 molecule due to higher
5f participation.62
Cerium is the only lanthanide to form tetravalent compounds, for
example, Cp3CeX. This is a stable compound that is a strong oxidizing
agent.63,64,65 In contrast, the +4 oxidation state is, by far, the most
common one for thorium and uranium organometallic compounds.5,17 The
organometallic chemistry of the Cp^^jAnXj, series is well known. The
Cp4Th and Cp4U compounds are prepared from the halides, as shown below.66'67 These molecules are pseudo-tetrahedral in shape, with all
benzene AnCl4 + 4 KCp ----- — > CpAAn + 4 KC1 (8)
benzene AnF* + 2 MgCp2 ------> Cp4An + 2 MgF2 (9)
An - Th, U
four of the Cp rings in i;5 conformations.68 The U-C distances in Cp4U
are somewhat longer than what is normally observed in U(IV) molecules.
The bonding of the Cp ligands in organoactinides is more covalent than
that present in lanthanides. This is evidenced by the lack of
reactivity of Cp3UCl with FeCl2 to form ferrocene (eq. 10).69
Conversely, the lanthanide cyclopentadienides are ionic and react with
FeCl2 to form ferrocene (eq. 11). PE spectra have shown that the 5f
Cp3UCl + FeCl2 --- > N. R. (10)
Cp3Ln + FeCl2 --- > Cp2Fe + . . . (11) 12 covalency is evident in the actinide compounds while no such evidence is found for the lanthanides.70,71 If the late actinides behave more like the lanthanides, then cyclopentadienide compounds containing these elements are expected to be more ionic. This view has recently been supported by theoretical calculations.72
An area where lanthanides and actinides behave differently is in the formation of homoleptic alkyl compounds. For example, the lanthanides are known to form a variety of anionic alkyl compounds of the general formula LnR*, (x - 4,6).73 These compounds are decidedly rare for the actinides but the neutral AnR474 and the Li2AnR675 species have been proposed. These are typically very unstable, decomposing well below room temperature. For this reason, the products have been largely uncharacterizable. A nearly homoleptic phosphoylide of uranium, CpU[ (CH2) (CH2P(C6H5)2]3 has been reported by Cramer and
Gilje.76 Also, a reasonably stable homoleptic actinide alkyl,
Th(CH3)73- was recently prepared.77 This compound is rather reactive despite the high degree of coordinative saturation.
From this discussion, one can see that the chemistry of the actinides and the lanthanides are very similar to each other at times, yet differ dramatically at others. It becomes clear that the late actinides represent a lanthanide-like f-element series. The early actinides, on the other hand, are able to employ their f orbitals for ligand bonding and can achieve higher oxidation states. Also, when in the +3 oxidation state, they mimic the lanthanides substantially. 13
Comparison of Transition Metal and Actinide Chemistry
As discussed earlier, the physical properties of the early actinides indicate that they may represent a new type of transition series. As an example, the oxidation states available to these elements are quite comparable to those for the transition metals. For these reasons it seems likely that the actinides will exhibit chemistry similar to the transition metals. At times, the early actinides parallel transition metal chemistry, at least in stoichiometry. One of the best examples of this is the formation of alkoxides of molybdenum(V),78 tungsten(V), and uranium(V)79’80. In each of these cases a dimer of the general formulation of M2(OR)10 is formed. However, there are some striking structural differences. In the molybdenum system the terminal Mo-O-R angles are non-linear, while in the uranium system the U-O-R angles approach linearity.
There is also a metal-metal bond in the molybdenum case while no such bond is observed in the actinide case. This interesting behavior will be explored in greater detail in Chapter 6.
Another example of similar behavior between the actinides and the transition metals can be found in the formation of Cp2MX2 compounds.
While Cp2AnX281 compounds are decidedly rare, it seems likely that this is due to the higher stability of Cp3AnX and CpAnX3. In general, the stability of the Cp2AnR2 molecules has been related to the bulkiness of the Cp ligands and the R ligands.82,83 Cp*2AnX2 compounds are known, for instance, and have similar geometries to their transition metal analogues.8*'85,86,87. A comparison of the U and Mo molecules reveals some subtle differences. The molybdenum compound is diamagnetic 14 whereas the uranium compound is paramagnetic. Xa-SW studies of these
two molecules have shown that the HOMO/LUMO gap in the molybdenum
compound is much larger than that in the uranium case. In fact, the
5f levels in the latter case are so close in energy that the last two
electrons are expected to be unpaired, and result in a paramagnetic
compound.w *88
Perhaps the closest similarities lie between the early transition metals and the early actinides where a variety of compounds are known
that are similar to one another. The compounds of tetravalent Ti, Zr,
and Hf for example, closely resemble Th compounds. In one example,
the Cp2M(X)RuCp(CO)2 type compounds can be prepared for thorium and
zirconium and have extremely similar structures.89,90,91 These will be discussed later in this section as well as in Chapter 3.
Even with all the similarities, there are still extensive differences between the actinides and the transition metals, these being: 1) the coordination number can be much higher for actinides,
2) M-Cp interactions are more ionic for the actinides, 3) a general lack of Cp2MCl2 chemistry for the actinides, 4) a general lack of formation of it acid compounds with the actinides, and 5) an overall lack of metal-metal bonding between the actinides.
Metal-Metal Bonding
One aspect of transition metal chemistry that does not have a parallel in the actinides is covalent metal-metal bonding. There are no metal-metal bound chain, layer, or cluster compounds as found in the lanthanides, such as GdaClg, ScCl, or Sc7Cl12.92 The closest is /?- 15
Thl3 which contains a short Th-Th distance, but this is still somewhat
too long to represent a metal-metal single bond.
A few compounds that contain a dative type interaction between a
formally cationic actinide fragment and an anionic transition metal
fragment have been recently prepared. These molecules are similar to
the analogous lanthanide compounds where a direct metal-metal
interaction is present.93 Often, the lanthanide compounds form
isocarbonyl or isonitrosyl linkages instead of a metal-metal
interactions.94’95’96 The earliest report of such a preparation for the
actinides was for the U[Mn(CO)5]A molecule, as shown below.97 While
the most straight forward structure would contain four U-Mn bonds, one
cannot rule out isocarbonyl linkages, such as found with lanthanides.
THF UC1A + 4 NaMn(CO)s ------> U[Mn(CO)5]4 + 4 NaCl (12)
More recent preparations have yielded unmistakable evidence of the presence of metal-metal bonds between actinides and transition metals.
The first of these are the phosphido bridged compounds prepared by
Ritchey, et al. and Hay, et al. as shown below.98’99 The second type
Cp*2Th(P^2)2 + Ni(cod)2 + 2 CO ---> Cp*2Th(/*-P^2)2Ni(CO)2 + 2 cod (13)
Cp*2Th(P$2)2 + Pt(cod)2 + PMe3 ---> Cp*2Th(/ii-P^2)2Pt(PMe3) + 2 cod (14)
of these are the unsupported compounds prepared by Marks, et al. which contain either a Cp3An or Cp*2AnX fragment (An - Th, U) that interacts with a CpRu(C0)2 or CpFe(C0)2 fragment (eq. 15). This type 16
CP*(2+n)AnX(2-n) + NaHCp(CO)2 — > Cp*(2+n)AnX(1_n)MCp(CO)2 + NaX (15)
X - Cl, I; An - Th, U; M - Fe, Ru
of a metal-metal interaction is much more lanthanide- or early-
transition-metal-like in behavior and will be covered in detail in
Chapter 3.100
One might now ask about the metal-metal bonding that results from
a covalent interaction between two metal atoms. It seems likely that
the best hope for such an interaction lies with the early actinides.
While this area of transition metal chemistry is only 25 years
old, a vast number of compounds within a variety of systems are known
to form metal-metal bonds.101 Those elements that are isovalent with
the early actinides are of particular interest. Within the transition metals, the highest metal-metal bond order reported so far is that for
the "naked" molybdenum dimer, which is proposed to have a sextuple bond.102 Since molybdenum and uranium atoms are isovalent (contain
the same number of valence electrons) they might be expected to behave
in a similar fashion. The inclusion of f-electrons does allow for higher symmetry interactions for the actinides. Corresponding Xa-SW calculations have been performed on U2 and Np2 and have shown that a
sextuple (ct2x46 V 2) and septuple (cr27rA$ V A) bond order might be present.72 Studies are currently underway where U2 is produced in the gas phase by laser desorption of uranium metal.103
A great deal of attention has been paid to the study of triple and quadruple bonds involving M2L6 and M2L8 systems. One of the most widely studied of these areas is the d3-d3 triple bonded M2L6 compounds 17
(M - W, Mo and L - amldo, alkoxyl, or alkyl groups). Within these
compounds a d3-d3 interaction of a27rV*°o*° configuration leads to a
formal metal-metal triple bond. These compounds exhibit similar
geometries with approximate D3d symmetry and exhibiting M-M-L angles
of 100 to 105°.
Since uranium behaves similarly to molybdenum in many
circumstances, why are there no uranium compounds known that contain
actinide-actinide bonds? Some experimental work with the early
actinides has been focused on methods that closely parallel the
approaches used to produced molybdenum and tungsten metal-metal bonds.
For example, the Mo2R6 species (R - alkyl) is prepared by the reaction
of MX5 with an alkyl lithium compound. With actinides, these
approaches have produced entirely different results. A similar
approach with UCl* results in reduction to uranium metal with the production of hydrogen and a mixture of organic species. In order to probe the possibility of forming a metal-metal bond and to search for
a rationale for this behavior, the electronic structure of a series of
An2Me6 molecules will be studied in Chapters 4 and 5. This approach will be extended to include the model compounds An2Cp6 in Chapter 6.
The latter compounds are more readily related to known organoactinide compounds and contain metal-ligand x interactions. This study should provide a comparison of the affects of Cp ligands on the metal based levels as well as a comparison of a and x donor ligands.
Another approach for forming actinide-to-actinide bonds takes advantage of the similarities between the Group (IV) transition metals 18
and the actinide Th. Recent reports of a single Zr-Zr bond have
appeared in the literature for the dimer [Cp2ZrMe]2 (eq. 16).104 This
[Cp2ZrCl]2 + AlMe3 ----- > [Cp2ZrMe]2 (16)
compound forms as a red oil that could not be as well characterized as
an analogous fulvalene bridged dimer which forms red crystals (eq.
17). If thorium is similar enough to zirconium then a thorium-thorium
interaction may be possible. For this reason, both terminal and
bridging conformations of Cp2MMe dimers (M - Zr, Th, U) will be
discussed breifly in Chapter 8.
I + 2 M e L i — {r______ir + 2 U a <1 7 >
= M . *Me3 ? Me
Molecular Orbital Method
In order to examine the bonding in actinide compounds it is necessary to find a theoretical approach that can be applied to these complex systems. The fact that f-orbitals must be included as both valence and core levels eliminates many molecular orbital methodologies from consideration. Computationally rigorous ab initio methods require an extremely long time to achieve reasonable results on greatly simplified model systems. Actinide elements also require 19
the incorporation of relativistic corrections if reasonable results
are expected.105,106,107 The Xa-SW methodology of Slater and Johnson
seems to fit these criteria.108’109 It is relatively simple
computationally, it incorporates f-orbitals, and it includes
relativistic effects. Several recent articles have reviewed the Xa-SW
methodology.109,110,111 The program employed for this study incorporates
quasi-relativistic effects after Wood and Boring and includes the
major relativistic effects (mass-velocity corrections and Darwin
shifts).112 While not as correct as the double group formalism
recently developed by Yang and Case,113,11A-115 it does preserve the
single group descriptors, making results easier to comprehend. This
method has been shown to yield reasonable results for a variety of
organometallic compounds as well as actinide compounds.
The earliest actinide molecules examined by molecular orbital
method were the M(cot)2 (M - Th, U)5* and M(ij6-C6H6)/iA+ (M - Th, U)116
systems. These molecules are highly symmetric, and, as such, were
greatly restricted in how they formed their orbital interactions. A
variety of methods have previously been applied to actinide problems.
Some of these are the semi-empirical LCAO Wolfsberg-Helmholtz, DV-
Xa,117 Xa-SW, extended Htickel,118,119 and ab initio120 methods. Valuable
insight has been gained by these previous calculations involving the
f-elements. The earliest calculations, coupled with PES data have
shown that relativistic effects are important and have demonstrated
trends in f-element covalency. Tatsumi et al. have diagrammed the
electronic interactions of the Cp3U unit with a variety of ligands.118
Recent advances in molecular orbital theory are making more rigorous 20
methods such as ab initio more useful toward the study of the
actinides.120 Calculations on M(BHA)4 (M - Zr, U) , performed by a
relativistic DV-Xat methodology have been shown to support the
relativistic Xa-SW (RXa) results.107,117 These results have both been
shown to correlate with the PES121 data when the relativistic effects
are included. Neglecting these effects, the metal-ligand interactions
appear to be more covalent. Molecular orbital calculations carried
out on a number of AnOB (An - Pa, U, Np ) compounds122 indicate that
the 5f levels drop in energy relative to the 6d levels as the atomic
number is increased.
The earliest application of Xa-SW to f-element problems were non-
relativistic studies on the halides MX^ and UX6 (M = Th, U; X -
F,C1)123'124'125 and on the actinocenes M(cot)2 (M - Th, U)59. More
recently, the lower symmetry model molecules, Cp2UCl2 and Cp2UMe2 were
studied by the non-relativistic and relativistic methods.126 Both
sets of the calculated ionization potentials were found to be in good
agreement with the PES spectra of the Cp* derivatives,127 but the
agreement was also shown to improve when the relativistic effects were
incorporated. In general, the addition of relativistic effects to the
Xa methods causes the metal-ligand interactions to appear more ionic
than in the non-relativistic calculations.117
A study of Cp2MoCl2 and Cp2UCl2 indicated that the 5f orbitals are not split nearly as much as the 6d orbitals. The HOMO/LUMO gap of the molybdenum complex was shown to be 2.5 eV while the uranium case was a
small 0.03 eV. This is consistent with the magnetic behavior of the
two compounds; molybdenum is diamagnetic while uranium is 21 paramagnetic.06,80 A study was also performed on the series CpAU,
Cp2UCl2, and UC14 in an attempt to gauge the difference between Cp and
Cl as ligands. The Cp ligand was shown to donate better than Cl, resulting in greater splitting of the uranium atomic orbitals.128’129
Recent studies of Cp3Th correctly predict a d1 configuration, which has been determined experimentally for Cp"3Th.130,131 While the
Xa-SW method is an approximate method, it appears to consistently yield reasonable results for the actinide molecules. For these reasons, it is expected to usually yield good results for the range of organoactinide compounds studied here. CHAPTER II
COMPUTATIONAL DETAILS
Computational Details
The Xa-SW method was chosen for the studies of these molecules.
This method has been previously applied to the study of transition
metal systems.101,108’111 As applied to actinide systems, it has been
shown to yield results that are consistent with experimental
data.86,100,117,126,131 The incorporation of relativistic corrections has
been realized as a necessary part in the study of these
systems.105,106,107 The advantages of Xa-SW over other methods lies in
the fact that it is computationally simple compared to a variety of
other methods available and still incorporates relativistic effects.
This simplicity allows for the study of larger and more chemically
correct compounds than ab initio type calculations would permit. The
general procedure for using the Xa-SW method is as follows:
The atoms in the molecule are idealized to maximize the symmetry
present in the molecule. In most cases, the ligands present in the molecules are left as chemically exact as possible. Large ligand sets
are sometimes simplified in order to achieve shorter convergence
times, however. The substitutions employed in this thesis are the use
of methyl for alkyl groups and Cp for substituted Cp ligands. The
effects of such substitutions have been studied previously, and have been shown to have a minimal effect for Cp.132,133
22 23
The neutral atom charge densities are created by use of the
Herman-Skillman procedure.134 The a exchange parameters were taken
from Schwartz and the actinide a values were extrapolated to
0.69200.135 These atomic potentials are then used to create a
molecular potential that is constructed as a series of overlapping (or
nearly so) spheres that correspond in radius to 89% of the generated
covalent radii of the neutral atoms.136 These are then contained by
an outer sphere that is tangential to the outermost spheres. The
inter- and outer-sphere regions employ an a value that is a valence-
electron weighted average of the atomic values. A set of symmetry
adapted linear combinations of spherical atomic orbitals is then
generated. The normalizing coefficients of the atomic orbitals in
these serve to define the basis functions for the molecule. The
largest 1 quantum number is chosen for each of the atoms and the 1 value for the outer sphere is generally chosen as the two largest 1 values added together plus one. The core levels of the atoms can be
chosen to be valence in type or can be calculated explicitly using
only the atomic sphere potential of the single atom. The latter
approach restricts the core level from interacting with any of the
levels and is computationally simple. However, in cases where one of
the core type levels rises above any other valence level, the former approach is required.
After the basis functions are chosen and the molecular potential
is created, a careful energy search is performed, in order to insure all the occupied levels are located. These values are then used as
the initial input to the iterative self-consistent field portion of 24 the program. The self-consistency convergence criterion used is a potential change (AE) value of < 0.001 R in most of the systems studied. In cases where any of the levels lie near in energy to the intersphere potential(IP), this AE criterion is difficult to achieve.
This is because levels that are near the IP may manage to cross over that value and thus drastically affect the AE value, presenting a difficult situation. In some of those cases a higher AE value was allowed to represent convergence.
For the molecules that contain actinides, a relativistic correction is applied. This has been well documented as necessary for the actinides and has been previously employed in a variety of MO methods. The Xa-SW program used here has an option for incorporating a quasi-relativistic correction factor, after Wood and Boring, for all the elements with Z > 69.112 An early approach taken for incorporating the correction into actinides was to first converge the molecule in the non-relativistic fashion and to then use the converged potential as the starting point for the relativistic calculation.
Unfortunately, the early cycles of the transition can be very difficult compared to the normal convergence procedure. Since the relativistic potential is regarded to yield a more accurate electronic structure than the non-relativistic potential this step was eliminated in later calculations. In this case the molecular potential constructed from the neutral atomic potentials is used as the starting point for the relativistic calculation. This proved to be computationally less demanding than the former method and could be further improved if a relativistically correct program could be 25
written to create atomic potentials for the heavier elements. Often,
numerous energy searches must be performed in the early cycles to
insure that none of the levels are lost.
In recent months, a few enhancements have been incorporated in
our version of the Xa-SW method. The first of these was the recoding
of the program to work on a Cray supercomputer. A factor of about 20
improvement was realized after vectorization, which turned out to be
invaluable in the study of the extremely large molecules which were
studied in this thesis. Another improvement made available in a new
version of Xa-SW developed by Bill Schneider reports the contributions
of the individual basis functions to the molecular orbitals. This was
previously not possible and often led to very difficult problems in
the assignment of the character of the molecular orbitals.
The converged potentials that are created in the methods above
were then used to construct potential files for fragments in a manner
consistent to that previously described. In this process, a new set
of basis functions must be created for the fragment. The fragments
are then constructed by removing the unwanted atoms from the converged
molecular potential. After all the levels of the new potential file
are located, it is run through one cycle of the self-consistent field portion of the program to yield decompositions of the fragment levels.
This is the sole purpose of this cycle. As such, the percent mix of
the energy values guessed at by the SCF cycle is kept low so that the
energy values will not change.
As the Xa-SW method allows for fractional occupation of levels, electrons can be split up among a number of levels. When this is done, the compound must be reconverged to obtain the new ordering of the levels. This method has been applied to the U2Me6 molecules discussed in Chapter 4. CHAPTER III
HETAL-METAL BONDING IN HETEROBIMETALLIC SYSTEMS
Introduction
A heterobimetallic compound is any compound which contains two
metal atoms that are different elements. This is in contrast to
homobimetallie systems where the two metal atoms would be the same
element. These compounds may or may not contain direct metal-metal
interactions. The heterobimetallic chemistry of the transition metals
is well known, but not nearly so well as their homobimetallic
chemistry.101 For the lanthanide elements the converse is true, and
the heterobimetallic chemistry that involves a dative type of
interaction is more prevalent than homobimetallic systems.17 In the
last few years, a number of compounds that contain a direct actinide-
to-transition metal interaction have been prepared.98,99,91,137
Previously, this aspect of organoactinide chemistry was unknown and
direct actinide-actinide interactions are still unknown. The first
compounds to be prepared were Cp*2Th(p-PPh2)2Ni(C0)2 and Cp*2Th(/i-
PPh2)2Pt(PMe3) as reported by Ritchey, et al. and Hay, et al.,
respectively.98,99 Both of these compounds were prepared in a similar
fashion, as shown below. The products of the reaction scheme shown below are suggestive of the presence of a cationic Th(IV) fragment and
an anionic Pt (or Ni) fragment. It was originally thought that both of these compounds contained a supported metal-metal bond. The basis 28
PhCH3 Cp*2Th(P^2)2 + PtCcodJa + PMe3 ----- > Cp*2Th(/u-P^2)2PtPMe3 + 2 cod (17) 8H,25°C
Cp*2Th(P^2)2 + Ni(cod)2 + 2C0 ----- > Cp*2Th(/x-P^2)2Ni(C0)2 + 2 cod (18)
for this conclusion was that the X-ray data gave a short 3.206 A Th-Ni
distance, compared to the expected distance between two phosphido bridged metal atoms of 3.7 A .98 In fact, molecular orbital studies were performed in order to discern whether a metal-metal interaction was present.138 It was concluded from these results that a weak metal-metal interaction was present. When the Th-Pt compound was prepared at a later date, the metal-metal distance was found to be
shorter (2.984 A ).99 Since the atomic radius of Pt is larger than
that for Ni it was suspected that the Th-Ni interaction must represent
a weak interaction between the two metals and that the Th-Pt distance
could represent a true metal-metal interaction. Molecular orbital calculations performed on a greatly simplified model compound of this
Th-Pt compound indicated direct donation of electron density from the
Pt(0) center to the Th(IV) center.99 Further evidence supporting the metal-metal bonding was discerned in the 31P NMR." The combination of these factors must be taken as very suggestive evidence of metal-metal bonding in the Th-Pt compound. In view of the evidence that resulted from studies on the Th-Pt compound it is now believed that the Th-Ni compound does not contain a considerable amount of metal-metal bonding character and the majority of the change in distance may be electrostatic.139 29
At the same time as this research was being performed, Sternal, et al. were approaching the same problem, except their goal was to prepare an unsupported actinide-to-transition metal bond.01 Success was reported shortly after the first of the phosphido bridged systems appeared in the literature. Their approach was to replace one of the halides in the organoactinide compound with a CpRu(CO)z" fragment in a simple metathesis reaction as shown below.
THF Cp*2ThX2 + NaCpRu(C0)2 ------> Cp*2Th(X)RuCp(CO)2 + NaX (19) 25°C,12h
X - Cl, I
The absence of bridging ligands in these first compounds and subsequent systems137 conclusively answered the question of whether actinide-to-transition metal interactions could exist. The chemistry that they employed to make their unsupported actinide-to-transition metal bond was different than that used by the Los Alamos group, but, as will be shown later, results in a similar type of compound. A crystal structure for the large Cp*2Th(I)RuCp(CO)2 molecule was obtained and the Th-Ru bond length was reported to be 3.03 A. This value was again supportive of the conclusion that the Th-Pt distance in the phosphido bridged compound did indeed represent a metal-metal interaction. Prior to the publication of the synthesis of these compounds, we became interested in studying the nature of the metal- metal bonds in organoactinide heterobimetallic compounds.
A number of different types of similar early-late transition metal compounds are known. These dimers are sometimes held together 30
by bridging ligands (A)73,140*141,142,143,144 or isocarbonyl linkages
(B),145,146,147 as is also often the case for the lanthanides.17 They
can also form metal-metal interactions where the metal-metal bond is bridged by ligands, such as carbonyl (C),140,149 or form unbridged
species (D) .89,90,146,150
o co O o c / \ \ / ./> M M M— M Nc (7 °s2 \x o O A B C D This latter case is found to have very similar chemistry and structure
to the thorium analogues discussed above. An example of the
transition metal chemistry that parallels the chemistry of Marks et al. follows in Scheme 1 below.89
.c h 3 / Cl ,OC(CH3)3 Cp2Zr, Cp2Zr/ Cp2Zr./
Cl Cl Cl f f t CH. 01 ,OC(CH3)3 / / Cp2Zr./ C p2Zr,\ Cp2Zr, \ RuCp(CO)2 RuCp(CO)2 RuCp(CO)2 Scheme 1.
Marks and coworkers have expanded on the zirconium chemistry and have reported the synthesis of a number of new actinide compounds in 31 which they have reacted R3AnX (R - Cp, Cp*; An - Th, U) compounds with
NaMCp(CO)z (M - Fe, Ru), as shown below.137 These molecules are found
to have very similar chemistry and structure to their titanium and
zirconium analogues. The products are extremely air sensitive; the metal-metal interaction is easily cleaved by alcohols and ketones.
R3AnX + NaMCp(C0)2 ---- > R3AnM(Cp) (C0)2 + NaX (20)
R - Cp, Cp*; An - Th, U; M - Fe, Ru; X - Cl
The carbonyl stretching frequencies for these compounds were
reported to follow the trend Cp3U < Cp3Th < Cp2Zr(Me)89 < Cp*2Th(Me) <
Cp*2Th(I) for both transition metals.137 The differences are fairly
small, with the largest occurring whenever a Cp is replaced by a halide or a methyl group. This tends to support the argument that the
character of the metal-metal interaction is similar in each of these molecules.
The purpose in studying these systems is three fold: 1) To
determine the role of actinide f orbitals in the metal-metal
interactions; 2) To determine whether these compounds are bound in a
covalent interaction or are in fact better considered as ionic; 3) To
see if we can correlate the CO stretching frequencies to the
electronic structure of these compounds. The information from these
systems will be explored later as to see how it might be extrapolated
to make homobimetallic systems of the actinides. 32
Comparison of Cp2M(I)RuCp(C0)2 (M - Zr, Th)
In order to explore the nature of the Th-Ru interaction, two Xa-
SW calculations were performed on models of the Cp*2Th(I)RuCp(C0)2
compound and the analogous isovalent zirconium compound. To simplify
this rather difficult Xa-SW molecular orbital calculation on these
large and low symmetry molecules, the molecules were idealized to Cs
symmetry and Cp ligands were used in place of Cp* ligands. The bond
angles and distances of these two idealized molecules were taken from known data for Cp*2Th(I)RuCp(CO)291 and estimated from the various Zr
compounds reported by Casey, et ai.89.i°°.i52
The Th-Ru and Zr-Ru distances for these two compounds are 3.03
and 2.94 A, respectively. The difference is probably best attributed
to the differences in the radii of the Zr and Th atoms. Otherwise the molecular structures of the two molecules are very similar to each other. At this point, we can address a few points that we expect to be able to discuss once the electronic structures of these compounds have been determined. In view of the great similarity in molecular structure of these two molecules, should we expect the electronic structures to also be similar? If they do show differences, can we attribute these difference to f orbital participation in the case of the actinide fragment? Is thorium behaving similar to zirconium in the well known d° Zr(IV) systems? Will the RuCp(CO)2" fragments be in the same environment, or differ, as was indicated by the CO stretching frequencies originally reported? Finally, is 5f/6d mixing important to the metal-metal interaction? 33
Let's start the analysis with the well known early-late
transition metal system model molecule Cp2Zr(l)RuCp(CO)2.89 In this
case we expect our calculations to indicate that Zr is present as
Zr(IV) with the RuCp(C0)2" fragment formally donating electron density
in a a sense to the cationic Zr fragment.
All of these systems contain a very large number of orbitals, but
only a few are of interest. The overall valence picture of this
molecule is shown in Figure 1, and demonstrates how the levels are
organized into numerous blocks. The lowest two orbitals are the C-0
3a orbitals at -38.03 eV. The next block of orbitals follows at
-32.18 to -32.07 eV and represents the Zr 4p metal core. The first of
the Cp C-C and C-H a levels follows at -25.66 to -25.39 eV. Primarily
the former in character, these three levels represent the highly
symmetric completely bonding combinations of these orbitals. At
higher energies (-21.02 to -20.69 eV), another block of Cp levels
which contain a higher degree of C-H a mixing are found. In this
case, the six orbitals are still primarily C-C a in character. Two
levels (-19.15 and -19.13 eV) follow next and represent the oxygen
lone pairs of the carbonyl groups. Ten levels are grouped together in
the range from -17.31 to -16.44 eV and represent another nine Cp C-C
and C-H a levels and one I n.b. level. The next two levels are Ru-CO
a interactions and contain an average of 34% Ru character. These
represent the lowest energy interactions that involve either of the metal centers. Four levels are found grouped together at higher
energy (-13.89 to -13.49 eV). These are the C-0 n levels and are
found to contain only a small amount of Ru character. Twelve more 34
-o. -
-5. - Zr4d 1, Zr-Ru, Cp 7t2, Ru 4 d
-10. - Cp 7t-|
I 1 Cp^C-H o
-15. - Ru-CO a eV n=i Cp C-C/C-H o O n.b. -20. “ Cp C-C a
-25. “ Cp C-C a
-30. ”
Zr 4p CO LO 1 1
C-0 a -40.
Figure 1. Molecular Orbital Diagram of the Valence Levels of Cp2Zr(I)RuCp(CO)2. 35
closely spaced levels fall in the range of -13.28 to -12.21 eV and
represent the last of the Cp C-C/C-H a interactions. Above this group
of levels lie the majority of the valence orbitals that contain metal
character.
An expanded energy diagram of these levels is shown in Figure 2
and their decompositions are presented in Table 1. Three levels are
grouped in the range of -10.78 to -10.08 eV. These are the 25a',
26a', and the 21a" and represent the Zr-Cp, Ru-Cp, and Zr-Cp rc1
interactions. These levels contain about 19% metal character, which
is primarily 5s in the a' cases and a mixture of 5p and 4d in the a" case. The next thirteen levels, from -8.70 to -5.64 eV, represent the
following interactions: Zr-Ru a, two Ru-Cp itz, four Zr-Cp it2, three
Zr-I, and three Ru 4 d-CO it* levels. These levels are not readily separable from each other as the low symmetry of the molecule allows for mixing between many of the orbitals. There are, however, orbitals that have a higher degree of one contribution over the others. The lowest two orbitals of this set (27a' and 28a') are a mixture of Ru-CO it* backbonding and the lowest Zr-I a level. The 22a" and the 29a' are primarily Ru 4d in character (ca. 70% Ru) and contain 9 and 18% CO it* character, respectively. The 23a" is one of the Zr-I it type donor orbitals, containing about 14% zirconium with the majority of the electron density present on the iodine atom (75%). The metal-metal interaction follows next (30a'), and is readily apparent as a dative type interaction. A high percentage (72%) is present on the ruthenium center and only 15% of this orbital is present on zirconium. The bond is formed by donation from a hybrid of the dz 2 and d^z on ruthenium to 36
35a' Zr4d
34a' 26a'
33a' Cp 71, 25a"
32a' 24a"
eV 31a' 30a’ Zr-Ru 23a" 29a' 22a" 28a' 27a'
- 10. 21a" 26a'
C p 7C-| 25a’ - 11.
Figure 2. Molecular Orbital Diagram of the Upper Valence Levels of Cp2Zr(I)RuCp(CO)2. 37
Table 1. Energies and Decompositions of the Valence Levels of Cp2Zr(I)RuCp(CO)2.
Orbital e(eV) %Zr %s %p %d %I %Ru %C0 %CpRu
36a' -2.85 30 1 0 99 3 28 10 21
27a" -2.96 5 - 2 98 0 7 75 10
35a' -4.49 76 0 3 97 10 6 2 3
34a' -5.64 25 13 13 74 0 9 18 44
26a" -5.94 25 - 7 93 7 10 2 10
33a' -6.29 27 0 29 71 6 2 0 2
25a" -6.36 16 - 3 97 14 15 6 27
24a" -6.94 30 - 1 99 7 10 2 18
32a' -6.96 36 0 14 86 4 2 1 0
31a' -7.72 5 1 15 84 78 13 1 1
30a' -7.82 15 9 8 83 4 72 5 2
23a" -7.92 14 - 5 95 75 2 1 0
29a' -8.15 7 2 4 94 7 60 18 7
22a" -8.45 4 - 2 98 1 79 9 5
28a' -8.70 16 19 23 58 47 25 4 6
27a' -8.73 14 4 10 85 31 41 6 9
21a" -10.08 16 - 56 44 0 0 0 0
26a' -10.25 1 1 25 74 0 19 2 67
25a' -10.78 21 79 6 15 3 1 1 1 38 a similar hybrid on the zirconium atom. The 31a' represents the other
iodine t t donor orbitals (5% Zr and 78% I). The last six occupied orbitals are primarily a mixture Zr-Cp and Ru-Cp jt2 orbitals. On the average, these contain 27% Zr and 8% Ru metal character. The
HOMO/LUMO gap is 1.15 eV between the HOMO (34a') at -5.64 eV and the
Zr 4d LUMO (35a') found at -4.49 eV.
Cp2Th(I)RuCp(C0)2
A comparison of the valence orbitals of the Th-Ru compound with the Zr-Ru compound is presented in Figure 3. The decomposition of the upper valence levels of this compound are located in Table 2. The differences in most of the levels are very minor, although the majority of the changes appear in the upper valence region where the presence of thorium 5f atomic orbitals becomes more important.
Within this region, the orbitals are organized in much the same fashion as in the ZrRu compound. The three Cp levels (26a' , 27a' , and 21a") are found from -10.31 to -9.64 eV, the middle level is the
Ru-Cp level. The next two levels (28a' and 22a") are almost pure Ru
4d CO t t * levels. This is in contrast to the ZrRu case because one of the Zr-I levels is closer in energy to the Ru 4d levels and readily mixes with them. The 29a' level at -8.82 eV is primarily located on iodine and is very similar in energy to the corresponding orbital in
ZrRu. The 30a' level is the last of the Ru 4d CO n* interactions. It contains 28% CO and 58% Ru character. In this compound, the orbital that contains the majority of the metal-metal interaction is the 31a'.
The orbital is still a dative type interaction just as in the ZrRu 39
Th 5f 35a' Zr 4d
34a’ 35a' -6. 26a' 33a’ 26a" 25a" Cp 7C. 34a’ 25a" 32a' 24a" 24a" 33a' eV 31a’ Zr-I 30a' Zr-Ru 23a" Zr-I 32a’ 29a' Ru4d/COn* I Th-I Th-Ru 22a" 30a’ Ru-COn’ 28a' Ru-COrcVZr-l 29a' , Th-I 27a' 22a" 28a' I Ru-COn’
21a'
- 10. 21a" 27a' 26a' 26 a'
25a'
Figure 3. Molecular Orbital Diagram of the Valence Levels of Cp2Zr(I)RuCp(CO)2 Compared With Cp2Th(I)RuCp(C0)2. 40
Table 2. Energies and Decompositions of the Valence Levels of Cp2Th(I)RuCp(CO)2.
Orbital e(eV) %Th %s %p %d %f %I %Ru %C0 %CpRu
47a' -1.67 41 7 0 91 1 17 1 17 8 46a' -1.83 55 3 0 91 6 13 7 6 4 35a" -1.88 4 - 1 73 25 0 50 23 15 34a" -1.97 60 - 0 88 12 0 7 6 2 45a' -2.12 53 8 0 78 14 5 6 7 3 33a" -2.20 61 - 0 77 22 3 4 5 2 44a' -2.28 28 1 0 89 10 14 5 27 6 32a" -2.49 27 - 1 55 44 1 31 16 14 43a' -2.53 59 0 0 65 35 12 8 4 3 42a' -3.40 1 0 0 18 82 0 31 61 6 41a' -3.49 5 1 0 36 63 1 27 51 17 31a" -3.50 13 - 0 4 96 0 11 73 0 30a" -3.93 4 ** 0 4 96 0 2 85 8 40a' -4.08 86 0 1 34 65 5 5 0 1 29a" -4.08 88 - 1 11 88 1 5 2 2 39a' -4.23 86 0 1 11 88 6 2 3 0 28a" -4.39 91 - 0 8 91 4 0 2 1 38a' -4.43 97 2 0 3 95 0 1 0 1 27a" -4.51 93 - 0 5 95 0 1 4 0 37a' -4.53 95 0 0 15 85 1 0 0 0 36a' -4.58 93 0 0 24 76 1 1 1 1 35a' -5.90 17 14 15 44 27 1 8 22 34 34a' -6.34 24 3 10 38 49 4 2 4 7 26a" -6.52 27 - 2 33 66 1 6 2 11 33a' -6.66 26 0 10 86 4 4 3 1 5 25a" -6.71 20 - 2 62 36 6 10 5 22 24a" -7.10 24 - 1 91 8 3 10 5 23 32a' -8.13 5 1 6 64 29 86 8 1 0 23a" -8.22 7 - 7 84 10 87 1 1 0 31a' -8.28 16 10 4 71 16 3 69 6 5 30a' -8.53 3 1 2 83 14 2 58 28 9 29a' -8.82 20 11 10 69 11 66 7 2 1 22a" -8.85 3 - 6 71 22 1 76 15 4 28a' -9.06 6 1 2 80 16 10 58 15 11 21a" -9.64 13 - 34 63 3 0 0 0 0 27a' -10.19 3 35 8 55 2 0 17 3 62 26a' -10.31 13 73 3 20 5 2 4 1 13 41
case, and is very similar to ZrRu. When the compositions of the two
orbitals are compared directly, the Zr and Th metal compositions are
seen to be very similar. In both cases the principal contribution
comes from the dz2 and d^ orbitals with only a minor amount of f z3
character (15.5%). The total amount of Zr and Th character is also
similar, 15.3% and 15.5%, respectively. Further, examination of the
Ru based character yields a close parallel between the two compounds.
The amount of Ru character is found to be 72.2% in ZrRu and 68.5% in
ThRu. The decomposition of the Ru character shows that the relative
contributions of 5s, 5p, and 4d are nearly identical in the two
compounds. It is best to describe the Th-Ru interaction as a dative
type interaction where the RuCp(CO)2 fragment acts like a "pseudo halide" and donates electron density in a a fashion to the Cp2Th(X)+
or Cp2Zr(X)+ fragments.
The 23a" and the 32a' levels are the other two iodine lone pair
orbitals. The next six levels in the range from -7.10 to -5.90 eV are
the metal-Cp it2 levels. These levels average 23% Th and 7% Ru in metal character with the majority lying in Cp based orbitals. The
decompositions of the metal character in these orbitals also indicates
that 5f participation is important to the Th-Cp «r2 interactions (34a'
and 26a") but not nearly as substantial as the 6d participation. The
HOMO (35a') lies at -5.90 eV and the LUMO (36a') lies 1.32 eV higher at -4.58 eV. The LUMO and the seven orbitals above it (-4.58 to
-4.08 eV) form a thorium based manifold consisting of a mixture of the seven 5f orbitals and one 6d orbital. 42
An examination of the electronic structure of the Cp2Th(I) and
RuCp(C0)2 fragments (Figure 3, Table 3, and Table 4) reveals that
energies of the orbitals do not change drastically on metal-metal bonding. Although a great deal of mixing occurs due to the low
symmetry, only one interaction changes substantially in energy. That
interaction is the metal-metal bonding level. It is evident
from Figure 3 that the Ru 4d level is the most important contributor
to this interaction. One problem that is discovered in the RuCp(CO)2
fragments is the fact that the Cp »r2 levels lie above the Ru 4d
levels. This is not expected and may be due to the lack of charge on
this fragment. If the number of electrons between the thorium and
ruthenium fragments were split up such that RuCp(CO)2" and Cp2Th(I)+ were present in the original molecule, then the situation might
change. The Ru 4d levels would probably rise in energy faster than
the Cp 7t2 levels and may resolve this problem.
A comparison of the PES, Xa-SW, and Fenske-Hall results for
Cp*Re(CO)3 reveal similar features.151 In this case the PE spectra has been assigned with the 5d levels above the Cp tt2 levels.Fenske-Hall predicts the same ordering of the levels but the correspondence to the actual energies is not very good. Xa-SW results are much closer to
the actual PES results, but the 5d levels are found lower in energy
than the Cp jt2 levels. It is felt that the Xa-SW method predicts too high an energy for the ir2 levels while Fenske-Hall predicts too low an energy. This may in part be due to the low overlap of the C atomic 43
Table 3. Energies and Decompositions of the Valence Levels of the Cp2Th(I) Fragment of Cp2Th(I)RuCp(C0)2.
Irbital e(eV) %Th %s %p %d %f %I
18a" -4.24 93 - 1 12 87 6
17a" -4.46 98 - 0 5 95 1
23a' -4.47 98 1 0 2 97 1
22a' -4.57 96 0 0 17 82 1
16a" -4.57 97 - 0 10 89 0
21a' -4.71 87 0 1 58 41 7
20a' -5.29 86 12 3 74 11 2
19a' -6.39 27 2 11 40 46 5
15a" -6.60 32 - 3 43 54 4
18a' -6.75 33 1 6 87 7 3
14a" -6.99 37 - 0 87 13 5
17a' -8.25 7 0 3 79 18 92
13a" -8.27 7 - 6 85 9 89
16a' -8.85 23 9 8 71 11 73
12a" -9.65 13 - 35 62 3 0
15a' -10.31 16 66 3 27 3 3 44
Table 4. Energies and Decompositions of the Valence Levels of the RuCp(CO)2 Fragment of Cp2Th(I)RuCp(CO)2.
dtal €(eV) %Ru %s %p %d %C0
17a' -4.70 35 8 6 86 26
16a' -5.72 18 7 29 63 36
11a" -6.81 28 - 24 76 14
15a' -7.87 81 5 2 92 8
14a' -8.45 65 1 2 97 25
10a" -8.77 80 - 0 100 15
13a' -8.99 69 0 0 100 19
12a' -10.21 20 10 13 77 4
9a" -11.89 0 - 16 84 99
8a" -12.16 5 - 0 100 89
11a' -12.27 5 0 6 94 92 45
-4.
Th 5 f 17a' -5. _
20a'
-6. -
-7. -
eV
-8. -I
-9. -
- 10.- Cp 7C1 * Cp
Cp2Th(l) Cp2Th(l)RuCp(CO)2 RuCp(CO)2 -11.- I
Figure 4. Molecular Orbital Diagram of the Upper Valence Levels of Cp2Th(I)RuCp(CO)2 and its Fragments, Cp2Th(I) and RuCp(C0)2. 46
spheres with the metal centers in these cases. Ferrocene for example
has some metal-carbon overlap and does not have the reversed ordering.
One can conclude that the CpRu(C0)2 fragments are nearly
identical in the two molecules and that they behave as organometallic
"pseudo-halides". The GpRu(CO)z fragment is best regarded as an
anionic fragment that donates electron density in a a sense to the
cationic Cp2M(X) fragments. One can also conclude that thorium is
present in the d°f° form, where it closely resembles a transition metal. Only a small amount of 5f character in comparison to 6d is present in any of the Th-L interactions, including the Ru-Th dative
interaction. Generally, the CpRu(CO)2 fragment should be able replace
a halide in a variety of actinide compounds just as in Zr compounds.
Also, the similarity of the electronic structure determined here led
to doubts about the CO stretching frequencies originally reported by
Harks and coworkers.91 Reexamination of the infrared spectra of the thorium compounds revealed an error that has subsequently been corrected.137 The spectra of the zirconium and thorium compounds were found to be very similar to each other.
Cp3ThRuCp(C0)2
A later paper by Sternal, et al. describes the preparation of another set of heterobimetallic compounds which are very similar to their earlier actinide compounds. In these compounds Cp3Th and Cp3U fragments replaced the Cp2AnX fragments and both RuCp(C0)2 and
FeCp(C0)2 anionic fragments were employed to make analogous dative compounds.137 The Cp3ThRuCp(C0)2 and Cp3URuCp(C0)2 compounds were 47
studied in an attempt to look at the differences in the electronic
structure between these molecules as well as the earlier Th and Zr
systems.
The major differences between the electronic structures of the
Gp3ThRuGp(CO)2 and Gp2Th(X)RuGp(C0)2 molecules is the presence of an
extra Cp ligand in place of a halide. This replacement makes the
calculation more difficult by adding more levels, but yields a cleaner picture since there are no halide levels. The lower valence levels
are organized somewhat differently than in the Cp2Th(X)RuCp(C0)2 calculation but fall at essentially the same energy.
The upper valence levels for Cp3ThRuCp(CO)2 as compared to those
in Cp2Th(I)RuCp(CO)2 are diagrammed in Figure 5 and their decompositions are given in Table 5. In Cp3ThRuCp(CO)2, there are
four Cp levels that are found in the -10.42 to -9.59 eV range. The three levels 33a', 26a", and 34a' are Ru 4d CO it* levels, which lie between -8.95 to -8.48 eV. These contain an average of 20% CO character. The orbital that contains the majority of the metal-metal interaction (35a') lies at -8.19 eV. The decomposition of this orbital indicates that both the Th and Ru fragments are very similar those in Cp2Th(I)RuCp(C0)2. There is a somewhat lower amount of Th character in this orbital (13%) as compared to the 31a' in
Cp2Th(I)RuCp(C0)2 (15.5%). The amount of Ru character is also somewhat higher, although this may be an artifact of mixing and may not accurately represent the metal-metal interaction. The next block of eight levels are all Th-Cp and Ru-Cp irz levels and include the HOMO
(39a' at -5.91 eV). Since there are now three Cp ligands on the Th 48
35a'
45a’ 34a" 44a’ Th 5f
36a'-40a' Th 5f
39a' 35a' 30a" 38a' 26a" 29a" 34a' 37a’ 25a" 28a" 24a" 36a' 33a' 27a" eV
32a' 35a' Ru-Th Ru-Th 34a' 30a’ Ru-CO 29a' 26a" ■ Ru-CO it1 22a" 33a’ 28a' 32a' 21a' 25a"
- 10. 27a’ 31a' 26a’ 30a'
- 11.
Figure 5. Molecular Orbital Diagram of the Upper Valence Levels of Cp2Th(I)RuCp(CO)2 Compared With Cp3ThRuCp(CO)2. 49
Table 5. Energies and Decompositions of the Valence Levels of Cp3ThRuCp (CO) 2.
Orbital e(eV) %Th %s %p %d %f %Ru %C0 %CpRt
49a' -1.71 60 0 0 69 31 2 9 1 38a" -1.75 62 - 0 75 24 6 5 2 48a' -1.94 65 0 0 64 36 7 5 1 37a" -1.97 7 - 0 24 76 51 21 15 47a' -2.21 57 2 0 47 52 16 6 5 36a" -2.45 14 - 0 24 75 35 28 17 46a' -2.49 47 1 0 47 53 7 28 4 35a" -2.71 82 - 0 0 100 0 0 0 45a' -3.33 4 0 0 6 94 31 64 2 34a" -3.37 30 - 0 11 89 8 61 1 44a' -3.42 9 1 0 8 91 27 43 19 43a' -3.65 95 0 0 7 93 3 0 0 33a" -3.66 84 - 0 4 96 7 7 0 42a' -3.86 94 2 0 0 98 1 2 2 32a" -3.91 5 - 0 18 82 3 81 11 41a' -4.02 94 0 0 9 91 1 1 0 40a' -4.04 90 0 0 18 82 3 1 2 31a" -4.11 80 - 0 10 90 3 13 0 39a' -5.91 12 13 33 43 12 10 22 25 30a" -6.00 31 - 0 0 100 0 0 0 38a' -6.22 23 4 8 65 24 4 5 16 29a" -6.35 22 - 4 51 46 9 3 14 37a' -6.47 28 0 5 63 32 3 0 1 28a" -6.74 19 - 10 89 0 10 5 19 36a' -6.92 31 0 4 94 2 4 1 0 27a" -7.17 22 - 0 95 5 11 5 26 35a' -8.19 13 9 5 72 15 73 7 5 34a' -8.48 3 0 1 85 14 60 28 7 26a" -8.77 3 - 3 79 18 75 16 5 33a' -8.95 2 5 7 70 18 65 17 14 32a' -9.60 13 0 29 69 2 0 1 0 25a" -9.62 13 - 29 69 2 0 1 0 31a' -10.19 2 29 10 58 4 17 3 71 30a' -10.42 13 83 1 13 4 2 2 6 50
atom, one of the 5f^ type levels Is well suited to Interact with one
of the Cp jt2 levels.118 The LUMO is the 31a" at -4.11 eV and
represents the lowest level in a block of Th 5f and 6d metal based
orbitals. One of the metal 5f levels (35a") is removed to higher
energy than the rest of the metal based block; this orbital is the
antibonding counterpart of the Th-Cp level.
In conclusion, this molecule contains approximately the same
metal based character in the thorium and ruthenium fragments
as displayed previously. Sternal, et al. reported the carbonyl
stretching frequencies for Cp3ThRuCp(C0)2 at 1939 and 1868 cm-1, and at
1968 and 1900 cm-1 for Cp*2Th(I)RuCp(C0)2. In this case, the
substitution of Cp for Cp* is not expected to make a substantial
difference between theory and experiment; Cp3Th < Cp2Th(I) «* Cp*2Th(I)
is the expected ordering of the yco values. Also, note that the Th 5f
orbitals are moved to higher energies in Cp3ThRuCp(C0)2, indicative of
the better donor abilities of Cp over I. A direct comparison of the
amount of Ru-CO n* levels in these two molecules is difficult to
determine due to the mixing between upper valence levels. Totalling all of the CO character in the upper valence levels in the two compounds yields a higher amount of CO ir* character in the
Cp3ThRuCp(C0)2 molecule. These values as well as the electron density present on the ruthenium atoms reflect the same trend as exhibited by the Vqq absorption bands. 51
Cp3URuCp(C0)2
In Cp3URuCp(CO)2, the actinide center has two more electrons than
in the thorium complexes. Will this influence the ability of the
CpRu(CO)2 fragment to bond to the actinide? Will this extra electron
density make the metal-metal interaction more covalent than in the Th
molecules? Examination of the lower valence orbitals of the ligands
revealed that the orbitals are very similar in the four
heterobimetallic CpnAnX(3_n)RuCp(CO)2 molecules studied, especially for
the Cp3AnRuCp(CO)2 molecules. The upper valence levels of this
molecule are very similar to those in the Cp3ThRuCp(C0)2, as shown
in Figure 6 and Table 6. The four lowest levels to be considered
(30a', 31a', 32a', and 25a") lie between -10.43 and -9.55 eV. These
represent the Cp interactions with uranium or ruthenium. Similar
to the Cp3Th case, the next three levels are Ru 4d CO t t * interactions
that lie between -9.13 and -8.62 eV. The 35a' U-Ru metal-metal a
interaction follows next at -8.52 eV. This level contains a higher
amount of actinide character (17%) and a lower amount of ruthenium
character than is present in the Cp3ThRuCp(CO)2 case. This would lead
one to conclude that the amount of charge transferred from the
ruthenium fragment to the actinide fragment must be higher in the
uranium case. This stands in direct contradiction to the vCQ values
reported by Sternal, et al. These data must be carefully analyzed, however, due to the low symmetry of the molecule. An analysis of the
total CO character in the upper valence orbitals and the charge on the
ruthenium atom yield results consistent with the i/co trends in the
actinide molecules. The next eight occupied valence levels are again 52
35a' Th 5f
CO CO' 34a’ U 5f Th 5f 33a1
31 a"-32a' U 5f
39a' 39a' 30a" 30a" 38a' 29aM 37a' 37a’ eV 28a" 28a" 36a' 36a' 27a" 27a'
35a' Th-Ru 34a' 35a' U-Ru 34a' 26a" ■ Ru-CO Tt’ 33a' 26a" Ru-CO Tt’ 33a'
32a' 32a' 25a" 25a"
-10. 31a' 31a' 30a' 30a'
-11.
Figure 6. Molecular Orbital Diagram of the Upper Valence Levels of Cp3ThRuCp(CO)2 Compared With Cp3URuCp(C0)2. 53
Table 6. Energies and Decompositions of the Valence Levels of Cp3URuCp(CO)2.
Orbital e (eV) %U %s %p %d %f %Ru %C0 %c Pru
45a' -3.39 2 0 1 57 42 29 66 1 44a' -3.50 3 0 0 17 83 29 45 21 35a" -3.52 8 - 0 1 99 6 76 0 34a" -3.68 72 - 0 0 100 0 1 0 33a" -3.95 2 - 0 0 100 3 85 10 32a" -4.35 87 - 0 4 96 6 1 2 43a' -4.38 91 0 0 5 95 3 0 1 42a' -4.68 96 1 0 7 92 0 0 1 41a' -4.76 98 0 0 1 98 0 0 0 40a' -4.80 97 0 0 2 98 0 0 0 31a" -4.84 95 - 0 3 97 0 3 0 39a' -5.96 12 13 33 36 18 9 21 27 30a" -6.07 45 - 0 0 100 0 0 0 38a' -6.13 21 3 7 46 44 4 5 20 29a" -6.26 29 - 1 27 72 7 2 11 37a' -6.36 33 0 2 41 57 4 1 11 28a" -6.64 22 - 9 91 0 5 3 8 36a' -6.73 27 0 6 93 1 3 0 0 27a" -7.17 14 - 0 91 9 16 8 39 35a' -8.53 17 8 6 64 22 69 8 4 34a' -8.62 4 0 0 76 24 59 28 7 26a" -8.97 5 - 3 68 29 73 17 4 33a' -9.13 5 6 7 62 25 63 16 15 32a' -9.55 11 0 31 66 2 1 2 0 25a" -9.60 11 - 33 65 2 0 0 0 30a' -10.21 4 44 7 45 4 16 2 58 31a' -10.43 11 90 1 5 5 6 4 18 the Cp jt2 levels. They are somewhat different from the Cp3ThRuCp(CO)2
case in that the 5f participation in the U-Cp is much more important.
The HOMO (31a") falls at -4.84 eV and represents one of the uranium
5fS based levels. An interesting detail about this molecule is that
while there is a lone pair on the uranium atom, the ruthenium anion
still effectively donates electron density. The other interesting
detail is that the HOMO doesn't contain any CpRu(C0)2 character
whatsoever. The LUMO (40a') is at -4.80 eV and is the lowest in a
block of 5f and 6d uranium-based levels which extends up to -4.35.
This orbital is the other 5fS orbital. This low degree of splitting
between the HOMO and LUMO could almost certainly result in partial
occupation of these two orbitals and possibly even spread out to
include more of the uranium based levels. Again, there is one
additional 5f level about 1 eV higher in energy than the rest of the
levels. The level is destabilized by this amount through an
antibonding interaction with the highest Cp ir2 level.
Conclusions
In each of the four heterobimetallic actinide compounds described
above, the metal-metal interaction can best be described as a dative
interaction. The RuCp(C0)2 fragment is present as an anion which
donates electron density in a a fashion to a cationic actinide or
transition metal fragment. The similarity of the t/co values reported by Sternal et al. and Casey et al. reflects this overall similarity in both the actinide and transition metal compounds. This description is again confirmed in the decompositions of the metal-metal bond, which are very similar in all four molecules, as summarized in Table 7.
When these numbers are compared for the cationic fragment metals the
amount of f character is shown to be the most important in the Cp3U
case with only 21.5% 5f character. Also, the amount of 5f character
simply replaces the loss in d character over the zirconium case. All
four molecules are even more similar in their anionic ruthenium
fragments. The similarity of the metal-metal a interactions in these
four molecules is also apparent in Figure 7, wherein only subtle
differences can be detected. The major differences between these
figures are due to the slight mixing of 5f into this orbital. In
general, the amount of the electron density present on the metal atom
of the cationic fragment is communicated to the carbonyls of the
anionic fragment, as evidenced by the j/co values (Cp3U < Cp3Th <
Cp2Zr(Me) < Cp'2Th(I)). The trends of the i/co stretching frequencies
are seen to reflect the better donor capability of Cp over halide
ligands. The choice of metals is also shown to affect these values U
< Th and Zr < Th. The vco values of the actinide compounds studied
have been shown to roughly follow the amount of Ru-CO 7r* character
located on CO. This also correlates well with the amount of Cp it
character on the actinide or ruthenium centers. These data support
the conclusion that the ionic character of the Cp it interactions is
Zr-Cp < U-Cp < Th-Cp. The i/CQ frequencies of Cp2Zr(I)RuCp(CO)2 molecule is not predicted correctly by the same models when compared
to the actinide molecules, however. It appears that even the small
changes in the i/co values can be predicted by the Xa-SW method as long as similar metals and geometries are compared. The most accurate 56
Table 7. Summary of the Decompositions of the Metal-metal Interactions of Cp2Zr(I)RuCp(CO)2, Cp2Th(I)RuCp(CO)2, Cp3ThRuCp(CO)2, and Cp3URuCp(CO)2.
%M %s %p %d %f
Cp2Zr(I)RuCp(CO)2 M - Zr 15.2 9.1 7.5 83.4 -
(30a' MO) M - Ru 72.2 6.6 0.8 92.5 -
Cp2Th(I)RuCp(CO)2 M - Th 15.5 9.7 3.6 71.8 15.5
(31a' MO) M - Ru 68.5 6.8 1.8 91.4 -
Cp3ThRuCp(CO)2 M - Th 13.4 9.1 4.7 71.5 14.8
(35a' MO) M - Ru 72.5 6.5 ' 1.7 91.9 -
Cp3URuCp(CO)2 M - U 17.5 8.2 6.0 64.4 21.5
(35a' MO) M - Ru 68.5 6.7 1.9 91.3 - 57
»#* *» • a O C ; X m -”! »
~ ...... ^ c
/ ; Zr>. v.^Ru {\\\ i S&Thtt L jPu-Vr:
* »*••** l 1 **** y • \ •*•••/ M ... \ * N-- / *... -•* / ..— . \ (c s\\. '1 'l w ..... Y " - ' i r \ ...... - " ■ i i l l )) ■ ( i h i i \ n 1
b ......
Y Y i r Y .. - i - w k S ' A 1 : r Z r K i :Ru ; S ! 1 i f \ u }
* ( 0 ) ''''...... •'*
® \ ) 9 N )
( n \ \ ( p i . §
i i I.. . j h u V i * ! : # U $ 5 i R u •.**. * '
**, / /\ /1 ♦ • * ## / * v / ^ | .. . ' *.. *
, . Q - • ..... 1 ! / « A *
\ ( ^ \ /".*•__ !'>, ... «.
; \ v . ® T h ,v I. H U } } ! \ I 'woV:-'.
Figure 7. Contour Diagrams of the Metal-Metal Interactions of Cp2Zr (I) RuCp ( CO) 2 1 Cp2Th(I)RuCp(CO)2, Cp3ThRuCp(CO)2, and Cp3URuCp(CO)2 in xz (a, c, e, g) and yz (b, d, f , h) Planes. 58
indication can be determined from the interactions within the cationic
fragment. More distant interactions show a smaller effect and as such
are subject to higher amounts of error.
Another set of observations is that Th(IV) and Zr(IV) are very
similar electronically. The two metals are similar because f-orbital
involvement is generally not very important in the Th cases. There is
a difference in the unoccupied orbitals, however. In these, the
unoccupied 5f and 6d hybrid orbitals are located in a tight band. The
Cp33- ligand field is better able to interact with the 5f atomic
orbitals, especially the highest lying occupied ligand orbital.
Uranium(IV) is different in that the 5f levels play a much more
important role in metal-ligand interactions than was present in the
Th(IV) cases. It is interesting to note that the U-Ru and Th-Ru
interactions are both nearly as ionic as reflected in both the
electronic structures and the j/co values.
Lastly, the presence of the isovalent fragment CpFe(CO)2‘ in place of CpRu(CO)z" in these compounds is shown to create almost no
difference in their chemistry. Likewise, the i/CQ values are only
slightly different in the two cases. For example, in Cp3ThFe(Cp)(C0)2
the values are 1927 and 1873 cm-1 and in Cp3ThRu(Cp) (C0)2 the values
are 1949 and 1886 cm-1. In fact, the difference here is almost
identical to the difference between Cp3ThRu(Cp)(C0)2 and
Cp*2Th(Me)Ru(Cp)(C0)2. This difference reflects the greater backbonding present in the iron compound and is consistent with the
slightly higher electronegativity of iron over ruthenium. Similarly,
the Hi-NMR and 13C-NMR are again very similar for these two molecules. These later data provide clear evidence that the Th-Ru and Th-Fe bonds are very similar. By extrapolation, it should be clear that the An-Ru and An-Fe interactions should also be similar. CHAPTER IV
HOMOBIMETALLIC SYSTEMS VITH a-ONLY LIGANDS
Introduction
In the previous section, several heterobimetallie compounds that
contain a direct actinide-to-transition metal interaction were
discussed.91’90'99,137 This type of metal-metal interaction was shown to
be dative in nature, with an anionic "pseudo-halide" fragment donating
electron density in a a fashion to a cationic actinide fragment.99,100
At this time, this is the only type of direct metal-metal interaction
demonstrated by the actinide elements within discrete molecules.91’137
There are no known compounds that contain either a covalent
interaction between an actinide and a transition metal, or a direct
actinide-to-actinide bond. This seems unusual when contrasted to the
plethora of transition metal compounds that contain metal-metal
bonds.5,101
A comparison of transition metal and actinide organometallics
shows many overall similarities and subtle differences. There seems
to be a close correspondence, for example, between Mo(IV) and U(IV)
chemistry, as well as for Zr(IV) and Th(IV).89,91,137 Two questions
that will be addressed in this section are: (1) Can the close
correspondence between uranium and molybdenum then be exploited to make an actinide-actinide bond? (2) If this is not the case then why
is this portion of the two chemistries so different?
60 61
Some of the Important differences between uranium and molybdenum
(or tungsten) compounds are summarized: (1) Cp*2MX2 compounds for
tungsten and uranium have been prepared, but the W(IV) compound Is a
d2 diamagnetic complex while for U(IV) it is paramagnetic and f2.86,88
(2) Uranium can acheive a higher coordination number than molybdenum,
as demonstrated by the formation of U(cot)2.5,17 (3) Oxide containing
molecules of Mo(VI) and U(VI) show a bent O-M-O structure in the
former and linear in the latter case.80,153 (4) Uranium alkoxides tend
to contain nearly linear U-O-R linkages while all except the early
transition metals show a bent linkage.80 These differences in the
chemistry of these compounds have been attributed, at least in part,
to the presence of f orbitals in the actinides. Could the presence of
f orbitals also be the rationale for the absence of actinide-actinide
bonds? If this is the case, does it reflect low overlap between the f-
orbitals and f-elements compared to d-orbitals or is some other factor
present?
In order to answer these questions, it is important to compare
and contrast the nature of bonding in both molybdenum and uranium
systems. It is instructive to first examine the bonding for the
simplest cases capable of containing covalent metal-metal
interactions, "naked" metal dimers. Later, this analysis will be expanded in steps to include more complex systems.
Mo2 has been produced in the gas phase and prepared by matrix isolation techniques.154,155 Spectroscopic studies of Mo2 have shown the Mo-Mo distance to be 1.929 A. This is considerably shorter than any quadruple bond length measured for molybdenum compounds and thus 62
Mo2 was expected to contain a higher bond order.156 Such "naked"
transition metal dimers have the capability of forming bonds in the
familiar a, it, and S symmetries from d orbitals. The use of only the
d-orbitals would dictate that a quadruple bond would be the highest bond order. In the case of diatomic molybdenum, Xa-SW157, Xa-
SW(PXa)158, and Cl159 calculations all agree that a sextuple bond would
form between the two metal centers. The ordering of bonding levels predicted by Xa-SW is found to be a2jr*$V2.157 This is different than
the normal metal-metal bonding picture in that two bonding
interactions of a symmetry appear. The first level is attributed to
the dz 2 orbitals while the upper arises from s-s interaction.
Normally the s orbital is removed to higher energy due to metal-ligand
interaction and does not appear in the metal-metal bonding manifold.
Calculations have indicated that it is this second a orbital that accounts for the majority of the bond shortening from the quadruple bond length.102,150 The it and 6 interactions are nearly pure d in
character arising from the d*z-dy2 and <^-<1*2 ^ 2 pairs respectively.102
To date, a very limited amount of experimental work has been reported for gas phase and none for inert matrix preparations of the
isovalent molecule U2.103 The dimer has been shown to exist in the gas phase by mass spectroscopy.162 Theoretical calculations have been previously reported for the hypothetical U2 and Np2.72 For such actinide systems, interactions of a , it, S , and now also 4> symmetry would be possible, due to the presence of f orbitals. The formation of these interactions from f orbitals is illustrated in Figure 8.72
It appears that the lobes of the a and it combinations of the f 63 o D * 0 | o
/»/•fz2x> f e y 71
fex2-y2)» fexy) S
fex2-3y2)> fey2-3x2) 0
Figure 8. Metal-metal Interactions of the f Orbitals. 64
orbitals can overlap substantially due to their directionality,
analogous to that for d orbitals. In addition, the f 5-forming-
orbitals, which contain a z component, are directed somewhat towards
each other, unlike d S forming orbitals. Xa-SW calculations have
confirmed the possibility that f S interactions are much stronger than
those arising from d 6 orbitals. For U 2 and Np2 the f
interactions would then replace the d S type interactions. In both of
these molecules, this interaction is similar due to the small degree of overlap expected between two orbitals that lie in parallel planes.
Molecular orbital calculations have indicated that these
Another calculation was performed on U2 under D3d symmetry and at a longer U-U distance of 2.6 A. (D3d symmetry was chosen in order to give a more direct comparison to the M2Lg molecules which will be discussed later.) This calculation indicates approximately the same result as that reported for the earlier compound. The interaction of the metal based orbitals to form the U2 valence levels is shown in
Figure 9. The decompositions of these orbitals are presented in
Table 8. This compound also exhibits a bond order of six with a 65
5f a’ 5f 5f 5 7s a 5f/6d 7i 6d/5f o Figure 9. Molecular Orbital Diagram of the Valence Levels of U2 (U-U - 2.6 A). 66 Table 8. Energies and Decompositions of the Valence Levels of U2 (U-U - 2.6 A). Level eV Occ %s %p %d %f 2e« -0.30 0 - 32 42 26 2a2u -1.38 0 6 1 16 78 la2g -1.66 0 - - - 100 3aig -1.66 0 --- 100 la2u -1.81 0 - - - 100 lalu -1.81 0 -- - 100 le6 -2.30 4 - - 10 90 2alg -2.96 2 90 2 0 8 l®u -4.05 4 - 2 43 55 ^•a lg -6.05 2 9 8 47 36 67 configuration. The f orbitals are found to play the dominant role in this compound while d orbitals play an important role in the higher symmetry interactions. The d contribution to the metal-metal bonding levels is 47% (a) and 43% (tt) . The metal s orbitals also play an important role for the higher energy a interaction and to a much lesser extent in the lower a orbitals. The calculations indicate that U2 and Mo2 should form similar metal-metal interactions. The two compounds differ in that the f orbitals on the actinide permit more bonding interactions to exist (these are unoccupied in this case). The f orbitals have a higher z component and are expected to interact more strongly than d orbitals. Thus, higher bond orders and perhaps stronger interactions should be attainable for actinide compounds. In order to examine the possibility of forming a bimetallic compound, we decided to undertake Xa-SW calculations on a series of hypothetical actinide dimers. In order to achieve good ligand-to-f orbital overlap, it is necessary to choose an early actinide of low oxidation state. The extension of f orbitals in such an actinide is much larger than that of the later or high oxidation state actinides.79 Based solely on structural studies, it has been previously argued that U(IV) homobimetallics would have little capacity for forming direct metal- metal bonds. However, it may be possible for U(III) to form this type of a bond.79 For this reason, U(III) was chosen as the starting point for the next series of complexes. There is also experimental evidence to suggest that /3-ThIa may have a metal-metal interaction, although the short Th-Th distance of 3.45 A seems a little long (by 0.15 A) for 68 a single bond.160 This distance is shorter than that observed in thorium metal. A problem exists in choosing thorium for model compounds in that it behaves too much like an early transition metal, in that it uses d orbitals in preference to f in bonding interactions.100 Using uranium is also more effective, since experimental and theoretical data is available for a variety of the isovalent molybdenum dimers.78*101,161,163,164,165 Numerous reports of a- bonded alkyl actinide compounds have appeared in the literature.17,166 While the majority of these are Cp3UR compounds, there are a few other types.166 The homoleptic alkyls are decidedly rare for the actinides and attempts to make them generally result in reduction to uranium metal. One homoleptic compound, heptamethylthorium(IV) anion,77 has been prepared as well as tetralkyl phosphine complexes of thorium and uranium.167 Recently the monomeric, pyramidal compound U[CH(SiMe3)2]3 was reported.40 For these reasons, the studies that follow were performed on molybdenum and uranium organometallic complexes. In order to keep the problem simple, methyl was chosen since it is a small, cr-only ligand. Computational Details of the U2Men System Xa-SW calculations were performed on a model of the known compound Mo2R6, R - CH2SiMe3168 and the analogous hypothetical model diuranium complexes U2Me4, U2Me6, and U2Me8. These calculations were performed in the manner previously described. Further parameters for each molecule are reported in Appendix B. Cramer, et al. reported a number of U-C distances to compare to their uranium-carbon double bond length of 2.29 A.160 The single a bond Interactions range from 2.45 to 2.66 A with the higher values present in multiple alkyl compounds.170,171 Since there are no compounds containing a direct uranium-uranium bond, all other parameters were estimated from the covalent radius of uranium (1.42 A) and the known structure of Mo2R6 (R- CH2SiCH3) .16B If two times the covalent radii is assumed to be the length of a single bond, then one arrives at a U-U length of 2.84 A, comparable to the known Th-Ru bond length of 2.9 A.91 Inasmuch as all the compounds Investigated are expected to have multiple bond orders, shorter U-U distances would be expected. If the metal f orbitals can interact in U2MeA, U2Me6, and U2Me8, then the metal-metal bond orders are expected to be 4, 3, and 2, respectively. Basing the length of the higher order triple bond on the ratio of a Mo-Mo triple bond to a Mo-Mo single bond, one obtains a length of 2.4 A. Calculations have been run on all of these hypothetical molecules at this U-U distance, as well as a slightly longer 2.6 A, which some have argued to be more valid.172 In the following section, the nature of the metal-metal and metal-ligand interactions in the Mo2Me6 molecule will be examined first. The next step will be to examine and contrast the structure of the hypothetical compound U2Me6 with the transition metal analogue Mo2Me6. Then we will show how changes in the geometry of the U2Me6 molecule from the Mo2Me6 structure affect the metal-metal and metal- ligand interactions. The last cases to be discussed will be those where some of the methyl groups were removed (U2Me4) or more were added (U2Me8) in order to gauge the effects of oxidation number, as 70 well as geometry, on the overall system. The discussions which follow will address the results of calculations obtained on all the hypothetical molecules listed in Table 9. MojMe6 Xa-SW(PXa) calculations were previously performed by Bursten, et al. on the model compounds Mo2R6 (R - Me, OH, and NH2).161 The Xa-SW calculation on Mo2Me6, when repeated, yielded results that are consistent with those previously published. There are some small differences between the geometry of Mo2Me6 chosen for the two studies, however. In this study, the Mo-Mo-C bond angle was set to 105° closer to the average value for Mo-Mo-L angles in Mo2L6 compounds than that found for Mo2Rg (R-CH2Si(CH3)3) .168 The electronic structure predicted by Xa-SW for the upper valence levels of Mo2Me6, as constructed from two MoMe3 fragments, is shown in Figure 10. The characters of the highest lying valence orbitals are tabulated in Table 10. As methyl is a a only ligand, one might expect the electronic structure of the Mo2Me6 molecule to be relatively simple. As the methyl ligands are added to a Mo2 core they would best interact with the s, d^y, and dx2 _ y 2 orbitals. This leaves the dz2 , d^, and dyZ orbitals available to form one a and two «• metal-metal orbitals. This analysis provides a simplified but fairly accurate picture of the more complex electronic structure. The majority of the a and jt interactions, as expected, can be attributed to overlap of the dz2 , dyZ, and d ^ orbitals on the two metal centers; the majority of the metal-ligand interaction is due to s, dx2 _y2 , and d^. on the metal 71 Table 9. Geometries of the Hypothetical U2Men Molecules Studied. Compound U-U U-C U-U-C geometry U2Me4 2.6 A 2.6 A 90° °2d U2MeA 2.6 A 2.6 A 135° °2d U2Me* 2.6 A 2.6 A 135° °2h U2Me6 2.4 A 2.6 A 1 0 5 ° D3d U2Me6 2.4 A 2.6 A 105° D3d U2Me6 2.6 A 2.6 A 90° °3d U2Me6 2.6 A 2.6 A 105° °3d U 2Me6 2.6 A 2.6 A 135° D3d U2Me8 2.4 A 2.6 A 105° U2Me8 2.6 A 2.6 A 105° °4h 72 Mo d eV Mo-C tc/Mo-C MoMe3 Figure 10. Molecular Orbital Diagram of the Valence Levels of the MoMe3 Fragment and Mo2Me6. 73 Table 10. Energies and Decompositions of the Valence Levels of Mo2Me6. Level eV Occ %Mo %s %p %d %C 7eu -0.92 0 65 - 17 83 28 6aig -1.05 0 8 3 10 87 90 6e8 -1.93 0 80 - 6 94 15 6eu -4.79 4 69 - 7 93 26 ^a2u -5.16 2 30 54 1 45 63 5alg -5.30 2 43 82 1 16 51 -*e6 -6.04 4 55 - 5 95 39 5eu -6.33 4 72 - 0 100 22 4aig -7.67 2 88 2 6 93 8 74 centers. There is, however, a degree of mixing between orbitals of similar symmetry.161 The mixing of the djr and S type orbitals results because the Mo-Mo-C angle is not equal to 90° or 135°, at which the interaction with it or S type metal orbitals would be optimized. As pointed out by Bursten et al., the HOMO (6eu orbital) is primarily it metal-metal bonding, but also contains a good deal of 6* character that weakens this interaction.161 This S* character, however, also serves to reduce, more substantially, the amount of Mo-C antibonding character in this orbital.161 Contour diagrams for the MoMe3 fragment frontier orbital that produces this orbital and the 6eu it metal-metal bond are shown in Figure 11. It is apparent from Figure 10 and Table 10 that the stabilization of this metal-metal it orbital over the fragment orbitals is > 1.4 eV. The next metal- metal orbital (4alg) is primarily a metal-metal a bonding orbital and lies below the metal-ligand interactions. This orbital is formed from a mixture of s and dzz on the metal centers, as contour diagrams of the fragment frontier orbital and the final 4alg a metal-metal bonding orbital indicate in Figure 11. As indicated in Table 10, Table 11, and Figure 10, the degree of stabilization of this metal-metal a bonding orbital is about 3.5 eV. The metal-ligand interactions are the 5eu, 5eg, 4a2u, and the 5alg. The 5eu orbital is primarily a Mo-C bonding orbital, and has a similar mixing of 6 and it type d orbitals as found for the Mo-Mo it interaction. The other degenerate metal-ligand orbital set is the 5eg. This orbital contains a high degree of metal character, while a metal-metal S interaction would not be expected to be occupied. This 75 Mo; Mo Mo Mo Figure 11. Contour Diagrams of the a and jr Metal-Metal Interactions of Mo2Me6 and Their Fragment Orbitals. 76 Table 11. Energies and Decompositions of the Valence Levels of the MoMe3 Fragment. Level eV Occ %Mo %s %p %d %C 7e -2.35 0 59 - 11 89 28 6ax -2.79 0 84 12 1 87 12 6e -3.39 2 94 0 100 2 5 a! -4.19 1 96 13 1 87 4 4aj -5.72 2 41 30 1 69 52 5e -5.89 4 54 * 4 96 41 77 is taken as evidence that the ligands effectively a donate their electron density to molybdenum in this orbital. The 4a2u and 5alg orbitals are also primarily metal-ligand bonding orbitals. Both orbitals contain a mixture of s and a small amount of dz2 metal character. The symmetry of the 4a2u orbital indicates that it contains some metal-metal antibonding character. It is important to note that the overall configuration of the metal-metal interaction is a2jrV*°o*0 formally resulting in a triple bond between the two metals. There are no orbitals available to form a metal-metal S bond, as the dx2 _ y 2 and d^ orbitals are employed in forming the metal-ligand interactions and are removed from the picture. It is interesting that the 5eg orbital is largely located on the metal centers, however. UgMeg Can the similarity between molybdenum and uranium be exploited to produce isostruetural uranium dimers analogous to Mo2Me6? Xa-SW calculations were performed on the hypothetical U2Me6 molecule where the geometry was taken to be analogous to that used for the model compound Mo2Me6. The molecule was again idealized to D3d symmetry with U-U - 2.4 A, U-C - 2.6 A, and U-U-C - 105°. A molecular orbital diagram representing the formation of U2Me6 from two UMe3 fragments is given in Figure 12. The decompositions of the orbitals of interest in the U2Me6 molecules and their fragments can be found in Table 12 and Table 13. 78 - 1.0 <|>,8, U - U 7C* -3.0 u a I9 U - U $ -4.0 u-c u-u 5 5a. eV U-C -5.0 - 6.0 U - U TZ -7.0 U - U c UMe3 fragment - 8.0 Figure 12. Molecular Orbital Diagram of the Upper Valence Levels of the UMe3 Fragment and U2Me6 (U-U - 2.4 A). 79 Table 12. Energies and Decompositions of the Valence Levels of U2Me6, (U-U - 2.4 A, U-U-C - 105°). Level eV Occ %U %s %p %d %f %C ®eu -1.57 0 87 - 5 65 30 11 7a2u -2.29 0 91 6 0 2 91 8 7aig -2.44 0 99 9 1 47 43 1 7eu -2.46 0 85 - 1 13 87 14 7*g -2.64 0 96 - 0 7 93 0 6 &2 u -3.08 0 93 2 0 6 92 7 6alg -3.14 0 98 7 0 0 93 2 2a2g -3.16 0 99 -- - 100 0 2alu -3.33 0 99 - - - 100 0 6®u -3.45 4 68 - 3 16 81 30 6es -3.51 4 34 - 10 90 1 61 5e« -3.90 0 99 - 0 2 98 0 5a2u -4.31 2 56 10 1 9 80 40 5aig -4.51 2 43 38 0 9 52 51 5eu -6.00 4 94 - 1 31 69 5 4alg -7.39 2 92 4 7 39 50 4 80 Table 13. Energies and Decompositions of the Valence Levels of the Fragment UMe3. Level eV Occ %U %s %p %d %f %C 8e -1.84 0 65 - 18 62 21 26 8at -2.61 0 91 3 1 14 82 8 7e -2.83 0 92 - 1 10 89 8 2a2 -2.95 0 99 - - - 100 0 6e -2.99 2 100 - 0 0 100 0 7a, -3.00 0 100 0 0 0 100 0 6a! -3.17 1 99 9 0 2 89 1 5e -3.79 4 51 - 5 41 54 45 5a! -4.55 2 45 42 1 10 48 50 81 The methyl groups can interact with the same orbitals in the uranium system as they could in the molybdenum system. However, the fS and the fx(x2 _3y2 ) orbitals are now also of proper symmetry to interact with the a only methyl groups. We are again left with six electrons to fill the metal-metal bonding orbitals. Filling the metal based valence levels according to the structure of U2 again leads to a a zid metal-metal bond configuration, although the overall nature of the bonding is difficult to gauge. The reason for this lies in the fact that more orbitals are available for metal-metal bonding and more f orbitals contain a z component. In fact, the Xa-SW method yields some unusual results for this molecule. The metal-metal levels computed by Xa-SW are the 6alg, 2a2g, 2alu, 5eg, 5eu, and 4alg and represent the interactions, respectively. The metal character in each of these orbitals is dominated by f orbitals and the ligands play a minimal role. It is readily apparent that the major differences between the electronic structures of U2Me6 and Mo2Me6 are a result of the presence of f orbitals in the former. The major result is the inverted ordering of the HOMO and LUMO for U2Me6, as well as the presence of a number of unoccupied orbitals near the HOMO. With regard to metal- metal orbitals, the metal bonding manifold becomes more complex with the introduction of the tf> and S interactions, which cannot exist in Mo2Me6. The Figure 13. Contour Diagrams of the U2Me6 Metal-metal a, jt, and Interactions as Formed from the Fragment Orbitals. 83 destabilization of the and antibonding orbitals (the 5eg and 7eu) are primarily f in character, partially due to the higher overlap allowed from f8 interactions as opposed to dS interactions. Another likely reason for this may be that the methyl ligands tie up the and <1*2 ^ 2 orbitals in this molecule, as was the case in Mo2Me6. This 6 bonding orbital and the fragment orbitals from which it is constructed, as shown in Figure 13, readily show how well they overlap to form this molecular orbital. Also, while it lies below the HOMO in energy, it is expected to be unoccupied, an unusual result. The a and jt metal interactions lie below the metal-ligand levels in energy and are represented by the 4alg and the 5eu orbitals. These filled metal-metal bonding levels are similar to interactions found in Mo2Me6 but consist of a mixture of both d and f character. The contribution of d character is the highest for the a (39%d, 50%f), followed by the w bonding interaction (30%d, 69%f). Contour diagrams of these two orbitals and their fragment orbitals are illustrated in Figure 13. If these figures are compared to the corresponding Mo2Me6 and MoMe3 orbitals in Figure 11, it is apparent how the inclusion of f character influences the shapes of these orbitals. The character of these two orbitals also shows a much lower contribution of ligand character than was shown for Mo2Me6. The additional f orbitals allow the metal-metal and metal-ligand interactions to mix less in this molecule, as indicated by the lower carbon character of these orbitals. In the case of the it and S bonds, this results from the 84 lowered contribution of the dir orbitals to the metal-metal interactions. When the differences between the fragment metal orbitals and the a and it energies are calculated for the molecules, these are found to demonstrate a slightly higher stabilization from their fragment orbitals than the corresponding interactions in Mo2Me6> as demonstrated in Figure 10 and Figure 12. This degree of stabilization does not preclude the formation of metal-metal bonding in actinides of this oxidation state. Clearly, however, this molecule would not be expected to be stable and the metals would probably be reduced by the ligands, filling the 6 interaction at the expense of the ligands. This view is consistent with the chemistry of uranium halides with alkylating reagents. Studies by Marks and by Evans have found the treatment of UC1A with a variety of lithium and magnesium alkylating reagents ultimately leads to the reduction of uranium to metal.173 The problem does not arise because the metal-metal bonds are too weak but rather, because they are too strong and too many of them are present, ready to steal as much electron density as they can at the expense of U-C bonding. The Mo-C interactions also have counterparts in U2Me6. In this molecule, the metal-ligand interactions are the 6eu, 6eg, 5a2u, and 5alg levels. The HOMO, the 6eu, is primarily U-C bonding and shows a large transfer of electron density from the methyl groups to the metal centers. This is apparent in the character of the orbital, as only 30% of the electron density is present on the carbon atom. Unlike the Mo2Me6 case, the orbital contains a large amount of f (81%) over d 85 character. This £ character is primarily S in type and is metal-metal antibonding. The presence of the metal-metal antibonding character is probably the main reason this orbital is the HOMO, since fS orbitals can have significant overlap compared to d5 orbitals. The 6eg is primarily 6d in character and represents a metal-ligand interaction. The next highest orbital is the 5eg) which is a metal dfi to ligand bonding interaction. Unlike the case found in Mo2Me6, electron density is not a donated as well to this orbital and the density still resides primarily on the ligand. In fact, a greater amount of electron density resides in the ligand than on the metal in this orbital. This orbital is similar to that in Mo2Me6 since the d^y and dx2 . y 2 dominate the metal portion of this interaction. The next metal-ligand orbital is the 5a2u, which is primarily a f The lowest of the metal-ligand bonding orbitals is the 6alg, which is a mixture of f, s, and d orbitals. This orbital is very similar to the 5a2u. A combination of so and £ The Mo-C and U-C bonds are different from each other in a variety of ways. The U-C interactions are formed primarily from a mixture of s, fx(x2 _3y2 j, d^y, d ^ ^ , and £6 orbitals instead of just the s, d^, and d^2 _y2 orbitals. The presence of the £6 orbitals allows the dn and d$ type orbitals to hybridize less severely. The degree of electron density transfer from the methyl ligands to the uranium atom is 86 lowered in the eg orbital and raised in the eu orbital. If the overall amount of electron density on the carbon atoms is summed for the metal-metal and metal-ligand interactions, the density on the ligands is higher in U2Me6 than in Mo2Me6. This analysis is not very valid for determining the ionic character of the U-C bond since the 8 orbital is lower in energy than the HOMO. Clearly, the uranium atoms want more electron density than this picture portrays. c27r462 Configuration of U.jMe8, U-U - 2.4 A Because the Xa-SW method allows for fractional occupation of levels, some of the electron density in an occupied orbital can be placed in another orbital. Since the uranium atoms want more electron density in their 8 orbitals, it seems reasonable that partial occupation of the metal-metal 8 bond would yield a more reasonable electronic description. To test this, two electrons were taken from the 5eg U-C level and placed into the 6eg. The characters of the levels in this new conformation can be found in Table 14. A molecular orbital diagram directly comparing this to the electronic structure of the ff2jr45° case, previously discussed, is shown in Figure 14. The new ordering of the levels is indeed closer to what one might expect in a compound of this type, but the calculation is still unusual in that it yields a quadruply bonded o2jt*62 system. In the Sz case, the HOMO becomes the U-U 8 bonding orbital(6eg). It is only partially occupied in this configuration and consists almost entirely of metal f 8 character, almost identical in character to the corresponding orbital in the a2ir46° configuration. This 87 Table 14. Energies and Decompositions of the Valence Levels of U2Me6 (Ti ll - 2.4 k, U-U-G - 105°) in a oVS2 Metal-Metal Configuration. Level eV Occ %U %s %p %d %f %C 2a2g -2.16 0 100 _ --- 100 0 6*18 -2.33 0 99 7 0 2 91 1 2alu -2.35 0 99 -- - 100 0 6e8 -2.97 2 99 - 0 2 98 1 6eu -3.00 4 55 - 7 23 71 42 5es -3.36 2 31 - 11 88 0 64 5a2u -4.00 2 40 20 2 16 62 55 5alg -4.31 2 35 57 0 9 34 59 5eu -5.24 4 91 - 1 36 64 8 4ais -6.45 2 96 4 6 41 49 2 88 - 2.0 -3.0 U-C u-u 8 -4.0 U-C eV -5.0 U - U K - 6.0 u-u a -7.0 U2Me( 2.4 A - 8.0 -9.0 Figure 14. Molecular Orbital Diagram of the Upper Valence Levels of U2Me6 (U-U - 2.4 A) in a 2ir*6° and a2jr462 Configurations. orbital is destabilized by 0.9 eV from its position in the 8° case. The SHOMO, the 6eu> is primarily U-C bonding, employing a mixture of primarily f and some d metal character. Uranium does not accept transfer of electron density from the ligand as well in this case, based on the higher amount of ligand based character of this orbital. This orbital is also destabilized by about 0.5 eV over the 5° case, apparently due to the higher electron density already present on U. The 5eg orbital follows next, being the other partially occupied orbital present in this configuration. It has character similar to that found in the a2jr*fi° case but is also destabilized slightly by about 0.2 eV. The next two orbitals are the 5a2u and 5alg metal-ligand bonding interactions; these orbitals are somewhat less perturbed by the partial occupation of the eg orbitals, being destabilized by 0,3 and 0.2 eV. The last two orbitals to be considered are the metal- metal 7r and a, 5eu and 4alg, bonding interactions. With the addition of electron density into the eg 8 bonding orbital, both of these orbitals experience a destabilization due to greater electron density present on the U atoms. The changes for these two orbitals are fairly large, approximately 0.8 eV for both. With this increased density, there appears to be less of a need to form the metal-metal interactions, which would serve to further increase the electron density on the U atoms. For this reason, the metal-metal interactions appear to be the most affected by the new 8Z configuration. In general, the promotion of two electrons into the 8 bonding orbital leads to overall destabilization of the total electronic structure. While adding electrons to the 8 bonding orbital 90 does place it above the metal-ligand bonding levels, this leads to an unfavorable destabilization of the metal-metal bonding orbitals as well as a partially occupied metal-ligand level lower in energy than the LUMO. In the case of the Mo2Me6 molecule, the eg and eu symmetry metal- ligand bonding orbitals show a large amount of electron donation from the ligand to the metal. For that reason it should not be surprising that the U atoms would also accept electron density from the a donor ligands. In the case of the actinide compounds examined so far, as well as those that follow, it is apparent that the f orbitals can form true S and of similar energy as the metal-ligand interactions of the same symmetry. As a result, these orbitals can mix with the metal-ligand orbitals to form bonding interactions, which can also serve to increase electron density on the metal centers through metal-metal interaction. Comparison of U2Me6 Where U-U - 2.4 and 2.6 A In general, the energetic ordering of the orbitals does not change very much in going from the 2.4 A to the 2.6 A case, as shown by Figure 15, Table 12, and Table 15. The only change occurs in the relative position of the 5eg and 6eg orbitals, which are U-U 6 bonding and U-C bonding levels, respectively. If the bonding picture is assumed to contain six U-C a interactions (alg + a2u + eg + eu) and three U-U interactions, we find that the U-U S bonding interaction, which would be expected to be unoccupied, again lies below the HOMO. 91 - 1.0 - 2.0 CF* -3.0 -4.0 ^ U-C -5.0 - 6.0 -7.0 U2Me, U2Me, 2.4 A 2.6 A - 8.0 105° 105° -9.0 Figure 15. Molecular Orbital Diagram of the Upper Valence Levels of U2Me6 (U-U - 2.4 A and 2.6 A). 92 Table 15. Energies and Decompositions of the Valence Levels of U2He6) (U-U - 2.6 A, U-U-C - 105°). level eV Occ %U %s %p %d %f %C 7a2u -2.33 0 95 7 0 2 92 4 7eu -2.42 0 92 - 0 11 89 8 7eg -2.55 0 100 - 0 6 94 0 *>a2u -2.88 0 97 2 0 4 93 3 2a2g -2.90 0 99 - - - 100 0 2aiu -3.00 0 99 --- 100 0 7ai8 -3.04 0 99 10 0 2 87 1 6eu -3.38 4 69 - 4 19 77 28 6eg -3.40 0 98 - 0 7 93 1 5eg -3.66 4 42 - 9 66 25 54 5a2u -4.38 2 49 21 .. 2 15 63 47 6aig -4.65 2 44 50 1 20 31 50 5eu -5.03 4 89 - 0 35 65 10 5aig -6.02 2 97 8 6 40 46 1 93 Not surprisingly, as the U-U distance is increased, the primary difference lies in the energy values of the metal-metal bonding orbitals. The a, tt, and 6 levels (4alg, 5eu, and 6eg) are destabilized by 1.37, 0.97, and 0.5 eV over that found in the 2.4 A complex, owing to decreased overlap at a longer distance. The 4> orbitals are also destabilized at the longer distance, but to a much lesser degree. These metal-metal bonding interactions all take on a lower f character as the U-U distance is increased. This result would be expected on the basis that f orbitals should not be as radially extended as the d orbitals, and as one lengthens the bonding distance the d orbitals can overlap better while the f orbitals lose overlap. The other observation that can be readily made is that the 4alg and 5eu gain more ligand character in the 2.6 A U-U distance than was present in the shorter case. The metal-ligand interactions are stabilized slightly on lengthening of the metal-metal bond, with the exception of the 6eu level, which is destabilized by 0.08 eV in this case. The 6eu orbital does not change appreciably in character, losing only a slight amount of f character. The 5eg is again dominated by dS forming orbitals and is stabilized apparently due to a lowered overlap of metal-metal antibonding interactions. The other two metal-ligand interactions also contain a slight amount of metal-metal antibonding character, which is probably the cause of the slight stabilization of these levels at the longer U-U distance. 94 Comparison of U^eg Where U-U-C - 90 and 105° The U-U-C angle was varied in order to gauge the effect of ligand perturbation on electronic structure. The valence electronic structure of U2Me6 at U-U - 2.6 A and at U-U-C angles of 90, 105, and 135° is shown in Figure 16. (See also, Table 15 - Table 17 for the character of the upper valence levels in these compounds.) As the U-U- C bond angle was varied from the 105° structure to 90°, all of the metal-ligand interactions are stabilized. This would be expected as the djy, d3t2 _y2 , and fx(x2 -3y2 ) orbitals, which are principly involved in metal-ligand bonding in the 105° case, are now directed in the plane of the methyl groups in the 90° case. The resultant degree of stabilization is large enough to shift all of the U-C bonding orbitals (5alg, 4a2u, 5eg, and 5eu) below the fS metal-metal bonding orbital (6eg). The 5alg U-C bonding orbital in the 105° complex is comprised of s, dz2 , and orbitals on the uranium atoms. In the 90° form, the amount of 5f/ligand overlap should increase since the fx(x2 _3y2 ) is directed at the ligand in this configuration, producing greater stabilization than seen at 105°. Xa-SW calculations support these results; a 0.34 eV stabilization as well as higher f character is evident in this orbital. In fact, this orbital loses metal-metal antibonding character that is present in the 105° case. What is somewhat surprising is that the f Rather, the increase in f character is due to an fz3 component which can mix but provide no stabilization since the ligands lie on a nodal plane. The analogue of the 5a2u in the 105° case should also exhibit 95 - 2 .0-1 -3.0- ien+6e, HJ-C -4.0- eV -5.0- -6.0 - 90° 105° 135° 2.6 A 2.6 A 2.6 A -7.0-I Figure 16. Molecular Orbital Diagram of the Upper Valence Levels of U2Me6 (U-U - 2.6 A) with U-U-C - 90°, 105°, and 135°. 96 Table 16. Energies and Decompositions of the Valence Levels of U2Me6 (U-U - 2.6 A, U-U-C - 90°). Level eV Occ %U %s %p %d %f %C geu -2.05 0 94 - 0 66 35 4 6a2u -2.68 0 98 4 0 2 93 2 ?eg -2.73 0 97 - 0 12 88 3 7aig -2.77 0 98 3 0 34 63 2 7eu -2.86 0 95 - 1 1 98 5 5a2u -3.02 0 94 4 0 6 90 5 2a2g -3.12 0 99 - -- 100 0 2*iu -3.22 0 99 - - - 100 0 6aig -3.23 0 99 9 0 1 89 1 6es -3.47 0 93 - 0 2 98 6 6eu -3.66 4 48 - 12 63 25 48 5eg -4.15 4 53 - 5 50 45 42 *a2u -4.68 2 48 21 0 14 65 47 5eu -4.90 4 94 - 0 23 77 5 5aig -5.00 2 52 60 2 0 38 43 4aig -6.17 2 95 4 5 47 45 4 97 Table 17. Energies and Decompositions of the Valence Levels of U2Me6 (U-U - 2.6 A, U-U-C - 135°). Level eV Occ %U %s %p %d %f %C 7es -1.30 0 94 - 1 22 77 5 7ai8 -1.51 0 93 0 0 33 67 4 6aZu -1.62 0 99 0 0 7 93 0 7eu -1.76 0 99 - 0 7 93 1 2azB -2.10 0 100 - - - 100 0 6alg -2.16 0 100 1 0 0 99 0 5a2u -2.18 0 97 2 1 0 98 2 2aXu -2.23 0 100 - - - 100 0 6eg -2.56 0 97 - 0 12 88 3 ^a2u -3.26 2 32 43 25 0 31 63 6eu -3.41 4 72 - 2 5 93 26 5eg -3.51 4 52 - 3 28 69 45 5alB -3.65 2 67 32 3 9 57 30 5eu -4.34 4 84 - 1 60 40 14 4alg -6.16 2 89 13 5 45 38 10 98 a similar behavior, as it is a mixture of the same metal orbitals. The 90° geometry shows a 0.30 eV stabilization for the 4a2u over the 5a2u in the 105° case, but, the relative percentage of d and f character on U does not change appreciably for this orbital. The relative amount of £ The 5eg U-C level is comprised primarily of d^ and in the 105° geometry. These orbitals should interact better in the 90° case, but the percentage of £n* character increases. The Xa-SW method predicts a stabilization of the orbital by 0.49 eV, which would be consistent with a higher overlap of d^ and dac2 _ y 2 with the ligand when at 90°. It appears that while the d character decreases, the total amount of overlap due to dfi and fjr* with the ligands increases, yielding the stabilization observed. The 6eu U-C level, which is primarily fsr and S in the 105° case, is stabilized by 0.26 eV at 90°, due to improved overlap of and d^z.y2 with the methyl groups. Overall, the eg and eu levels, which are comprised of the same atomic orbitals, possess greater d character in the 90° conformation. The changes in bonding with geometry should also be reflected in the metal-metal bonding orbitals. The U-U a bonding orbital (5alg in 105°, 4alg in 90°) shows a slight influence from the partial metal- ligand bonding character in this geometry. This orbital is stabilized by 0.15 eV in the 90° geometry, while the nature of the contributions from the metals do not change appreciably. The U-U tt bonds (5eu) exhibit a slight destabilization of 0.13 eV in the 90° case, due to the fact that the fz2y, is directed at one of the methyl groups and possesses partial metal-ligand antibonding character. The unoccupied 99 U-U S molecular orbitals are primarily dependent on the fS forming atomic orbitals, as dfi overlap should be minimal. The geometry change should not greatly affect the 6 forming f orbitals as they are not directed at the ligands in either conformation. Indeed the S orbitals show a small stabilization of 0.07 eV in the 90° case, which is consistent with a lowered interaction of these f orbitals with the ligands since they lie along a nodal plane. Comparison of U^MeB Where U-U-C - 105 and 135° When the geometry is changed from U-U-C - 105 to 135°, the overlap of the ligands with the dS forming orbitals is expected to decrease in favor of an interaction with the drr and fS orbitals. The major effects on the metal-metal bonding should be the destabilization of the it bond due to d orbital interaction with the methyl ligands. The same result will occur with the S bond due to the interaction of f6 forming orbitals with the methyl groups. This result is evident in Xa-SW calculations; both the it (6eu) and S (6eg) are destabilized, by 0.68 and 0.84 eV respectively. However the reasoning for the result is somewhat different than expected, as the dit orbitals are not principally involved in metal-ligand interactions. The a bond on the other hand, is stabilized slightly by a small amount of U-C bonding character in this geometry. The metal-ligand bonds are the 5alg, 5aZu, 5eg, and 6eu in the 105° conformation and the 5alg, 4a2u, 5eg, and 6eu in the 135° case. In general, the U-C bonding orbitals are dominated by the fz3, fz2 x, fz2 y, fjcy-j, fz(x2 _y2 ) , fx(x2 .3y2 ) and s in the 135° compound instead of the d ^ 100 and <1*2^2, as found in the 105 and 90° cases. This suggests that the interaction of the dir forming orbitals and the alkyl ligands is minimized. The interaction of the fjr, fS, and f in a destabilization of the metal-metal interactions in this geometry. The 5alg and 4a2u orbitals are destabilized in the 135° case by 1.01 and 1.12 eV over the 105° case. This destabilization is not surprising since the overlap with the f^ orbital is decreased at the 135° angle. The 5alg orbital has considerable metal-ligand anti bonding character, which serves to destabilize the overall U-alkyl interaction. The metal-metal a interaction (4alg) contains some U-C bonding character. The 5eg orbital is destabilized by 0.15 eV and is predominantly f in character on U. This is different from the 105° case in which this orbital was predominantly d in character. This orbital is fjr antibonding and fS bonding on the U centers. The 6eu also possesses much lower d character, but is stabilized by 1.12 eV. This stabilization is due to the domination of fjr character orbital on U, which can interact with the ligand better in this geometry as well as with the metal center. Calculations on UgMe* In the U2Me4 molecules the uranium atoms would be present in a lower oxidation state than in U2Me6. Instead of three metal-metal bonds as in the U2Me6 case, a a zir‘t62 configuration would be expected (if the relative energetic ordering is a < ir < 6 < Xa-SW calculations were performed on three different hypothetical geometries for a U2Me4 molecule. These geometries were chosen to maximize ligand-metal d orbital interactions in order to determine what effect the U-Me ligands have on the electronic structure. The three geometries chosen were D2d U-U-C - 90°, D2d U-U-C - 135°, and DZh U-U-C - 135°, as shown in Figure 17. The data from these three calculations can be found in Table 18 - Table 20. A molecular orbital diagram comparing these three sets of data can be seen in Figure 17. I. Daa U-U-C - 90° The overall configuration of the metal-metal bonds in this compound is found to be o2jr4S2. The HOMO (2^) is one of the uranium- uranium f5 orbitals. This orbital is almost 100% f in character, supporting our belief that £6 overlap is more substantial than d5 overlap. The SHOMO (7e) consists of an interaction between the ligands and the £ Below these lie the 6e, which is the metal-metal it bond. This orbital is mainly f in character, with a slight degree of metal-ligand bonding character. The 5bz is a a interaction between the d^y on the metal center and the ligands. The 6ax is the lowest energy metal-ligand bonding orbital, and is primarily s in character. The lowest energy orbital, to be considered here, is the metal-metal a bonding orbital (5ax) which is 51% d and 39% f in character. The higher amount of d character could be the effect of the oxidation state of the metal centers. This seems unlikely when the results for the other UzMe4 cases are included, as they show a lower amount of d character. It 102 - 1.0 - 2.0 " 8 2b. 7a„8 7e+5b. -3.0 eV -4.0 - -5.0 - a 5a - 6.0 - Me Me. Me U— U .U-U, I 4 -7.0 J Me Me Me 90° 135° 135° Figure 17. Molecular Orbital Diagram of the Upper Valence Levels of the Three U2MeA Molecules 1) D2d, U-U - 90°, 2) Dzd, U-U = 135°, and 3) D2h, U-U - 135°. 103 Table 18. Energies and Decompositions of the Valence Levels of U2 Me4 (D2d, U-U - 2.6 A, U-U-C - 90°). Level eV Occ %U %Us %Up %Ud %Uf %C 2 bj. -2.13 2 1 0 0 -- 5 95 0 7e -2.67 4 58 - 1 0 13 77 39 6 e -3.61 4 95 - 1 32 6 8 4 5b2 -3.99 2 39 2 1 0 73 6 56 6 ax -4.04 2 49 67 3 2 0 1 1 46 5at -5.12 2 95 5 5 51 39 4 104 Table 19. Energies and Decompositions of the Valence Levels of U 2 Me^ (D2d, U-U - 2.6 A, U-U-C - 135°). level eV occ %U %Us %Up %Ud %Uf %C 2 b i -2 . 0 1 2 1 0 0 -- 6 94 0 7e -2.87 4 6 8 - 4 8 8 8 30 5b2 -2.92 2 43 18 1 1 1 1 60 52 6 ax -3.15 2 65 30 3 13 54 32 6 e -3.77 4 8 6 - 1 56 43 13 5*i -5.69 2 92 1 2 6 45 37 7 105 Table 20. Energies and Decompositions of the Valence Levels of U2 MeA (D2h, U-U - 2.6 A, U-U-C - 135°). level eV Occ %U «Us %Up %Ud %Uf %C 4b 2u -1.59 0 1 0 0 - 0 0 1 0 0 0 2 b l 8 -1.93 1 1 0 0 - - 7 93 0 6 0 2 7a8 -2.03 1 98 1 93 5b3u -2.70 2 89 - 3 2 96 1 0 3b2u -3.13 2 1 0 0 - 2 35 63 0 6 ag -3.16 2 79 15 3 23 59 19 5blu -3.18 2 41 26 14 2 1 40 54 4b2 8 -3.26 2 46 - 8 44 48 50 4b3u -3.93 2 64 - 0 91 9 33 5as -5.20 2 85 2 1 4 44 32 13 106 seems more likely that reduction In the number of ligands present and geometry Interact to produce this result. II. U-U-C - 135° As seen in the previous geometry, the configuration here is again a V 6 2 with the HOMO being the 8 bonding orbital (2b!). As has been the case for all the molecules examined so far, the 8 interaction is dominated by f character, indicating that this is not due to the ligands removing the dS orbitals but rather is due to the higher overlap of the U f6 orbitals compared to the d6 . The 7e is the highest lying M-L interaction and contains primarily 5f metal character. The next orbital (5b2) is a metal-ligand bonding orbital, which contains some metal-metal 8 character. This orbital is dominated by f6 character and a large amount of the electron density is donated from the ligands to the metal center. The 6 ax is the lowest lying M-L interaction and consists of 5f and 7s character. The next lower orbital (6 e) is a metal-metal it bonding orbital consisting of d (56.3%) and f (42.5%) character. The 5ax and 6 ax orbitals both consist of a mixture of d and f on the metal centers, with the 5ax being metal-metal a bonding and the 6 ax being metal-ligand bonding. These two orbitals mix slightly so that each contains some metal-metal and metal-ligand bonding character. In going from the 90° to the 135° conformation for the molecule, the a and the it metal-metal bonding orbitals (5ax and 6 e) show some stabilization. The reason for the stabilization of the a and it 107 orbitals Is that the ligands can bond better with the fw orbitals at the 135° angle. It would be expected that this type of an interaction would normally lead to destabilization of the metal w bonds. However, the lack of a dw ligand interaction allows both a higher metal-metal dm contribution as well as metal-ligand dw bonding character in the predominantly metal-metal it orbital. The 6 ax and 5b2 are destabilized at the 135° case as a result of poorer metal-ligand orbital interactions. The 7e orbital is also stabilized for similar reasons as the 6 e level. The in the 90° conformation is now replaced by fir character in the 135° case. Last, the 2bx orbital shows very little change in the two conformations. III. Da U-U-C - 135° As the molecule is changed from D2d to D2h symmetry, each of the e sets split into two levels. In this case, the HOMO and LUMO (7ag and 2 blg, respectively) are both fS bonding orbitals and are split by 0 . 1 0 eV. Since they are close in energy, it is possible that each would be partially occupied, resulting again in a a 2ir*Sz configuration (as shown). The next lower occupied levels are the two metal-metal ir orbitals. These orbitals are mainly fir in character with a little dw character (35%) in the 3b2u orbital. The next four orbitals represent the U-C bonding orbitals 4b3u, 4b2g, 5blu, and 6 ag. These orbitals vary greatly in character and contain a large amount of metal character. The 4b3u has 64% metal character and is formed from the metal dm orbital that accepts electron density from ligands. The 6 ag also 108 contains a high degree of metal-metal bonding character. The lowest level Is the 5ag, which Is the metal-metal a bonding orbital. The Interesting observation to make In the comparison of the D2h and D2d calculations at 135° is how symmetry restrictions can apparently play a large role in the overall ordering and character of the levels. In particular, the ir interactions and related M-L orbitals demonstrate a strong preference for or against the incorporation of d or f character. As shown in Figure 18 the 5b3u is almost entirely f in character, the 4b3u is almost all d, and the 4b2g is a mixture between the two. The ligands use up all the d character they can in M-L interactions leaving the f orbitals to form M-M interactions. The other interesting observation is how the metal- ligand interactions contain a high degree of metal-metal bonding character. UgMeg Calculations Calculations were also performed for two hypothetical U2 Me8 molecules. The geometry for each of these was based on the structure used for U2 Me6, as previously discussed. The overall symmetry of the molecule was chosen to be DAh. The difference between the two U2 Mea molecules is the length of the U-U interaction which was set at 2.4 or 2.6 A, just as for U2 Me6. The character of the upper valence orbitals is similar and can be found in Table 21 and Table 22 for the 2.4 and 2 . 6 A cases, respectively. Figure 19 is a molecular orbital diagram that compares the two U2 Me8 calculations. 109 Figure 18. Contour diagrams of the 5b3u, 4b3u, and 4bZg orbitals of the D2h case for U2MeA. 110 Table 21. Energies and Decompositions of the Valence Levels of U2 He8 (U-U - 2.4 A, U-U-C - 105°). Level eV Occ %U %Us %Up %Ud %Uf %C 2 b lu -1.91 0 1 0 0 -- 2 98 0 -3.00 0 99 - - 3 97 0 6 e u -3.09 0 73 - 4 0 96 25 2 b2g -3.14 2 1 0 0 - - 6 94 0 5es -3.21 4 48 - 8 4 8 8 49 3b2u -4.27 2 37 - - 63 37 58 5 a 2u -4.28 2 28 41 4 33 2 2 6 6 3blg -4.54 2 35 - - 95 6 60 6aig -4.79 2 30 80 0 8 1 2 63 5eu -5.35 4 8 8 - 0 37 63 1 1 5 ai8 -6.25 2 97 3 5 42 51 1 Ill Table 22. Energies and Decompositions of the Valence Levels of U 2 Me8 (U-U - 2.6 A, U-U-C - 105°). Level eV Occ %U %Us %Up %Ud %Uf %C 3b2g -1.08 0 8 8 - - 78 2 2 3 8 eu -1.46 0 74 - 14 54 32 2 2 4b2u -1.85 0 96 - - 4 96 4 7e8 -1 . 8 6 0 78 - 5 5 91 2 1 7eu -1.97 0 87 - 1 17 82 1 1 6 a 2u -2 . 0 0 0 98 3 0 5 92 1 6 es -2.08 0 99 - 0 6 94 0 2 b lu -2 . 2 0 0 1 0 0 - - 1 99 0 7ai8 -2.44 0 98 1 0 1 37 52 2 ^big -2.91 0 99 - - 4 96 0 2 b 2 8 -3.04 0 1 0 0 - - 5 95 0 6 eu -3.10 2 81 - 4 1 96 18 5es -3.36 4 52 - 9 5 8 6 45 3b2u -4.48 2 41 - - 69 31 54 5a2u -4.49 2 33 47 4 32 17 61 3blg -4.68 2 39 - - 91 9 55 5e„ -4.76 4 82 - 0 39 61 16 6 alg -4.96 2 40 46 2 46 5 53 5alg -5.27 2 96 1 1 4 37 48 3 112 - 2.0 -3.0 -4.0 eV -5.0 - 6.0 -7.0 Figure 19. Molecular Orbital Diagram of the Upper Valence Levels of U2Me8 with U-U - 2.4 A and U-U - 2.6 A. 113 The anticipated picture for this uranium dimer would exhibit a bond order of two instead of three, as in the U2 Me6 cases previously described. The electrons would be distributed in a o 2ir2 sense, with the rest of the occupied upper valence orbitals involved in metal- ligand binding. Examining the two cases, it becomes apparent that the ffi forming levels no longer lie below the HOMO in the 2.6 A case but one is still occupied and is the HOMO in the 2.4 A case. In both cases, the 5eu and 6 eu, which represent the metal-metal jr interactions and one of the metal-ligand bonding representations, mix with each other, resulting in a similar situation as observed for the f6 bonding set in the U 2 Me6 calculations. In the 2.4 A case, the compound was not converged with the electrons in the orbitals that they would be expected to reside in, but rather where they fell by energetic ordering this case the 6 eu is predicted to be the LUMO but this orbital is a metal-ligand bonding orbital that we normally be filled. It is almost entirely f in character with a large portion of the orbital residing on the U atoms. The HOMO in this case is predicted to be the 2b2g, which is one of the metal-metal fS bonding orbitals. This orbital is expected to be unoccupied in this case, and in fact, the tt metal-metal bonding orbital is only expected to be partially filled. The next orbital is the 5eg, which is metal-ligand bonding and the electron distribution is approximately half on U and C. This orbital is high in f character, in contrast to the U 2 Me6 molecule, wherein the eg orbital 114 was primarily d in character. The metal character for this orbital consists primarily of f * bonding type orbitals. This orbital is followed by the 3b2u, which is primarily d£ in character and is metal-ligand bonding. There is a small amount of fS antibonding character in this orbital. The next orbital is the 5a2u, which is a mixture of s, d, and f on U and is also a metal-ligand bonding orbital. The next lower orbital is the 3blg, which is the metal-ligand bonding orbital and is primarily d5 in character. The 6 alg, which is a metal-ligand interaction, is the metal-metal bonding counterpart of the 5a2u. The 6 alg is a metal-metal a bonding orbital and has a slight metal-ligand antibonding character. The calculation for the U 2 Me8 molecule with U-U - 2.6 A do not have the problems with the 6 bonding orbitals taking electron density from the metal-ligand interactions. As observed for the U 2 Me6 case, as the distance is increased between the two metals two things happen: the metal-metal bonding orbitals rise in energy, and the amount of d character stays nearly constant. It is interesting to note that the rise in energy is not as great as observed in the U2 Me6 calculations. The actual bond order predicted by Xa for this case is more difficult to ascertain. The problem in the interpretation arises from the 6 eu and 5eu, which should be metal-ligand a and metal-metal jt bonding orbitals. These two orbitals mix heavily resulting in two orbitals that do not look very different from one another. The 5eu is only slightly higher in metal character than the 6 eu. The electronic structure of this molecule is perhaps best interpreted as a tr2 jr3 115 interaction, with the U-U bond somewhere between a double and a triple bond. Conclusions The uranium and molybdenum compounds studied are similar, but have some interesting differences. While it seems unlikely that any of the U2 Men species studied would be able to exist, the result that a, 7r, and even 6 interactions between f orbitals can have energies lower than those of U-C levels is promising. The stabilization of the S bonding orbitals for uranium is observed to be predictably high due to better overlap from the fS forming orbitals. These findings, coupled with the observation that the degree of stabilization of metal-metal interactions is similar to that found for Mo-Mo interactions, suggest that U-U bonds might exist between U(III) centers, Another important finding is that both the d and f orbitals play an important role in metal-ligand and metal-metal interaction in these uranium compounds. This yields a number of orbitals from which the metals can choose to form interactions with the ligands. As a result, almost all of the ligand conformations investigated were quite similar to each other. The large number of orbitals makes it difficult to find an-orientation where interactions would be completely unfavorable. The unusual result that unoccupied U-U bonding orbitals in U2 Me6 and U 2 Me8 lie below the HOMO is consistent with the observation that uranium is reduced when alkylation of the actinide metals is 116 attempted.77,167’173 Indeed, even the metal-ligand interactions often became more metal-metal in character than would be expected based on comparisons with Mo2 Me6. For this reason, it is doubtful that these compounds can be prepared. Also, from these calculations it is apparent that the unoccupied f orbitals are relatively close in energy to the HOMO. In order to form stable compounds that contain an actinide-actinide bond these orbitals must be "tied up" or the U-L interactions must demonstrate a higher degree of stabilization. In the case of U2 Me6, only six of the total number of orbitals available are potentially removed from metal-metal interactions. A higher number of donor orbitals would not only serve to split the orbitals but could be used to provide more electron density to the metal center through a larger variety of orbitals. It is apparent that strong donor ligands are important as they enhance the ability of metals to form metal-metal bonds by increasing extension and therefore metal- metal overlap. This certainly would be affected to a certain degree by the type of donor ligand present. Exactly what happens when a ligand such as Cp is employed will be discussed later. Another way of removing the fS interaction from the picture is to fill the level by choosing a higher actinide. In order to test this hypothesis, further calculations were performed on other actinide alkyls. This then leads us into the next section, where we will discuss the effect of changing the actinide metal. CHAPTER V STUDIES OF THE HYPOTHETICAL SERIES Introduction Since it is difficult to imagine the existence of any of the U 2 Men (n - 4, 6 , and 8 ) molecules discussed in the previous section, we are interested in looking at similar hypothetical compounds containing other actinides and comparing these to their transition metal analogues. It is hoped that either the 5f and 6 d orbital energetics or the overall bond order of the compounds investigated will yield results that do not contain the inverted HOMO/LUMO ordering found in the U2 Men series. In the previous section, the U2 Men molecules were based on an idealized structure of known Mo2 R6 compounds.78,16A*168 Since other M2 R 6 compounds are known for the transition elements, 1 0 1 it is most instructive to extend the study on the series An2 Me6 and compare it to M2 Me6, where An - actinide and M = transition metal. Because we are particularly interested in the nature of actinides that have f orbitals readily available for metal- ligand interactions, only the early actinides will be explored, especially since the later actinides behave more like the lanthanides. 1 1 , 8 0 Calculations using the Xa-SW method were performed on the hypothetical molecules Th2 Me6, U2 Me6, and Pu2 Me6. The electronic structure of these molecules will be compared to their second row 117 118 transition metal analogues, Zr2 Me6, Mo2 Me6, and Ru2 Me6. First, the electronic structure of the transition metal compounds will be examined. This should give us some insight into the nature of the bonding and progression we might expect in the corresponding actinide systems. It is of interest to note that Mo2 R678,164,168 and Ru2 R6 1 7 4 compounds are actually known while the direct Zr(III) analogues are not as yet known. There are some Zr(III) compounds that purportedly contain a Zr-Zr bond .104’175,176'177 Many of these compounds have only recently been prepared and are found to decompose readily.104,175’177,176 Like zirconium, the (+3) oxidation state is relatively rare for thorium with +4 being the preferred oxidation state. In contrast, the +3 oxidation state is relatively common for uranium and plutonium. 1 7 For these calculations, an idealized structure of the Mo2 R 6 compounds, which have been determined by x-ray analysis, 1 6 8 formed the basis for the overall geometry of the Zr2 Me6, Mo2 Me6, Ru2 Me6, Th2 Me6, U2 Me6, and Pu2 Me6. In the case of Zr2 Me6, the Zr-Zr distance was estimated to be 2.9 A based on covalent radii and the Zr-C distance was set to 2.3 A based on the Zr-R distance in Cp2 ZrMe2 . 1 7 8 The structure of the model compound Ru2 Me6 was idealized from the known structure of Ru2 R6, (R - CH2 CMe3 ) . 1 7 4 The Ru-Ru distance was set to that of the compound (2.31 A) while the Ru-Ru-C angle was set at 105° to allow a closer comparison to the other molecules. The structure for Th2 Me6 is based on the covalent radii of thorium, yielding a Th-Th distance of 3.3 A and the Th-C distance of 2.6 A was based on known thorium-alkyl compounds.17,166,170,180 The structure of Pu2 Me6 was chosen to be completely analogous to the 119 previously studied U 2 Me6 case where M-M - 2.6 A and M-C - 2.6 A. The Fu-Pu distance would be estimated as shorter than the U-U distance based on a slight decrease in atomic radii and higher metal-metal bond order, however. Further computational details for each of these molecules can be found in Appendix C. Studies of M ^ e g (M - Zr, Mo, Ru) As shown in the qualitative diagram presented in Figure 20, we can consider an M2Lg molecule to be constructed from two ML3 fragments. The ML3 fragments can in turn be constructed from a metal atom and the three ligands. In this diagram, the metals are assumed to have only s and d orbitals readily available, limiting us to a total of 6 metal based orbitals from which to construct all the metal- metal and metal-ligand interactions. Previous Mo2 Me6 calculations have shown that metal 5p orbitals are insignificant in the bonding in this type of a geometry and can thus be ignored.102,161,163 As three a-only ligands are allowed to approach a metal atom at a 105° angle to the C3 axis, it is apparent that the d^, d^i_y2, and an equatorially enhanced s and dz 2 hybrid are the metal based orbitals that can most effectively interact with the ligands. This interaction causes the d^ and <3*2 .^ orbitals to fall out of the picture, leaving the axially enhanced s/dz 2 hybrid, dxz, and dyZ orbitals as the important frontier orbitals of the fragment. When another ML3 fragment is allowed to approach along the C3 axis, forming a M2 L6 molecule, the s/dz 2 hybrid is in good position for forming a a interaction and the d^ and dyZ are in position to form a degenerate 120 M-L’ M-L’ G* 71* M-L M-L MM@3 Figure 20. General Bonding Scheme for Transition Metal M2 L6 Systems Where L is a Terminal a-only Ligand. 121 set of 7r interactions under D3d symmetry. This simple picture leaves us with an ordering of metal-metal bonding and antibonding states that is best described as o < it < n* < cr*. Calculations are necessary to decide whether the metal-metal bonding interactions will energetically lie above or below the metal- ligand interactions. There is a further complication to the picture that arises from the symmetries of the metal-metal and metal-ligand interactions. Under D3d symmetry, the six M-L a interactions in a M2 L6 molecule transform as alg + a2u + eg + eu. The metal-metal a , it, it*, and a* also transform as alg, eu, eg, and a2u, respectively. By symmetry, the metal-metal orbitals are allowed to mix with the corresponding metal-ligand interactions. For that reason, the qualitative picture described above is rather simplistic compared to the case predicted for Mo2 Me6 (Figure 22 and Table 25) . The dit and d8 orbitals mix to form two e hybridized sets of orbitals that serve to increase metal-ligand overlap. The qualitative diagram is instructive, however, since it yields the correct metal-metal bond order for Mo2 Me6 and a very similar overall electronic structure. In this dimer, the two Mo(III) atoms have a total of six electrons left after the metal-ligand orbitals are filled up. Placing these six electrons in the lowest orbitals in the metal-metal bonding scheme yields a o 2it* configuration with a bond order of three. Likewise we can use the qualitative diagram to predict the metal- metal bonding in analogous zirconium and ruthenium M2 L6 compounds. In the case of Zr2 Me6 we only have two electrons to fill the metal-metal orbitals. Since the a orbital is the lowest we would predict the a z 122 configuration as the result. Similarly, for Ru2 Me6, we can again use the same scheme to obtain a ct2 7tSt*a configuration which would yield a bond order of one, contrary to that predicted by Wilkinson, et al . 1 7 4 The results of the Xa-SW calculations performed to confirm the qualitative predictions follow. Zr2 Me6 The calculations on the hypothetical compound Zr2 Me6 indicate that the energetic ordering of the metal interactions is metal-ligand < a < jr < it* < a * . This yields a er2 configuration and is in agreement with our qualitative approach. The metal-metal a bond is the HOMO and the LUMO is a metal-metal tt bonding orbital. The HOMO/LUMO gap present in this compound is fairly large (1.2 eV) but is not as large as that present for Mo2 Me6. Since only a single metal-metal bond is present in this molecule, and this orbital is not very stabilized over the metal based ax ML3 fragment energy (see Figure 21, Table 23, and Table 24), it should be much easier to break the metal-metal bond. Indeed, the Zr(III) dimers that contain a metal-metal bond are found to be very sensitive to decomposition.104,175,176,177 The low stabilization of this a bonding orbital compared to Mo2 Me6 is caused by a lowered overlap of the dz 2 orbitals at this long of a metal-metal distance as compared to the triple bond distance in the latter compound. Due to the nature of the HOMO and LUMO this compound would be expected to be unstable to oxidation, as this decreases the metal- metal bond order. If this compound is allowed to undergo a one 123 -2.- Zr-Zr 71* Zr-Zr 71 eV Zr-Zr a Zr-C Zr-C -5.- ZrMe Figure 21. Molecular Orbital Diagram of the Valence Levels of Zr2Me6 as Constructed from ZrMe3 Fragments. 124 Table 23. Energies and Decompositions of the Valence Levels of Zr2 Me6. Level eV Occ %Zr %s %p %d %C 6e« -1.36 0 91 - 4 96 5 fieu -2 . 6 8 0 92 - 4 96 5 5aig -3.88 2 98 1 1 5 84 1 5eu -4.43 4 49 - 9 91 46 - 1 1 5 e 8 -4.50 4 49 89 47 ^a2 u -4.63 2 40 62 3 35 54 4 al8 -4.98 2 42 8 6 0 13 51 125 Table 24. Energies and Decompositions of the ZrMe3 Fragment. Level eV Occ %Zr %s %p %d %C 7e -1.62 0 92 - 2 98 4 6 a 1 -1.97 0 96 1 0 1 90 1 6 e -2.08 0 93 - 2 98 4 5ai -3.11 1 93 7 3 90 7 5e -4.46 4 48 - 1 1 89 47 4ax -4.83 2 41 72 1 26 53 126 electron oxidation the bond order would fall from one to 0.5. One electron reduction, on the other hand, would yield a metal-metal bond order of 1.5, improving the metal-metal bonding. MozMe6 Calculations on the Mo2 Me6 molecule were presented earlier and will only be briefly recapped here. These resulted in a a < metal- ligand < 7T < it* < a* energetic ordering (See Figure 22, Table 25, and Table 26). In this case the HOMO is a ir metal-metal bonding orbital while the a is now much lower in energy than that found for the Zr analogue. This is expected, since this compound formally has a metal- metal triple bond and therefore calculations were run at metal-metal distance much shorter than the Zr-Zr single bond distance. It is interesting to note that oxidation or reduction of this molecule should have the same result, both reducing the metal-metal bond order, either by taking away jt bonding electrons or by adding electrons to the ir* LUMO. For a more detailed description of this molecule refer to the preceding section. Ru2 Me6 Calculations on Ru2 Me6 resulted in essentially the same ordering as that presented for Mo2 Me6. The decompositions of these levels are in Table 27 and Table 28 and the electronic structure is shown in Figure 23. The calculational results for the valence levels places the metal-metal a orbital within the energy region of the metal-ligand interactions with the metal-metal tt and if levels at higher energy. 127 Mod Mo-C Figure 22. Molecular Orbital Diagram of the Valence Levels of Mo2Me6 as Constructed from MoMe3 Fragments. 128 Table 25. Energies and Decompositions of the Valence Levels of Mo2 Me6. Level eV Occ %Mo %s %p %d %C 7eu -0.92 0 65 - 17 83 28 6 alg -1.05 0 8 3 1 0 87 90 6 e 8 -1.93 0 80 - 6 94 15 6 eu -4.79 4 69 - 7 93 26 ^a2 u -5.16 2 30 54 1 45 63 5ais -5.30 2 43 82 1 16 51 -6.04 4 55 - 5 95 39 5eu -6.33 4 72 - 0 1 0 0 2 2 ^aig -7.67 2 8 8 2 6 93 8 129 Table 26. Energies and Decompositions of the Valence Levels of the MoMe3 Fragment, Level eV Occ %Mo %s %p %d %C 7e -2.35 0 59 - 1 1 89 28 6 a! -2.79 0 84 1 2 1 87 1 2 6 e -3.39 2 94 - 0 1 0 0 2 5ai -4.19 1 96 13 1 87 4 4ax -5.72 2 41 30 1 69 52 5e -5.89 4 54 .. 4 96 41 130 -1. n o* -2. - Ru-C* -3. - Rud -4. - eV -5. H Ru-C -6. - Ru-C Ru-C Ru-C ■7. " -8. J Figure 23. Molecular Orbital Diagram of the Valence Levels of Ru2Me6 as Constructed from RuMe3 Fragments. 131 Table 27. Energies and Decompositions of the Valence Levels of Ru2 Me6. Level eV Occ %Ru %s %p % d %C 5 a ig -0.95 0 7 0 17 83 84 6 eu -1.26 0 59 2 63 74 34 4 a 2u -1.78 0 87 2 9 89 9 5e s -3.22 4 91 - 1 99 4 5eu -4.65 4 8 6 - 2 98 8 4 a is -5.56 2 57 65 1 34 38 3a 2u -5.76 2 40 44 2 55 54 - 4 e s -6.57 4 58 5 95 37 3 a is -6.64 2 79 0 4 96 17 4 ®u -6.70 4 62 - 2 98 32 132 Table 28. Energies and Decompositions of the Valence Levels of the Fragment RuMe3. Level eV Occ %Ru %s %p %d %C 6e -2.14 0 41 - 33 67 43 5ax -2.93 0 76 11 2 87 18 5e -3.95 4 93 - 0 100 2 4ax -4.25 1 92 15 0 85 7 3ax -6.12 2 46 31 2 67 47 4e -6.60 4 59 _ 4 96 36 133 The configuration of the metal based levels is The unoccupied levels that were located are the 6eu and the 4a2u. The 6e„ level is a metal-carbon antibonding interaction between the metal S orbitals and resides primarily on the metal centers. The LUMO (4a2u) is the metal-metal a* orbital that results from the dz 2 orbitals. The HOMO of the compound is the metal-metal it* orbital, the 5eg. The next lower level is the 5e„ which is primarily metal-metal it bonding in character. Both of these levels contain some metal S character in a bonding or antibonding form as demanded by symmetry. The next three levels (4alg, 3a2u, 4eg) are primarily metal-ligand bonding orbitals. The alg and a2u levels contain a mixture of s and dz 2 character. The next orbital is the 3alg and is predominantly metal-metal o in character. This orbital is formed almost entirely from the metal dz 2 orbitals. The last orbital is the final metal- ligand bonding interaction and is apparently at a lower energy due to the incorporation of some metal-metal it bonding character. It is of interest to note that the relative stabilization of the metal-metal orbitals of this compound are somewhat less than the that found in Mo2Me6. This would be expected on the basis that a bond length more approximating a single bond length for this compound would be assumed compared to the triple bond in the Mo2Me6 case. Addition of an electron to the a* orbital on reduction of the molecule would result in a decrease in the metal-metal bond order and probable decomposition. Oxidation on the other hand would result in the increase of the bond order of the metal-metal bond, through removal of 134 metal-metal n* density. For this reason, Ru2R6 compounds would be expected to behave conversely to the Zr2R6 compounds, one being unstable to reduction and the other to oxidation. It is interesting to notice how well the transition metal M2L6 molecules studied have followed the qualitative metal-metal and metal- ligand bonding scheme. The three molecules conformed well to the a2, <72*rA, a27r*jr*4 metal-metal bonding descriptions for Zr2Me6, Mo2Me6, and Ru2Me6. This scheme would not be expected to hold as well for cases where the ligand had the capacity to form n interactions with the metal. Also, this scheme doesn't hold for the U2Me6 calculations presented in the last section. The conclusion reached in the last chapter was that mixing of 6d and 5f were the cause of this deviation. The 5f orbitals transform as 2alg + a2g + 4eg + 4eu + alu + 2a2u under D3d and, as in the case of U2Me6, will complicate the picture. Calculations on An2Me6, An - Th, U, Pu Calculations on the Th2Me6, U2Me6, and Pu2Me6 series yield an even more interesting progression than that presented for their transition metal analogues. A significant complication that appeared in the case of U2Me6 was that unoccupied metal-metal bonding levels appear lower in energy than the metal-ligand interactions. The fact that these actinide compounds are isovalent with the transition metal compounds discussed above could lead to the conclusion that these molecules should be similar. Can we modify the simple scheme used to predict the bonding in the transition metal compounds to incorporate the actinides? We have previously shown that 135 the U2Me6 case would be anomalous to any reasonable bonding scheme. Th2Me6 and Pu2Me6 might still follow a reasonable scheme, however. In diatomic U2 we have shown that the ordering of the f-f bonding orbital interactions is such that a < 7r < 6 < considered important and that the number of electrons available for the metal-metal bonding in the three An2Me6 cases are 2, 6, and 10, then we can fill up these f-f interactions in each of the three actinide compounds forming a single, triple, and quintuple bond, respectively. We have already seen that this presents a poor picture for two reasons. For one, the metal-ligand bonding levels are not necessarily below the HOMO. Secondly, we have also assumed that the primary metal-metal interaction is dominated by f character. For U2Me6 we have already shown that the levels are a mixture of d and f. ThjsMeg In the case of Th2Me6, a single bond distance of 3.3 A was assumed, based on a thorium covalent radii of 1.65 A. A calculation by the Xa-SW formalism predicts the formation of a Th-Th single bond resulting from a a2 configuration for this molecule, as shown in Table 29, Table 30, and Figure 24. The relative ordering of the metal-metal and metal-ligand interactions was metal-ligand < da < d/f^r < fa < far* < f^ < fn. The contour plots of the a and n orbitals are presented in Figure 25 in order to illustrate the nature of these orbitals, as summarized in Table 29. It is readily apparent that the overall electronic structure of this molecule is quite different than was seen for the U2Me6 molecules previously studied (see Figure 24 136 a* Thf Thf Thd - 2. Thd -3. eV Th-C Th-C ThMe, Figure 24. Molecular Orbital Diagram of the Valence Levels of Th2Me6 as Constructed from ThMe3 Fragments. 137 Th Figure 25. Contour Diagrams of the Th-Th a and jr Interactions in Th2Me6. 138 Table 29. Energies and Decompositions of the Valence Levels of Th2Me6. level eV Occ %Th %s %p %d %f %C 7eu -1.08 0 98 - 0 11 89 1 6a2u -1.28 0 99 8 0 0 91 1 6e* -1.34 0 98 - 0 8 92 1 7al6 -1.86 0 94 3 0 6 92 5 6eu -2.02 0 90 - 6 63 32 8 6ai8 -3.15 2 95 12 5 76 8 5 5eg -3.81 4 44 - 11 83 6 51 5*u -3.97 4 50 - 5 79 16 46 5a2u -4.44 2 42 48 2 25 24 52 5alg -4.67 2 46 78 0 4 18 48 139 Table 30. Energies and Decompositions of the Valence Levels of the Fragment ThMe3. level eV Occ %Th %s %p %d %f %C 7ax -1.54 0 97 19 0 14 68 97 6e -1.79 0 82 - 11 58 30 14 6ax -2.88 1 84 14 5 59 23 15 5e -3.85 4 45 - 9 81 10 50 5ai -4.54 2 43 62 1 59 23 14 140 and Figure 26). It Is also quite apparent that the amount of d and f mixing in this compound is drastically different than that seen for the U2Me6 molecule. The highest level located was the 7eu (see Table 29) and is primarily comprised of 5fit type orbitals and is of proper symmetry to contain some £5* character. It is seen to interact in an antibonding sense with the ligands and demonstrates a situation of poor metal- metal overlap between f orbitals. The next level is the 6a2u, which is primarily comprised of f The level which follows is the 6eg, which consists of f6 and some far* type character on the metal centers. Again, the overlap between the two metal centers is negligible in this metal-metal bonding level. Analogous to the 6eg in U2Me6, this level contains almost no d 6 metal bonding character. The next level is the 5fo bonding level, the 7alg. It is interesting to note that this level contains < 6% dz 2 character. Additionally, some f in the overall energetic ordering of d and f levels in Th as compared to U. The level that follows is the 6eu and is one of the tt metal-metal bonding levels. This orbital is almost entirely centered on Th but has the highest amount of ligand interaction for any of the metal- metal orbitals. It also has a higher percentage of dn character than f?r character. This is very interesting when compared to the U2Me6 case, which only had higher d than f character in the 5eg metal-ligand bonding orbital. The 6eu corresponds to the 5eu in the U2Me6 case and is shown to have much lower overlap to form the metal-metal bonding interaction. The HOMO of the compound, the 6alg, is almost entirely composed of Th orbitals and is predominantly 6dzz in character. This is also quite different from the U2Me6 case previously described. In this case the molecule is best represented by a d1-d1 interaction, whereas, in the case of U2Me6 a d-f hybrid interaction is preferred. Is this a result of the longer bonding distance or d-f energetics? In order to answer these questions we must inspect the ThMe3 fragment to see where the d and f orbitals are located. Clearly, these calculations indicate that the d orbitals fall well below the f orbitals within the fragment. This is very strong evidence that the energetics of the d and f orbitals in Th and U are very different from each other. While it is possible to say that energetics are responsible for the d1-d1 configuration, it is not possible to separate this from the conclusion that the long Th-Th distance enhances the 6d-6d bonding in preference to 5f-5f. In fact, both effects must be operative, since poor overlap 142 of the f orbitals Is apparent In the contour diagrams of the f orbital interactions. The last four valence orbitals to be discussed are the metal- ligand bonding interactions. The 5eg and the 5eu levels are almost entirely 6d in character and contain a small amount of 5f character. In fact, these orbitals look almost identical to the corresponding Zr- Me orbitals in Zr2Me6. The same can be said for the 5a2u and the 5alg which are primarily Th 7s and 5da in character. The 5a2u contains the highest amount of f orbital influence, owing to the ability of the f orbitals to reduce the amount of metal-metal antibonding character. It is interesting to note that the metal-ligand levels are at about the same energy in each of the actinides investigated, with the uranium case being the most deviant of the three. It appears that the overall f orbital influence in Th(III), as well as in Th(IV), is much lower than that expected for U compounds in both metal-metal and metal-ligand interactions. It would be expected that at the longer bond distances d orbital interactions would become more favorable by consideration of the relative overlaps available. On the other hand, in going from U2Me6 to Th2Me6, the An-C bond distance was not changed. It must also be true that a better energetic match of the Th d and C hybrid orbitals must also exist than is present in the case of U2Me6. UjjMee U2Me6 was discussed partially in the previous section, so the metal-metal interactions will only be briefly discussed here (see 143 Figure 26, Table 31, and Table 32). We saw that the compound U2Me6 is not expected to be stable due to an inverted HOMO/LUMO arrangement. Also, the d and f orbitals of the uranium centers are able to mix effectively to form hybrids that result in more favorable levels. The ordering and character of the levels of U2Me6 can be found in Table 31. The highest level located in this calculation was the 7a2u and is the fz3 a * interaction. The next level (7eu) is interpreted to be the fS* interaction. Below that we find the 7eg which is the fn* interaction. The next four orbitals, the 6a2u, 2a2g, 2alu, and 7alg, represent the 4> and $* metal-metal interactions. Up to this point, none of the orbitals has contained a substantial amount of metal d or ligand character. The next orbital, which is the 6eu, now contains a higher degree of ligand character (28%) and represents, primarily, an interaction between the ligands and a metal based f S orbital. The next orbital is the 6eg, which is a metal-metal 5 orbital. Below this are the other three metal-ligand interactions and lastly the metal- metal 7r and a interactions. These orbitals all contain certain degree of d character ranging from 15 to 65%. It is clear that both d and f character is important to the nature of the electronic structure for U2Me6. Pu2Me6 Pu2Me6 would be expected to have a bond order of five based on the f-f orbital configuration of o27r464 as derived from U2Me6. If this were true, then we could certainly avoid the inverted HOMO/LUMO arrangement presented in U2Me8. Partial occupation of the S orbitals 144 a* U5f eV u-c ► u - c UMe3 Figure 26. Molecular Orbital Diagram of the Valence Levels of U2Me6 as Constructed from UMe3 Fragments. 145 Table 31. Energies and Decompositions of the Valence Levels of U2Me6 (U-U - 2.6 A, U-U-C - 105°). level eV Occ %U %s %p %d %f %C 7a2u -2.33 0 95 7 0 2 92 4 7eu -2.42 0 92 - 0 11 89 8 7*6 -2.55 0 100 - 0 6 94 0 6 a 2u -2.88 0 97 2 0 4 93 3 2a 2g -2.90 0 99 - -- 100 0 2aiu -3.00 0 99 - - - 100 0 7a ig -3.04 0 99 10 0 2 87 1 6 e u -3.38 4 69 - 4 19 77 28 6e6 -3.40 0 98 - 0 7 93 1 5e* -3.66 4 42 - 9 66 25 54 5a2u -4.38 2 49 21 2 15 63 47 5 a lg -4.65 2 44 50 1 20 31 50 5®u -5.03 4 89 - 0 35 65 10 6 a lg -6.02 2 97 8 6 40 46 1 146 Table 32. Energies and Decompositions of the Valence Levels of the UMe3 Fragment. Level eV Occ %U %s %p %d %f %C 8e -1.84 0 65 - 18 62 21 26 8ax -2.61 0 91 3 1 14 82 8 7e -2.83 0 92 - 1 10 89 8 2az -2.95 0 99 - - - 100 0 6e -2.99 2 100 - 0 0 100 0 7a, -3.00 0 100 0 0 0 100 0 6a! -3.17 1 99 9 0 2 89 1 5e -3.79 4 51 - 5 41 54 45 5ax -4.55 2 45 42 1 10 48 50 147 would cause them to rise in energy and, even if they don't rise above the metal-ligand interactions, they would still be occupied in this case. To test this hypothesis, calculations were performed on this molecule using the same metal-metal distance as that chosen for U2Me6, although a shorter metal-metal distance might be expected from a quintuple bond involving f-f 8 interactions. As has been reported, the shortening of metal-metal bonds due to dfi interactions is very slight, but f6 interactions should be able to overlap better and perhaps provide more incentive for the metal-metal bond to shorten.72 As shown in Figure 27 and Table 33 (see also Table 34), the a, m and 8 interactions all are lower in energy than the HOMO in this molecule. This leaves the HOMO, which is the 6eg metal-ligand bonding orbital, very close energetically to the LTJMO, which is a metal-metal likely that the HOMO and LUMO would each be partially occupied. This loss of electron density from the metal-ligand based orbitals could present a pathway to decomposition of the metal-ligand bonds if this compound could be prepared. A brief description of the metal-metal and metal-ligand interactions follow. The highest level located was the 8eu representation. This orbital is interesting in that it is almost entirely dm bonding in character with a slight degree of fm and f8* character. This is just the opposite of the case shown for Th2Me6 where the d-d bonding interactions were at low energy, while here they are at high energy. The next lower level is the 8alg level, which consists of primarily d a bonding character 50.8% with a high amount of 148 - 2. Pu5f •++■*■*+■ 4^4=1 L Pu-C Pu-C Pu-C -5. PuMe Figure 27. Molecular Orbital Diagram of the Valence Levels of Pu2Me6 as Constructed from PuMe3 Fragments. 149 Table 33. Energies and Decompositions of the Valence Levels of Pu2Me6. ^evel eV Occ %Pu %s %p %d %f %C 8eu -1.39 0 90 - 6 75 19 7 8aig -2.38 0 96 7 2 51 41 4 7eu -2.58 0 81 - 2 11 88 18 7a2u -2.59 0 95 6 0 1 93 5 -2.78 0 99 - 0 4 96 0 6a2u -3.03 0 96 2 0 4 94 4 2a2g -3.10 0 100 - - - 100 0 7aig -3.16 0 100 7 0 1 92 0 2alu -3.18 0 99 -- - 100 0 6eg -3.31 4 60 - 5 41 55 37 6e« -3.34 4 76 - 3 10 87 22 Seg -3.55 4 82 - 1 4 95 17 -4.27 2 51 20 2 12 67 44 6aig -4.52 2 52 31 1 27 41 43 5eu -4.58 4 89 - 0 24 76 10 5aig -5.32 2 96 13 4 28 55 3 Table 34. Energies and Decompositions of the Valence Levels of the PuMe3 Fragment. .evel eV Occ %Pu %s %p %d %f %C 8e -1.70 0 60 - 24 63 14 29 6ax -2.56 0 81 2 3 20 75 18 7e -2.87 0 79 - 2 17 82 20 2a2 -3.12 1 99 - - - 100 0 6e -3.15 4 100 - 0 0 100 0 5e -3.62 4 63 - 3 19 79 34 5ax -4.45 2 46 43 0 7 50 48 151 f character, which is primarily fz3. Overall this mix results in what appears to be a nearly non-bonding type of interaction. The next level is the 7eu which is now higher in f character (87.6%) than d character (11%). This orbital is primarily an fS* metal-metal interaction with a slight degree of metal-ligand antibonding character. This level is followed by the 7a2u level, which represents a mixture of primarily metal-metal f z3 o* and a slight degree of £ which is primarily a metal-metal fir* level. Next, is the 6a2u which is similar to the 7a2u but shows a higher degree of compared to fa* character. The 2a2g is 100% f other orbital combinations are of appropriate symmetry to interact with this orbital. The 7alg is also primarily f 2alu is the LUMO in this compound and consists of 100% £ character. The occupied levels follow appropriately in this model compound's structure with the HOMO being the 6eg followed by the 6eu levels. Both of these levels are primarily metal-ligand bonding levels but are able to incorporate some metal-metal character from the jt and S metal- metal interactions. The 6eg contains a mixture of d and f (40% and 55%), which are primarily dS and f5 in character. The 6eu is primarily f in character (87%) and is dominated by fS* character. The next level is the 5eg, which is primarily an fS metal-metal bonding level with some metal-ligand bonding character. The following two levels, the 5a2u and 6alg, are metal-ligand levels. These levels are both a mixture of s, dz2 , fz3, and f orbitals. The major difference between the two results from the presence of metal-metal bonding character in the latter while the former contains some metal-metal antibonding character. The lowest two valence orbitals to be considered, are the 5eu and the 5alg, which are the metal-metal rr and a bonding representations. These levels are primarily f in character with a low <30% amount of d character. This result is also interesting when compared with the result for U2Me6; in this new compound the d orbitals don't play as significant a role in either the metal-metal or the metal-ligand interactions. Comparison of Th^eg, U2Me6, and Pu^eg In Figure 28, the molecular orbital diagrams for the three actinide compounds are shown. It is clear from this figure that the f-orbital energies drop as one goes from Th to Pu. The largest of the energy decreases occur in the transition from Th-U with a smaller one between U and Pu. This effect has also been observed in a variety of other calculations11,122,131 where f orbital energies drop as the atomic number is increased. The effect that has not been seen before is the rise in d orbital energies from left to right. In the calculations presented in the literature, the d-orbital energies are fairly fixed as one goes from the early to late actinides.122 The result seen here is apparently due to an actual crossing of the d and f orbitals. This crossing has recently been supported by experimental determination of a d1 ground state for Cp"3Th.130 This result has been corroborated by molecular orbital calculations.130,131 153 Thf 7 e u '7 a 1g Pud 8e„-8a -2 " Puf -3 - 7e,,-2a U-C Pu-C *4 ’7e,,-2a Th-C ►6e,,-5a 5V 5aig eV -5 H -6 - -7 - Th2Me6 UgMSg Pu2Me6 2.6 A 105° Figure 28. The Electronic Structure of An2Me6 (An - Th, U, Pu). 154 Another effect that Is exhibited by this system is the increased degree of hybridization of f and d orbitals for uranium, whereas both thorium and plutonium lie more toward one extreme or the other. This hybridization appears to be able to form metal-metal orbitals that can gain more stabilization than the corresponding orbitals in the Th and Pu cases. As the Th-Th distance was not as short as the U-U distance, it is equally probable that the effect seen is just the effect of increased radial extension of the orbitals in the lower actinides. The last effect easily noticed from Figure 28 is that the metal-ligand orbital energies, which do not change much, are more stable for the Th case than for Pu. This is probably best explained as solely a function of the metal radius. Another effect that is not quite as apparent is found when the amount of electron density present on the ligand in each of the metal- metal and metal-ligand interactions is summed. It is found that the ligands have about half of the electron density in the case of the Th2Me6 calculation, while the Pu2Me6 calculation shows significantly less electron density on the C atoms. As the methyl groups should be negatively charged in these three cases, this is indicative of a more ionic type of interaction in the Th case while the Pu case is covalent. This is the opposite of what would be expected for An-Cp interactions. Conclusions In the comparison of the transition metal and actinide series there are some important differences. In the case of the transition 155 metals the metal-metal and metal-ligand interactions are formed almost exclusively from the 5s and 4d orbitals. Within the M2L6 framework, when L is a a only ligand, a a < 2tt < 2jt* < a * metal-metal manifold is formed. By simply counting electrons left over after formation of the M-L interactions the metal-metal bond order can be determined. This simple scheme is not readily converted to the actinide series. Within the actinide series a single, triple, and quintuple bond order would be predicted for Th2Me6, U2Me6, and Pu2Me6. As it turns out, there are problems with this description for U2Me6 and the two ends of the series have drastically different electronic structures. In conclusion, it must be realized that, of the actinides studied here, Th behaves the most like a transition metal and is in fact probably best described as a having d1-d1 configuration in this Th(III) system. In contrast, plutonium has been shown to utilize little d character in either the metal-metal and metal-ligand interactions; this causes plutonium to behave more like an f-element than the earlier actinides, as it has a purer set of f orbital Interactions. In the case of uranium the best of both the d and f interactions can be employed in bonding. This leaves uranium as an oddity; it doesn't behave exactly like an actinide, or a transition metal, or even somewhere in between the two. It has even more orbitals available than the other two cases can, leading to a much more complex picture, and in the case of U2Me6 to a catastrophic electronic structure. It is important to realize that, while the atomic radius of Th is much larger than that for U or Pu, the extension of the f orbitals is 156 not sufficient to create good overlap at the intermetallic distances chosen for these molecules. It is also important to note that the later actinides appear to form more ionic bonds with the alkyls than the early actinides. CHAPTER VI THEORETICAL STUDIES OF An2Cp6 COMPOUNDS Introduction The electronic structure of hypothetical dinuclear actinide compounds that contained a -only donor ligands has been addressed. Now we will turn our attention to hypothetical organoactinide dimers containing more complex ligands that can have both a and ir interactions with the metal center. There are a host of ligands that have this capability, the most important of which in the organometallic chemistry of the actinides being cyclopentadienyl.17 For this reason, we have chosen to explore how the electronic structure of a metal-metal bound dimer of Cp3An units would compare to the An2Mee calculations already presented. This may seem at variance to what is already known about Cp3An compounds, since there are no experimental indications of a metal-metal interaction.17 It is still instructive to see whether our calculations would indicate some rationale for not forming a metal-metal bond. Would, for example, the donation of electron density from the Cp ligands destabilize metal- metal interactions sufficiently to prevent metal-metal bonding? A wide variety of chemistry is known for organoactinides that contain Cp ligands and it has been the subject of several reviews. Many of these compounds contain three f;5-bound Cp ligands.17,118 Further, numerous accounts of early An(IV) compounds of the general 157 158 formulation Cp3AnX stand in comparison to a relatively small number of Cp3An compounds where the actinide is in an An(III) oxidation state.17 The majority of these An(III) compounds are uranium compounds17 while the Th(III) compounds17,181, are much rarer. In fact, a number of crystal structures have been reported for U(III) compounds. These include [UCp*20i-Cl)]3,*6 [UCp"z(M-X)]2 (X - Cl, Br),47 [UCp*2H(dmpe) ],182 [UCp3(thf)] ,183 [U(^-indenyl)3],184 [U(^-C5H4SiMe3)3- CNEt] ,25 [UCp3(n-Bu)]~,181 [UCp"2(/i-Me) ]2,48 and ThCp"3.39 Most of the An(III) compounds are known only in adduct form but a few examples of free Cp3An compounds are known. The crystal structures of two of these, Cp"3Th39 and Cp'3U, are known and both indicate a nearly planar structure. The nature of these Cp3An species behavior in solution is much more uncertain.185 In general, AnR3 compounds have been shown to have a pyramidal structure.25,40 We have chosen to model the Cp6An2 system after the known crystal structures of pyramidal Cp3AnR molecules instead of the planar Cp3An structures. As Cp ligands are bulky, it seems likely that the pyramidal form would minimize ligand-ligand repulsions in a metal- metal bonded molecule. Also, Cp6An2 can be thought of as Cp3AnR where the R group represents the second Cp3An group.17 It also seems likely that the D3d conformation would be preferred over D3h for steric reasons. We have chosen to study these large molecules for thorium and uranium. 159 Cp3M Compounds: Symmetry Considerations The bonding interactions that arise between a metal and the Cp ring are a a type interaction and two jr interactions based primarily in the Cp tt system; these will be referred to as the jtx and tt2 orbitals, where the ir2 is doubly degenerate. These three interactions are bonding interactions in which electron density is donated from the Cp" ligand to the metal orbitals. Two other interactions may arise from back donation of electron density into S symmetry orbitals on the Cp ring. When three Cp ligands are distributed about a central point in C3v symmetry their and ir2 orbitals are split as into an ax + e and an + a2 + 2e, as diagrammed in Figure 29. This has been discussed in detail for U(IV) by Tatsumi et al.118 Upon dimerization of the Cp3An compound to form a Cp3AnAnCp3 compound with D3d symmetry, we introduce another source of ligand-ligand interaction that serves to split the Cp 7rx and v 2 orbitals further. We can consider these effects by looking at the Cp68- ligand field in the absence of the metal centers. Under D3d symmetry the six 7r3 orbitals are formed from two sets of the C3v Cp33" group orbitals of ax and e symmetry splitting further, forming group orbitals of alg, a2u, eg, and eu symmetry. The twelve it2 orbitals split similarly under D3d to form alg, a2g, 2eg, alu, a2u, and 2eu representations. As has been shown previously for the Cp33- ligand field, the 7r3 orbitals are neither stabilized or destabilized appreciably with respect to the tTj of the free ligand. The ir2 orbitals, on the other hand, show that the a2 and one of the e representations are 160 Figure 29. Splitting of the jt Orbitals of Cp", Cp33-, and Cp66- 161 destabilized while the a! and the other e are stabilized with respect to the it2 levels of the free ligand.118,131,186 Within a Cp66- ligand field, the majority of the ligand splitting of the Cp it orbitals will be most likely due to the interligand interactions within the Cp33- moieties on one metal center. There should be a smaller contribution to the overall splitting of the orbitals due to interactions between the Cp it orbitals present on the two different metal centers. This splitting is expected to rise as the metal-metal distance is decreased. It is therefore likely that the it1 orbitals will again show little splitting while the it2 orbitals will show substantial splitting. For Cp66- the ordering of the it2 orbitals is expected to be aig, a2u, eg, eu, eu, egJ alu, a2g. In this case the alg and a2u orbitals are expected to be interligand bonding within their Cp33' groups while the a2g and alu orbitals form the corresponding interligand antibonding interactions. The interaction of the Cp ligands with the metal center will be different from the previously discussed a only ligands. In the case of the Cp ligands the jrx interactions are analogous to the a-only interactions while the ir2 interactions will be a new addition for the Cp ligands. Lauher and Hoffmann have previously discussed how a transition metal interacts when placed at the focus of three Cp ligands.188 A transition metal cannot interact with the a2 Cp it2 orbital and as such should be an inappropriate template since it fails to stabilize the highest lying orbital of the ligand set. On the other hand, an actinide has an ample number of orbitals with which to 162 interact with the Cp33- ligand field. Also, actinides have an f- orbital of proper symmetry to allow the stabilization of the highest lying Cp33- tt2 orbital of a2 symmetry. The metal-ligand interactions will serve to remove some of the potential for forming metal-metal interactions. The fz3 and the dz 2 orbitals are of proper symmetry to interact with the ligand 7rx and jt2 interactions but do not have a high degree of overlap with the ligand and as such do not play a significant role. Likewise the djr orbitals can overlap with the ligands better than the fjr orbitals. This in effect leaves a choice of dz2 or fz3 for a metal-metal interactions while the n , 6, and The symmetry labels of the Cp66- levels are quite similar to those used for the lower symmetry Cp33" levels. The two ax orbitals become the source of an alg and an a2u orbital. The two a2 levels split into an a2g and an alu level. Each set of the degenerate e levels split into an eg and an eu orbital. Cp6U2 Dimer U-U - 2.84 A As has been demonstrated by previous calculations on the monomer, the extension of the 5f based, frontier orbitals of a and 7r symmetry in Cp3U is substantial.118,131 It was considered likely that this orbital extension might be sufficient to yield good overlap between two metal atoms. This overlap would then enable the monomers to dimerize, forming metal-metal bonding interactions of a and tt symmetry. In this system the uranium centers can be considered as 163 U(III) , analogous In that manner to U2Me6 and to Mo2Me6. As such, six electrons would be left over from the metal-ligand interactions to form metal-metal interactions. The major differences that would be expected between U2Me6 and U2Cp6 would arise from the ir2 donation of electron density from the ligands to the metal and a weaker a donation. The donation of ligand based electron density into these levels would be expected to raise the metal based anti-bonding counterpart in energy, perhaps sufficiently to remove it from the metal-metal bonding picture. A further complication to this effect is the splitting of the Cp ligand levels due to Cp-Cp through space interactions. In this respect, we have performed RXa calculations on both the single bond distance (2.84 A) and a shorter distance (2.6 A) for Cp6U2 and on Cp6Th2 (3.30 A). These molecules are very large, and computationally difficult problems to solve with the Xa method, each requiring about an hour of CPU time on a supercomputer. By fragmenting the converged molecules in a variety of ways we can study the interaction of two Cp33- units with each other as well as the metal-metal energetics. We will first examine the MO results for the single bond distance of 2.84 A. This Cp6U2 dimer (U-U - 2.84 A) exhibits all the features found in the Cp3U monomer, with the additional features of metal-metal splitting of the metal based orbitals and additional, but slight, interligand splitting of the Cp it levels. The HOMO for the dimer (the 10alg) contains a high degree of metal d character as shown in Table 35. This orbital and the lower energy 9alg both contain a 164 Table 35. Energies and Decompositions of the Valence Levels of U2Cp6 (U-U - 2.84 A ) . 18e„ -2.00 0 68 1 71 28 6 a 2g -3.40 0 77 - -- 100 6 a lu -3.47 0 79 - -- 100 16eg -3.53 0 86 - 0 0 100 17eu -4.14 0 97 - 0 3 97 H - a2u -4.16 0 98 1 0 5 94 12alg -4.33 0 99 1 0 0 99 l<>a2u -4.43 0 96 0 0 4 96 H a i s -4.53 0 96 2 0 5 95 10alg -5.09 2 59 13 14 62 12 15eu -5.17 4 91 - 1 4 95 5 a 2g -5.78 2 41 -- 100 9 a 2u -5.78 2 14 1 30 19 51 14eg -5.85 4 23 - 2 18 89 5 a lu -5.91 2 39 -- 100 14 eu -6.35 4 24 - 7 65 28 13 eg -6.68 4 26 - 7 78 15 13eu -7.07 4 48 - 1 90 9 9 a lg -7.16 2 71 7 0 54 39 lle6 -9.37 4 11 - 28 71 1 12eu -9.43 4 9 - 45 51 5 8a2u -9.87 2 12 66 7 21 6 8 a ig -10.26 2 8 90 0 6 4 H e u -10.84 4 9 - 48 46 7 6 a ig -10.92 2 3 82 0 18 1 165 degree of metal-metal a bonding character formed from the mixing of the dz 2 and fz3 metal orbitals. The 9alg contains some metal-ligand bonding character while the 10alg is apparently more metal-ligand non bonding. The SHOMO (15eu) is a metal-metal n bonding interaction that is primarily 5f in character. It is instructive to note that the primarily metal-metal a bonding interaction (9alg) contains a high degree of dz 2 character (54%d and 39%f) and the 15eu tt level is primarily f in character. This follows well from the observation that dn- orbitals should be able to overlap better with the ligands than the fjr orbitals while neither the dz2 or fz3 (or type) orbitals can overlap substantially with the ligands. It appears that the choice of the source for the metal-metal jt bonding orbital is entirely ligand driven. If this were not the case, the 6d and 5f contribution to the 7r level would be much more similar to the a orbital. The LUMO is a metal-metal f8 bonding orbital, as expected from a qualitative argument. This molecule does not display the inverted HOMO/LUMO ordering of the UzMee molecule. Could this result be due to some destabilization of the 8 orbital by ligand tt2 donation into this orbital? The answer is no: the f 8 orbitals, as we will see later, are not greatly destabilized by the Cp ligands. The rest of the metal based orbitals lie at higher energies and demonstrate perturbation from their positions in the Cp3U monomer. If two U atoms were brought together in the absence of a ligand field and other metal based orbitals, the f orbitals would be expected to yield metal-metal interactions ordered as a < ir < 8 < dimer the ordering is 6a < fn < £8 < fa < £ £ the metal-metal bonding scheme in the absence of a ligand field are the presence of the two location of the a and a* orbitals. The reason for the position of the a orbitals is that the dz2 and fz3 can both effectively form a a interaction. Mixing of these leads to a four orbital interaction that yields one bonding, one antibonding, and two essentially non-bonding interactions. The two f If the two if) orbitals that cannot effectively interact with the ligands (10a2u, 12alg) are considered to be the barycenter of the f- orbitals, then we can calculate the splitting of the metal based f orbitals due to the metal-metal interactions and displacement due to the metal-ligand interactions. For this compound the center is calculated as -4.38 eV. The £ The barycenter analysis provided above can be verified by fragment analysis of the U2Cp6 molecule. On fragmenting the molecule into Cp3U units, the effects that are due to the ligands are easily separable from the metal-metal interactions, as the metal-metal effect is removed. The data from these studies are found in Table 36. In Figure 30 the construction of the Cp6U2 molecule from two Cp3U units is shown. The metal-metal splitting is found to be essentially the same as that calculated by the barycenter analysis. An encouraging result is the relatively large H0M0/LTJM0 gap of 0.52 eV for the single bond distance in this uranium dimer. In this case the HOMO is a level that contains a high degree of metal-metal it character while the SHOMO Is primarily metal-metal a in character with some Cp tt2. The degree of stabilization of the a and it interactions between the two Cp3U fragments is also high. This leads to the conclusion that forming metal-metal bonding interactions between two Cp3U moieties would be favorable at even the single bonding distance and should increase at shorter distances. This result would not have been expected from experimental observations on the Cp3An systems which are known. So there must be some readily available conformation that the Cp3An monomers find energetically more favorable conformation than the metal-metal bound dimer. Cp6U2 Dimer D-U » 2.6 A As the U-U distance is shortened, the metal-metal interactions are expected to intensify due to increased overlap of the orbitals on the two metal centers. This same reasoning may be applied to the 168 18e-15e U5f U5f U-U % U-U a 12e-8a. 11 e„-8a UCp. U2Cp6 U-U = 2.84 A Figure 30. Molecular Orbital Diagram of the Valence Levels of U2Cp6 (U U - 2.84 A) as Constructed from UCp3 Fragments. 169 Table 36. Energies and Decompositions of the Valence Levels of the UCp3 Fragment of U2Cp6 (U-U - 2.84 A). .■evel eV Occ %U %s %p %d %f 12ax -1.62 0 4 69 15 14 2 20e -1.69 0 65 - 1 79 20 7a2 -1.79 0 5 - - - 100 19e -2.07 0 49 - 1 80 19 18e -3.28 0 23 - 0 6 93 6a2 -3.46 0 78 --- 100 llax -3.52 0 14 1 1 26 73 10ax -4.06 1 90 6 3 35 56 17e -4.29 0 98 - 0 5 94 16e -4.49 2 98 - 0 1 99 15e -4.84 0 81 - 0 6 94 5a2 -5.86 2 40 -- - 100 9ax -6.24 2 16 0 34 11 55 14e -6.35 4 27 - 6 63 32 13e -6.68 4 27 - 2 93 4 12e -9.42 4 10 - 38 60 2 8ax -10.28 2 13 84 1 13 3 170 interligand interactions between the two sets of Cp33" ligands. It is readily apparent on examination of Figure 31 and Table 37 that this is indeed the case for the metal based orbitals. All the metal based orbitals experience a greater shift from the barycenter due to the increased metal-metal overlaps, thus resulting in changes in the ordering of the levels. The new ordering of the levels is do < fjr < £8 < fa < £ located). This still leaves a a 2^ triple metal-metal bond configuration, in contrast to the U2Me6 calculations. The 5 orbitals still lie above the HOMO in this molecule. By decreasing the metal- metal distance by 0.24 A the occupied a interaction (9alg) is stabilized by 0.71 eV and the 15eu jt orbital by 0.06 eV. These increased stabilizations indicate that the metal-metal bonding interactions are indeed stronger at the shorter metal-metal distance. This also results in a H0M0/LUM0 gap of 0.69 eV which is slightly larger than that calculated at the longer metal-metal distance. The character of these orbitals does not change greatly on shortening of the bond. This result is also in contrast to the changes that appeared when the U-U distance was shortened from 2.6 to 2.4 A in the U2Me6 molecule. In part, the insensitivity is due to the use of d orbitals by the Cp ligands leaving primarily f orbitals available for metal-metal bonding. If the same barycenter approximation is used to find the degree of metal-metal and metal-ligand splitting of the metal based orbitals, some differences are discovered. The f orbital energies are again estimated as originally at -3.96 eV. The metal-metal splitting of the 171 U5f 1.0 eV U2Cp6 U-U = 2.6 A U2Cp6 U-U = 2.84 A Figure 31. Molecular Orbital Diagram of the Valence Levels of U2Cp6 for U-U - 2.84 A and 2.60 A. 172 Table 37. Energies and Decompositions of the Valence Levels of the U 2Cp6 (U-U - 2 . 6 0 A). Level eV Occ %U %s %p %d %f 18eu -1.71 0 71 1 64 35 16eg -3.01 0 85 - 0 2 98 6a2g -3.04 0 79 - -- 100 6alu -3.15 0 81 --- 100 17eu -3.54 0 96 - 0 2 98 H a2u -3.59 0 97 1 0 7 93 12alg -3.89 0 99 0 0 0 100 10a2u -4.02 0 95 0 0 4 96 H a i 8 -4.17 0 96 6 0 26 68 15eg -4.24 0 92 - 0 6 94 10alg -4.93 2 48 9 19 41 32 15eu -5.23 4 80 - 2 4 95 9a2u -5.42 2 15 1 27 9 64 U e g -5.45 4 35 - 2 11 87 5a28 -5.45 2 37 - - - 100 5aiu -5.65 2 36 - - - 100 14eu -6.02 4 26 - 4 50 47 13eg -6.43 4 26 - 7 68 25 13eu -7.12 4 64 - 2 76 22 9aig -7.87 2 86 8 2 52 38 lleg -9.11 4 11 - 26 73 1 12 eu -9.19 4 9 - 42 48 10 8a2u -9.47 2 11 61 10 21 8 ®a ig -9.91 2 6 88 0 6 6 173 orbitals is larger, the 2.2 eV, and the upper lying ^ orbital pair shows a splitting of 0.11 eV. The upper lying In looking at Figure 32 and Table 38, the Cp3U fragments interact to form a Cp6U2 molecule in the same way as described for the 2.84 A case. The most marked differences which can be seen in this figure are the degrees of metal-metal interactions and the splitting of the Cp ?r2 orbitals. The latter differences arise partially from the through space interactions of two Cp33- groups. The two Cp3U fragments should appear very similar to each other since both of these effects are eliminated by fragmenting the molecule. These two effects can be accounted for by using the Cp66- fragment diagrams to gauge the degree 174 Table 38. Energies and Decompositions of the Valence Levels of the UCp3 Fragment of U2Cp6 (U-U — 2.60 A). .evel eV Occ %U %s %p %d %f 7a2 -1.48 0 6 --- 100 19e -1.72 0 38 0 2 82 16 18e -2.94 0 25 - 0 6 93 6a2 -2.98 0 81 - - - 100 12fll -3.19 0 16 3 2 4 91 llai -3.59 1 91 6 3 34 57 17e -3.76 4 98 - 0 5 95 16e -3.92 4 98 - 0 1 99 15e -4.50 4 74 - 0 5 94 5a2 -5.49 2 36 - - - 100 ioai -5.77 2 33 7 2 60 31 9ai -5.93 2 14 0 40 9 52 14e -6.01 4 25 - 5 63 32 13e -6.32 4 25 - 3 93 4 12e -9.12 4 10 - 41 57 2 8a! -9.88 2 13 84 1 12 3 175 18e-15e > U5f >• U5f eV 12e-8a U2Cp6 U-U = 2.6 A Figure 32. Molecular Orbital Diagram of the Valence Levels of U2Cp6 (Ti ll - 2.60 A) as Constructed from UCp3 Fragments. 176 of Cp33- splitting effects in the 2.84 A and 2.6 A cases. This will be examined later for all these molecules. Cp6Th2 Dimer Calculations were also performed on the dimer Cp6Th2. This dimer would formally contain a single metal-metal interaction expected to be a in symmetry. In most respects the Th dimers are expected to be mimic the U dimers with four fewer electrons present in the MO diagram. One difference between Th(III) monomers of this type that has been noted is the presence of a d1 ground state. This result has been noted experimentally and in theoretical calculations.130,131 The Th2Me6 molecule, as reported earlier, also exhibits a d1-d1 interaction and each ThMe3 fragment has a d1 ground state. This result is expected to appear for this molecule as well. On examination of Table 39 and Figure 33 it is apparent that while the Th and U dimers are similar, there are some important differences in the bonding description between the two actinides. The metal-ligand interactions in these two compounds show that there is less splitting of the Cp it2 levels by Th than by U. The overall ordering of the Cp n 2 and the Cp n x levels is relatively unchanged, however. The only real change is in the presence of the 10alg at lower energy than its corresponding orbital in the Cp3U dimer (10alg). It is also readily apparent that the character of the metal basis of the interaction with the Cp jt orbitals is much higher in d character than for the U dimers. In the Th dimer only the 5a2g and 5alu are 100% f in character and only one of the eg levels (14eg) has 48% f 177 >■ Th 5f U5f U-U K Th-Th G 1 n eV U-U a U2Cp6 U-U = 2.84 A Figure 33. A Comparison of the Electronic Structures of Th2Cp6 and U2Cp6 (U-U - 2.84 A). 178 Table 39. Energies and Decompositions of the Valence Levels of Th2Cp6. Level eV Occ %Th %s %p %d %f 16eg -2.63 0 94 - 0 1 99 12a2u -3.33 0 98 2 1 2 96 13alg -3.33 0 98 1 0 0 98 16eu -3.36 0 95 - 0 7 94 12alg -3.48 0 96 0 2 0 98 H a2u -3.61 0 95 0 1 18 82 15eg -3.68 0 94 - 0 2 91 15eu -4.02 0 92 - 1 11 88 H al8 -5.13 2 56 13 11 75 1 3fl2g -5.70 2 28 - - - 100 1 0 fl2 u -5.74 2 15 10 32 40 28 5alu -5.76 2 28 - - -' 100 14eg -6.03 4 26 - 4 48 48 14eu -6.40 4 25 11 78 12 10ai8 -6.61 2 52 7 0 70 22 13eg -6.75 4 29 6 89 5 13eu -7.00 4 42 - 0 98 2 12eg -9.39 4 13 - 28 70 1 12eu -9.39 4 12 - 37 61 2 9a2u -10.03 2 14 70 4 23 4 9ais -10.31 2 14 86 0 10 3 179 Th 5f > Th 5f 12a 15e Th-Th c > n. Figure 34. Molecular Orbital Diagram of the Upper Valence Levels of Th2Cp6 as Constructed from ThCp3 Fragments. 180 Table 40. Energies and Decompositions of the Valence Levels of ThCp3 Fragment. -evel eV Occ %Th %s %p %d %f 20e -1.53 0 72 - 0 53 47 14a1 -1.71 0 5 60 16 16 7 7a2 -1.83 0 45 - - - 100 19e -2.16 0 50 - 1 43 57 6a2 -2.31 0 86 - - - 100 18e -3.25 0 89 - 0 13 87 17e -2.44 0 97 - 0 1 99 13ax -3.56 0 80 2 0 3 95 16e -3.62 0 94 - 0 15 85 12ax -4.38 0 86 9 2 78 11 15e -4.72 0 46 - 1 13 87 5a2 -5.76 2 27 --- 100 llai -5.99 1 68 13 6 81 1 10ai -6.17 2 22 2 16 53 29 14e -6.38 4 26 - 9 73 18 13e -6.81 4 33 - 2 95 3 12e -9.40 4 13 - 33 65 2 9ax -10.25 2 14 80 1 16 3 181 character. All the other metal-Cp it interactions present contain less than 30% f character. For the U dimer the case was different. The 5a2g and the 5alu are demanded by symmetry to have 100% f character on the metal centers. For the Cp3U dimer (U-U - 2.84 A) the 9a2u contains 50.9% f and the 14eg representation contains 80.3% f character on the metal centers. In essence, this preference for metal d in the metal-ligand interactions for the Th dimer should lead to a cleaner separation of the d and f portions of the metal based orbitals. In fact, this separation of d and f, as well as the preference of d over f in Th compounds, is based in the original energy of d and f levels in Th. The fragment analysis of this molecule in conjunction with the Th2Cp6 energy levels again support a crossing of d and f levels in this molecule. If we now address the metal-metal interactions in the Th dimer it is apparent that ordering of the levels located is da < fit < fS < fa* + £ character present in these metal-metal interactions does not change appreciably except for the a and it interactions. The HOMO in this compound is the metal-metal a bonding orbital (10alg). This orbital is similar to the orbitals found for the U dimers but now contains a higher degree (75.0%) d character and lower (1.1%) f character. The LUMO is the metal-metal it bonding 15 eu interaction present at -4.02 eV. This results in a HOMO/LUMO gap of 1.11 eV, about double that in the U dimer calculations. Analysis of the Th2Cp6/ThCp3 orbital energy diagram shows that the degree of stabilization of the a orbital in this molecule is not as high compared to the U2Cp6 molecules. This is 182 expected due to the lower degree of metal-metal orbital overlap in this case. Comparison of the Cpe6~ Fragments The degree of splitting of the Cp jt levels can be examined by comparing the Cp66- fragments for the three cases studied, as shown in Figure 35 and Tables 41 - 43. As the ligand coordinates were not changed for the Cp ligands in these three cases, with the exception of the z axis, we can see the additional degree of splitting is due to two Cp33- ligands sets coming into close proximity. The degree of splitting of the n 1 levels is shown to increase slightly in changing from the thorium case to the uranium cases, but there is little difference between those two cases. The ordering of the ir2 levels is also seen to change, but again the overall splitting of these levels is about the same in all three cases. Evidently the Cp ligands do not have a significant interaction even at the 2.6 A U-U distance. For this reason, the differences between the n splittings in the An2Cp6 dimers must be metal dependant. This dependency is based in the covalency of the Cp-An interactions and the electron density on the metal centers. Conclusions While none of the Cp3An compounds are known to form a metal-metal bound dimer, it is interesting to see that Xa-SW calculations give an electronic structure that one might expect for a stable molecule. Molecular orbital calculations cannot be used to determine if a 183 Cp *a Cp %2 levels levels Cp 7^ Cp TCj levels levels Th 3.30 A U 2.84 A U 2.60 A Figure 35. Molecular Orbital Diagram of the Valence Levels of Cp6 Fragments of Th2Cp6, U2Cp6 (U-U - 2.84 A), and U2Cp6 (U-U - 2.60 A). 184 Table 41. Energies and Decompositions of the Valence Levels of the Cp6 Fragment of Th2Cp6. Level eV 5a2g -4.93 5aiu -5.04 13e8 -5.18 13eu -5.57 12eu -5.68 ®a2u -5.74 12eg -5.79 8aig -6.17 H e , -9.28 H e u -9.44 7a2u -9.74 7aig -9.99 185 Table 42. Energies and Decompositions of the Valence Levels of the Cp6 Fragment of U2Cp6 (U-U - 2.84 A). Level eV 5a2g -4.94 14eg -5.07 5 a lu -5.11 14eu -5.63 9a2u -5.68 13eu -5.69 13eg -5.90 9 a lg -6.26 12eg -9.25 12eu -9.53 8a2u -9.65 8 a ig -10.04 186 Table 43. Energies and Decompositions of the Valence Levels of the Cp6 Fragment of U2Cp6 (U-U - 2.60 A). Level eV 5a2g -4.80 14eg -4.82 5aiu -5.02 14eu -5.40 9a2u -5.44 13eu -5.56 13eg -5.79 9aig -6.12 12eg -9.07 8a2u -9.40 12eu -9.42 8aig -9.86 187 molecule would be expected to exist. Since no Cp6An2 molecules are known, there must be some reason why this molecule is less preferred than another form. Two reasons come to mind immediately. Could charge density on the An centers be high enough to preclude metal- metal distances which are as short as necessary for metal-metal bonding in preference to ligand bridging? Could some geometry change result in sufficient stabilization of orbitals such that metal-metal bonding is energetically unfavorable? If the former of these two is true then formation of metal-metal interactions might be achieved through the use of a template where the actinide atoms are held in close proximity. A ligand that seems to fulfill the potential for this application is fulvalene, shown below. 2- Fulvalene ligands have incorporated as a bridging ligand in many transition metal compounds, e.g. (fulv)2M2 compounds where M - V, Cr, Mo, Fe, Co, Rh, a n d Ni.187-188-188'wo.wi, 192,193 It may be p0Ssible to construct a (fulvalene)3An2 dimer where the fulvalenes each act to replace two Cp ligands. This molecule would be very similar to the molecules studied in this section except that the Cp33_/Cp33“ interactions would be large in the fulvalene case. Also, the overall symmetry would be D3h instead of D3d. These differences should cause the Cp and Cp jr2 interactions to be split further, but not sufficient to greatly change the picture presented here. Another character that fulvalene ligands permit is the ability to form planar or non-planar conformations. This would allow the nearly planar conformation seen in some Gp3An compounds to exist in the metal-metal bound case as well. Another ligand that is closely related to the fulvalene ligand is as-indacene, shown below.194 The distance between the centers of the five membered rings is 4.0 A in the ligand, and it has been shown to bend when coordinated to two metal centers.194,195 It is likely that this ligand would be much stiffer than the fulvalene ligand, however, and could not achieve its flexibility.194 If the planar form for Cp3An is more stable than the pyramidal form, this may be the basis for preventing metal-metal interactions. Lanthanide work suggests that a pyramidal form is more stable in solution. Another method to reach a metal-metal bond could then be to tie back the Cp ligands into a permanent pyramidal form. This might be achieved by making a ligand where three Cp ligands are bound to a central atom, as shown above. In each of these cases, the overall molecule would be of the Cp6An2 form studied here. Since, these calculations indicate no problems with the Cp6U2 electronic structure, perhaps limiting the more favorable forms seen for Cp3An could yield a metal-metal bonded species. On the other hand, since thorium and zirconium compounds are similar to each other and some metal-metal bound zirconium compounds104,175,176,177 are known, perhaps this line of reasoning could be applied to yield a Th-Th bond. CHAPTER VII MOLECULAR ORBITAL STUDIES OF BIMETALLIC ACTINIDE(V) COMPOUNDS Introduction The group 6 transition elements are known to readily form metal- metal bonds in compounds that contain a variety of ligands.101 Molybdenum and tungsten(III-V) are known to regularly exhibit bond orders of 3, 2, or 1 and stand in contrast to the actinide alkoxides which are not known to exhibit any metal-metal bonding.79,153 While uranium is a group 6 element and would be expected to have similar chemistry to the transition elements, the nature of the valence shell is somewhat different because of 5f participation. This may well impart sufficient changes in the electronic structure to direct the very different structure and chemistry of the actinide alkoxides. A variety of actinide alkoxides have been reported, primarily for the higher oxidation states. The Cp2U(0R)2 compounds and other Cp compounds are known82 as well as other types of U(IV) alkoxides. In general the U(IV) alkoxides are unstable with respect to oxidation and decompose forming U(V) alkoxides.196*197 Bradley et al. showed that the alkoxides are oligomeric as exhibited by U3(OMe)15198 and U2(OR)10199, and the tendency is dependent on the size of the alkoxide groups. In an investigation of the possibilty of making a U(III) alkoxide, a mixed-valent compound was reported in which both U(V) and U(VI) were present.196 A DV-Xa calculation was performed on the U(VI) U(OMe)6 190 191 molecule and has indicated that the oxygen ir orbitals are split to a larger extent than that in transition metal compounds.200 Electronic structures of a series of bimetallic U(V)-U(V) complexes were investigated to determine whether electronic features might be attributed as the cause of the different chemistry of the actinides. Uz(OR)10 Compounds Cotton and coworkers recently reported the preparation and structural determinations of three uranium alkoxide dimers. These are K[U2(OCMe3)9] *C6H1A, U2(OCMe3)g, and U2(OCHMe2)i0i representing U(IV)- U(IV), U(TV)-U(V), and U(V)-U(V) type compounds.80 The first two of these form confacial bioctahedra while the latter has edge sharing bioctahedra.80 Each of the three compounds are similar in that the uranium atoms occupy an octahedral site. The crystallographic structures of these compounds exhibit some details which are unusual when compared to other group 6 alkoxides. All the U -• -U distances are long, approximately 1 A longer than the predicted length of a U-U single bond.80 In the case of the latter compound, the U -• -U distance is 3.789 A and the central U202 unit is distorted. An idealized edge sharing bioctahedron would have U-O-U angles of 90° instead of the 111.4° reported. Is this distortion due to some orbital effect caused by the p -OR ligands or due to a uranium-uranium charge repulsion? Another unusual detail is that the terminal alkoxides are nearly linear with U-O-C angles between 160° to 176°.80 Transition metal alkoxides typically exhibit a M-O-R angle, 135°-145°, which corresponds more to an sp2 hybridized oxygen atom.79’153 In order to investigate these effects, Xa-SW calculations were carried out on two sets of hypothetical molecules based on U2(OR)10. These two sets of model molecules are U2H10 and U2(OH)10. In the former case only a interactions between the metal and the ligands exist while in the latter case both a and jt interactions are allowed. By performing these two sets of calculations the influence of the a and 7r type interactions can be separated. In each set, three different geometries were assumed for the molecules. The first of these is the D4h case where two ML5 fragments are allowed to approach each other such that all the ligands are terminal and a single metal- metal bond is assumed. In the other two, D2t, symmetry was assumed for an edge-sharing bioctahedral structure. In these two structures the U ,,,U distance is varied to reflect the distance expected for a U-U single bond (2.90 A) while the other reflects the actual nonbonded distance of 3.789 A. The structure that contains the short U-U distance also contains a much more acute U-R-U angle of 80°. Structural Symmetry Considerations In each of these hypothetical compounds, the uranium atoms lie at the center of an essentially octahedral arrangement of ligands. In this type of environment, the typical assignment of f orbitals is not convenient to describe the a and n metal-ligand interactions. It is much more appropriate to consider the f orbitals to be formed from the fj c y z i f y 3 , fz ( x 2 - y 2 ) > ^ x ( z 2 - y 2 ) > and i y (z 2 - x 2 )1 shown in Figure 36.101 In this scheme, the fx3, fy3, and fz3 orbitals (tlu) can interact in a a sense with the ligands while the last three orbitals 193 t2u set y=z plane y --z plane a2u orbital Figure 36. The f-Orbitals Under Oh Symmetry. 194 form the t2u n set. The a2u f ^ orbital is then left over to form any 6 interactions with the six ligands. This type of a description is most readily applicable to the hypothetical U2R10 molecules with DAh symmetry. Under this lowered symmetry, the same orbitals are important to the a, n, and S type interactions of the f orbitals. The major difference lies in the reduction of the degeneracy of the f levels. Under this symmetry, the fa type interactions are given by the fx3 and fy3, forming an eg + eu set, and the fz3 which forms an alg + a2u. The metal-ligand it type interactions of the t2u set give rise to a eg + eu + b2u + blg. Lastly, the fS a2u gives rise to an a*, set. The D2h molecules can be approached in a similar fashion by simply rotating the ML5 fragments of the D^h molecule such that two of the ligands end up in the bridging positions. A diagram representing this type of an action and its effect on the orbitals is shown in Figure 37. If the distance to the ligands is approximated as constant in this process, it is easy to see that the overall nature of the orbitals remains unchanged in the process. The major differences are simply the position of two of the axis and the nature of any metal- metal interactions. In forming the D2h molecules from the D*h molecules, it is necessary to rotate the MLS groups through 45° about the x axis. 195 l,-'L M— L y=-z t1u set t1u se t y=z z=-y z=y y=“z t2u se t t2u s e t y»z plane y«-z plane a2u orbital a2u orbital Figure 37. Rotation of an ML5 Fragment from DAh to D2h Symmetry. 196 u 2h 10 In order to examine the influence of ligand a bonds on the electronic structure, the details of the U2H10 calculations will be studied first. The three molecules studied are diagrammed below: H H H H H H It is easiest to examine the electronic structure of the non bridging cases and then to examine the bridged compounds. In the unbridged case, the U-U distance was set to a bonding distance of 2.84 A and the U-H distance was set to 1.9 A.7A The molecular orbital diagram of the valence levels of this compound is shown in Figure 38. It is apparent that these levels are split into two distinct groups. The lowest energy of these are the U-H a bonding levels with the U 5f levels lying at higher energy. The energies and decompositions of all of these orbitals are reported in Table 44. The metal based orbitals are further split into a lower set that contains no ligand character and an upper set that contains metal-ligand antibonding character. Under D^h symmetry the metal-ligand orbitals transform as 2alg + lblg + eg + 2a2u + lb2u + eu. The alg and a2u orbitals account for the interactions with the axial hydrogens, while the other orbitals account for the rest of the interactions. The metal-metal interactions of the 5f orbitals transform as a: alg + a2u, it: eu + eg, 8: blg + b2g + blu + b2u, and eu + eg. It is clear that only a few of these orbitals (b2g and blu) are forbidden, by symmetry, to interact 197 0.0 - 1.0 U levels 2b“ 3b; - 2.0 -3.0 eV -4.0 -5.0 -6.0 U-H levels -7.0 - 8.0 -9.0 ;U:-- H Figure 38. Molecular Orbital Diagram Displaying the Valence Orbitals of U2H io Under D^h (left) and D2h Symmetry (right) (U-U - 2.84 A). 198 Table 44. Energies and Decompositions of the Valence Levels of U2H10 (D4h, U-U - 2.90 A). The 4alg Orbital is the HOMO. Orbital eV %U %s %p %d %f % H eq - 4 e g -0.67 90 8 8 83 0 10 4aZu -0.85 96 0 4 1 95 3 1 4eu -1.52 97 - 6 11 83 0 3 3 e g -1.85 100 - 0 1 99 0 0 2b2u -1.86 100 - - 0 100 0 0 lblu -1.89 100 - - 0 100 0 0 2bl8 -2.19 100 - - 0 100 0 0 lb2s -2.23 100 - - 1 99 0 0 3eu -2.44 100 - 1 6 93 0 0 4 a ig -2.81 100 3 1 7 90 0 0 2e8 -5.07 56 - 18 0 82 0 44 2eu -5.56 54 - 18 0 82 0 46 3a2u -5.82 44 22 19 12 47 51 4 3 a lg -6.70 44 42 5 37 17 52 4 2a2u -7.41 38 34 2 58 6 6 57 2 a ig -8.14 40 21 0 78 0 7 54 lb2u -8.19 29 -- 100 0 0 61 199 with the ligands. It is also possible for the two sets of eg and eu orbitals to mix, forming new hybridized orbitals. These f orbitals can form two degenerate sets that are best described as a metal-ligand a set (fx3 + fy3) and a metal-ligand it set (fx(z2.y2) + fy(z2_3C2)).101 The metal 5f orbitals that are expected to interact with the ligands in a a sense are the alg + eg + a2u + eu orbitals. These are the only f orbitals that have lobes which are directed at the ligands. Under D4h, the f orbitals o f the eg and eu sets are the metal-metal antibonding and bonding combinations o f the f x3 and f y3 orbitals. Since these orbitals form a bonding interaction with the ligands in the 3alg, 3a2u> 2egl and 2eu orbitals, they have a metal-ligand antibonding counterpart at higher energy. Of these four combinations, only the eg and the eu orbitals are primarily f in character, with the a2u and alg consisting of hybrids. The metal-metal orbitals influenced by this metal-ligand anti-bonding character are the a: 4alg, a*\ 4aZu, and it + Turning our attention to the molecules, the orbitals of this system can be thought of as originating from the orbitals of the D4h case, as previously diagrammed. Examining the electronic structure for U2H10 at the short U-U distance, as presented in Figure 38, it is apparent that the ordering of the levels is very similar to that under 200 D*h. This time the f orbitals are the a : ag + b lu, jt: b2u + b3u + b2g + b 3g, 8: ag + b lu + au + b lg, and 4>: b 2u + b3u + b 2g + b3g. It is still better, however, to envision these orbitals as arising from the metal-ligand a and metal-ligand it orbitals, as the metals are in a relatively unchanged ligand environment. For this reason, the metal- ligand orbitals of the D2h compounds can be derived from the DAh case by rotation of the MLS fragments by 45° about the x axis. As in the D4h case, the metal based levels that contain the highest amount of ligand character are the highest lying orbitals. The energies and decompositions presented in Table 44. In this reduced symmetry the metal-metal interactions within the f-block become more complex than those presented in the D4h case. In addition to the 5fit/ This interaction serves to weaken the already weak U-U 5fa interaction. The lowered splitting (0.96 eV) of the a/a* set (4ag, 3blu) best evidences this when compared to the 1.96 eV splitting of the 4alg and 4a2u in the D4h case. What is most important to notice, however, is that the c-only ligand set only slightly perturbs the f-block and the metal-metal a < it < 8 manifold remains essentially intact in the lower f levels. This will become important in deciding whether the U-L a or it interactions are more important to the electronic structure in U2(OR)10. The hybridization of the levels in the D2h case also causes an important reduction in the metal-metal bonding capacity relative to the D^h case. 201 Table 45. Energies and Decompositions of the Valence Levels of U2H10 (Da,, U-U - 2.90 A). The 4ag Orbital is the HOMO. orbital eV %U %s %p %d %f %U1 %h 2 %h 3 3b2g -0.65 93 _ 7 9 84 0 7 0 3b3g -0.95 98 - 2 2 96 2 0 0 -0.98 2 3 26 72 2 0 1 5 a B 97 *blu -1.35 99 1 1 2 96 0 0 0 3blu -1.44 100 1 0 3 96 0 0 0 3b3u -1.51 99 - 4 10 86 0 1 0 2b2g -1.60 100 - 0 1 98 0 0 0 -1.63 100 - - 1 99 0 0 0 2b3g -1.73 100 - 0 0 100 0 0 0 3b2u -1.98 99 - 0 1 99 0 0 1 lblg -1.99 100 -- 1 99 0 0 0 2b3u -2.25 100 *• 1 10 89 0 0 0 4 a s -2.40 99 0 1 6 92 0 0 0 lb2g -5.13 53 22 0 78 0 47 0 2b2u -5.49 47 - 27 1 72 30 0 23 lb3u -5.65 52 - 21 0 78 0 48 0 3 a g -5.86 47 2 12 8 77 26 0 26 2blu -6.05 41 32 24 9 36 56 2 0 lb3g -6.77 44 - 15 53 32 56 0 0 -7.85 32 0 31 2 a 8 98 2 0 23 14 Ibiu -8.03 38 14 1 84 2 3 59 0 lb2« -8.58 38 - 0 100 0 29 0 33 -8.78 40 0 99 8 46 6 l a 8 0 1 202 0.0 Metal based 6d and 5f levels - 2.0 " -3.0 - -4.0 - -5.0 " - 6.0 " U-H bonding levels. -7.0 - - 8.0 “ -9.0 Figure 39. Molecular Orbital Diagram Displaying the Valence Orbitals of U2H10 Under D2h Symmetry Where U-U - 2.84 A (left) and U-U - 3.79 A (right). 203 Table 46. Energies and Decompositions of the Valence Levels of U2H10 (D2h, U-U - 3.79 A). The 4ag Orbital is the HOMO. Orbital eV %U %s %p %d %f %HX %h 2 %h 3 ^b2u -1.13 98 . 7 0 93 1 0 1 3b3g -1.19 99 - 1 5 94 1 0 0 5as -1.19 97 0 2 10 88 1 0 2 3b3u -1.24 99 - 5 5 90 10 10 10 ^blu -1.36 100 2 0 2 95 0 9 0 3blu -1.48 100 0 0 0 100 0 0 0 2bZg -1.51 100 - 0 0 100 0 0 0 -1.52 100 -- 0 100 0 0 0 2b38 -1.53 100 - 0 0 100 0 0 0 3b2u -1.54 100 - 0 0 100 0 0 0 lblg -1.58 100 -- 0 100 0 0 0 2b3u -1.59 100 - 0 0 100 0 0 0 4a« -1.65 100 9 0 2 98 0 0 0 3ag -5.05 47 4 33 11 53 34 0 19 lb2g -5.12 54 - 22 0 78 0 46 0 2b2u -5.17 47 - 17 0 83 17 10 37 lb3„ -5.21 53 - 23 0 77 0 57 0 2blu -6.16 43 41 17 7 34 55 2 0 lb3g -5.73 46 - 14 59 27 55 0 0 lb2u -7.42 41 - 4 92 4 41 0 19 2a6 -7.89 37 63 2 33 2 3 47 13 lb iu -8.08 40 12 1 85 1 3 56 0 la8 -8.71 39 9 1 86 6 17 18 26 204 Very few changes in the electronic structure appear on changing from the short U-U distance to the longer U " ’U distance within the D2h conformation as shown in Figure 39 and Table 46. The most important of these is simply the reduction of any metal-metal bonding or antibonding interactions as well as the reduction of any potential for repulsion between the two U(V) centers. U2(OH)10 The model system, U2(OH)10 was examined in three geometries: 1) The all terminal D4h geometry where U-U - 2.84 A. 2) The bis-bridged D2h geometry where the U-U was long (3.79 A). 3) The bis-bridged D2h geometry where the U-U distance was short (2.90 A). Of these three geometries the second one resembles the situation actually found in U2(OR)10 compounds. In each of the models the M-O-H angles of the terminal ligands were assumed to be linear since the M-O-H angles in the known uranium alkoxides is nearly linear. These model systems, coupled with the a only U2H10 models should allow a good comparison of the OH it effects on the metals in both the supported and the unsupported cases. Once again, we will begin the analysis with the less complicated D*,h complex and then simply rotate the U(0H)5 fragments to explain the transformation to cases. In the D4h case, three sets of valence orbitals can be seen readily in Figure 40. These three groupings represent the U-0 a bonds, the U-0 it interactions and the U 5f orbitals, which are grouped together in the highest lying 205 -3.0 -5.0 U5f -7.0 -9.0 eV U-O tc - 11.0 -13.0 -15.0 U-0 a -17.0 -19.0 | ,'0H | , P H OH, I O,. I ,,%'OH HO— U JJ-OH ...... HO I HO' OH Figure 40. Molecular Orbital Diagram Displaying the Valence Orbitals of UZ(OH)10 Under D*h (left) and D2h Symmetry (right) (U-U - 2.84 A). 206 set. Table 47 and Table 48 list the energies and the decompositions of U 5f block and the U-0 n interactions, respectively. If we look first at the 5f block in U2(OH)10 and compare this to U2H10, as shown in Figure 41, some dramatic differences appear. In the U2H10 case a a < it < S ordering appeared while here the two lowest orbitals represent 6 and 6* interactions. As the geometry of the overall molecule has not changed and only the ligands bound to the metal have changed, it is easy to conclude that M-L interactions must contain the reason for this difference. On inspection of the decompositions of the U 5f levels, the amount of ligand character is found to be substantial in all but the two lowest orbitals (2b2g and 2blu). In fact, all of the U 5f orbitals (except the 2b2g and 2blu) demonstrate a shift to higher energy of about the same magnitude. The next question is whether this shift is due to the a or the tt donor capabilities of the two ligands. If the U2H10 molecule can be taken as a guide in this case, then the relative importance of a donation must be low. Evidence of this is the relative amounts of ligand present in the U 5f orbitals of the two species (cf. average U 2H10 98% U, U2(OH)10 89% U). This evidence prefers the description that most of the new contribution to the U 5f orbitals must be in the form of U- OH t t antibonding character, hence there is a destabilization of the orbitals affected and concomitant displacement of the a < jt < S sequence from the lower portion of the U 5f manifold. The 2b2g and 2blu as previously mentioned are 5 and S* interactions between the U 5f orbitals. These are formed from the U 5ffi orbital that has S symmetry along all the U-U and U-L axis in the Table 47. Energies and Decompositions of the Metal Based Levels of U2(OH)10 (D^h, U-U - 2.90 A). The HOMO is the 2b2g. Orbital eV Type® %U %OH 8es -5.38 <£*A* 86 14 8e« -5.61 *b2u -6.32 S* 86 14 7e6 -6.38 */ 4bl8 -6.51 6 88 12 7eu -6.89 n/4> 88 12 7aig -7.07 a 90 10 2blu -7.09 6* 100 0 2b28 -7.33 S 100 0 “Refers to U-U interaction type. 208 Table 48. Energies and Decompositions of the U-0 ?r Levels of U2(OH)10 rbital eV % U % f % d % p % s % O H 6e8 -9.65 3 66 3 31 0 97 l a lu -9.83 0 0 0 0 0 100 l a 2g -9.95 0 0 0 0 0 100 6eu -9.95 6 63 2 35 0 94 6 a 2u -10.13 8 58 2 39 1 92 5es -10.56 11 63 8 29 0 89 3b2u -10.58 18 100 0 0 0 82 5eu -10.81 15 89 1 10 0 85 4es -10.99 13 51 48 1 0 87 6alg -11.03 13 75 7 14 4 87 3blg -11.28 17 100 0 0 0 83 lblu -11.53 10 0 100 0 0 90 4eu -11.59 11 3 97 0 0 89 lb28 -11.77 11 0 100 0 0 89 209 Figure 41. Molecular Orbital Diagram Displaying the Valence Orbitals of U2H10 (left) Compared With U2(OH)10 (right) Under D*h (U-U - 2.84 A). D4h case. As such, these orbitals are forbidden to interact with any of the a and n ligands under D4h symmetry. These two orbitals are split a very small amount, 0.24 eV, and would not be expected to form a true HOMO and LUMO. Rather, there would most likely be partial occupation of these two orbitals leading to destabilization of the already weak metal-metal S interaction. The U 5fa bonding level (7alg) lies next highest in energy and contains 10% OH character. The hydroxide character in this orbital is primarily U-OH n* and destabilizes the 7alg substantially. The antibonding interaction arises between the inner doughnuts of the fz3 orbitals and the OH n orbitals that lie equatorial to the z axis. Although such an interaction may seem unfavorable, the orbital contains 9% equatorial OH character and only a very small amount of axial character. This interaction results in substantial destabilization of the 7alg (-0.9 eV) from the position in the a only complex. As a result this level rises above the S interactions and provides a very sound reason for discouraging the formation of a D^h type U2(OH)10 molecule. It should also be pointed out that in a planar MoMe3 molecule, the dz 2 ax interaction is more important to M-L bonding than the s orbital. This represents a similar case where overlap would be expected to be low. The fact that the D4h molecule was preferred to have the stronger metal-metal interactions in the U2H10 molecules leads to the conclusion that the influence of the U-OH it interactions are catastrophic toward the formation of any metal-metal bond in the U2(OH)10 molecules. 211 It is also of interest to examine the U-OH it bonding levels in this molecule. These interactions are summarized in Table 48 and reflect substantial 5f interaction in several levels. It is interesting to note that in this geometry only two of the multitude of OH it levels don't contain any metal character, while most of the levels contain greater than 10% uranium. By symmetry, these two levels are unable to interact with any of the uranium atomic orbitals. The importance of these levels will be addressed later on in the discussion of the D2h molecules. An explanation for the near linearity of the U-OR groups is already in hand, however. An alkoxide ligand can have a hybridization which lies between the sp2 and the sp extremes. In the linear case, the OR group is considered to be sp hybridized and have two pjr orbitals with which to form interactions with the metal. In the bent case, the M-OR group can have an sp2 hybridized oxygen atom. In this case there is of course, only one oxygen -pit orbital available to bond to the metal. This means that in an ML5 fragment the ligand set would have a total of 10 it orbitals in the sp case and only 5 in the sp2 case. The maximum number of atomic orbitals available to a transition metal is nine (assuming s, p, and d) while an actinide has sixteen (assuming 5f in addition). If the M-OR a orbitals are subtracted from the total orbitals available, then the transition metal has only four orbitals left while the actinide has twelve. Clearly, a transition metal would not have the number of metal orbitals necessary to restrict an alkoxide to the sp hybridized form in an M2(OR)10 molecule while the actinide might have an excess of orbitals available for this 212 purpose. This, of course, will only be the case if the 5f and 7p metal based orbitals can fulfill the proper symmetry requirements, and if they are available. As is readily apparent in Table 48, the 7p, 6d, and 5f all readily form interactions with the alkoxide ligands in the D4h U2OR10 molecule, and the first requirement is met for the majority of the alkoxide ligands in that only two orbitals cannot interact within this geometry. As in the case of the U2H10 molecules, the D2h geometry for U2(OH)10 can be arrived at by simple rotation of the ML5 fragments by 45° about the x axis through each uranium atom. Two of the terminal hydroxide groups must end up in a bridging position and cannot be linear any longer. The valence MO diagram of the D2h alkoxide dimer with a short U-U distance of 2.90 A is shown in Figure 40 and the decompositions of the 5f block and the U-0 it bonding levels are presented in Table 49 and Table 50, respectively. As in the DAh case, the valence orbitals are split into three distinct groups of U-0 a, U-0 it, and U 5f metal-metal levels from lowest to highest energy. The 5f block contains greater mixing in this reduced symmetry as the a and S are allowed to hybridize in addition to the it and ^ orbitals. Once again, the 5f block is shown to interact substantially with the alkoxide it orbitals. As in the DAh molecules, the amount of metal character in the metal based orbitals is lower in the alkoxide dimer than in the hydride (cf. alkoxide - 89%, hydride - 99%). This is taken as evidence that the U-OH it interactions are much more important than the U-OH a interactions, as previously discussed. The destabilizing effect of the ligand it Table 49. Energies and Decompositions of the Metal Based Levels of U2(OH)10 (D2h, U-U - 2.90 A). The HOMO is the 7b3u. Drbital eV Type* %U %0Ht %m -o h 7b28 -4.97 ?/** 86 14 0 8b3u -5.12 10blu -5.18 S*/a* 83 9 8 11a, -5.44 S/a 88 9 3 8b3g -5.46 87 10 3 9b2u -5.58 n/4> 87 9 4 9blu -5.74 a*/S* 88 11 0 * /A * 7b38 -5.97 * /9 90 10 1 4blg -6.14 S 87 8 5 8b2u -6.17 n/ 3 a u -6.21 S* 92 8 0 6b28 -6.58 * / f 100 0 0 10ag -6.59 a/S 92 7 0 7b3u -6.90 n/ "Refers to primary U-U interaction type. 214 Table 50. Energies and Decompositions of the U-0 ir Levels of U2(OH)10 (D^, U-U - 2.90 A). Orbital eV %U %f %d %p %s %0Ht % 0 H (1 5b2g -9.26 2 _ 62 7 31 98 0 3bl8 -9.41 1 -- 0 100 75 24 6b3u -9.44 1 - 60 7 33 87 11 6b3g -9.49 7 - 89 1 11 65 28 7b2u -9.58 8 - 24 1 75 90 2 2ay -9.59 8 -- 3 97 92 0 5b3g -9.70 7 - 89 1 11 65 28 8blu -9.85 12 0 25 3 73 83 5 5b3u -9.96 4 - 75 2 23 26 70 2blg -10.27 15 -- 1 99 41 44 6b2u -10.37 15 - 9 0 90 85 1 7blu -10.38 17 5 1 12 83 63 20 9ag -10.43 10 1 20 55 24 87 2 4b2g -10.52 10 - 19 79 2 90 0 ^b3g -10.56 19 - 2 5 94 55 26 8as -10.70 16 0 1 14 84 84 0 lau -10.86 12 -- 72 28 88 0 6blu -10.93 17 6 9 32 52 26 57 lblg -11.22 11 -- 100 0 64 25 *b3u -11.30 12 - 0 100 0 64 14 215 orbitals on the metal 5f levels Is demonstrated In Figure 42 where the metal 5f block of the D2h molecule Is compared to the U2H10 case. As in the DAh case the alkoxide w interactions significantly destabilize all but the lowest two of the f-based metal-metal interactions (6b2g and 7b3u). If these orbitals are considered from the point of the ligands, they have S symmetry. These orbitals then correspond directly to the S and S* metal-metal orbitals of the D4h case, which could not interact with the ligands either. They are simply rotated through 45° about the x axis. Under the D^, geometry, these lowest two levels represent a n/ The decompositions of the U-OH i t bonding levels, which can be found in Table 50, show a great degree of similarity to those in the D*h case previously described. The one important change lies in the fact that all the hydroxide n levels can interact with the uranium atomic orbitals in the reduced symmetry of D^. For this reason, the desire for the U2(O-iPr)10 molecule to exist in the bridged conformation appears to be created by two reasons: 1) The metal-metal interactions are weak and, as such, the D4h molecule is not favored 216 10b T 0.5 eV i 1u *1u *3u au 10a, 3g 3u 3u 4a, lg HH OH OH OH OH Figure 42. Molecular Orbital Diagram Displaying the Valence Orbitals of U2H io (left) and U2(OH)10 (right) Under D2h Symmetry (U-U = 2.84 A). 217 over the D2h form. 2) The metal-alkoxide tt Interactions are improved slightly in the lower symmetry case. Increasing the metal-metal distance from 2.90 A to 3.789 A yields the last UZ(OH)10 D2h molecule to be considered. This conformation represents an idealized form of the known structure for U2(O-iPr)10. In this geometry, lengthening the metal-metal distance affects the metal-metal a and ir interactions much less than would be expected in the DAh case. The main reason for the low effect is the mixing of the a and S as well as the ir and Rather, the local pseudo-octahedral environment about each uranium atom largely controls the hybridization of the metal atoms. As a result, not only are the metal-metal bonding levels destabilized by alkoxide w interactions but they are further diminished by the local symmetry effects of the ligand field on the hybridization of each metal center. Figure 43, Table 51, and Table 52 exhibits the effects of lengthening the metal-metal distance on the 5f orbital energies. For example, the splitting of the U-U a interactions decreases from 1.0 eV to < 0.1 eV at the longer distance. At this distance, it is best to assume that no interaction exists between the two metals. Conclusions These studies have resolved a number of questions about the nature of bonding in the uranium and molybdenum alkoxides. The relatively large U-O-R angle has been explained in terms of nearly sp hybridization of the oxygen atoms in contrast to sp2 hybridization of 218 10b 10b: 0.5 eV 10a, ' > < 10a, OH OH OH„. I ,,0„. | OH OH OH OH*'' | ^ | ^OH Figure 43. Molecular orbital diagram diplaying the valence orbitals of U2(OH)10 under D2h symmetry with U-U - 2.84 A (left) and U-U - 3.79 A (right). 219 Table 51. Energies and Decompositions of the Metal Based Levels of U 2(OH)10 (D2h, U-U - 3.79 A). The HOMO is the 7blu. Orbital eV Type* %U %0Ht %o h m 8b3« -4.74 4/* 86 13 1 7b28 -4.79 87 13 0 00 CO to -4.87 */ 10blu -4.99 6*/o* 84 9 7 9b2u -5.18 */ lla8 -5.37 S/a 89 10 1 8b2u -5.73 ir/ 7b38 -5.75 **/t* 89 11 0 4big -5.81 S 90 7 3 * /r* 9blu -5.83 a /o 92 7 1 10ag -5.93 a/S 94 5 1 3a« -5.99 S* 92 8 0 6b28 -6.38 */4>* 100 0 0 7b3u -6.40 ix/ “Refers to primary U-U interaction type. 220 Table 52. Energies and Decompositions of the U-0 tt Levels of U2(OH)10 ( D ^ U-U - 3.79 A). The HOMO is the 4blu. Drbital eV %U %f d P s %OHb %OHm 5b2g -9.17 2 28 3 69 0 98 0 6b3u -9.17 1 32 0 68 0 93 6 3ble -9.23 2 96 4 0 0 82 16 6b38 -9.29 5 25 1 74 0 87 8 7b2u -9.38 6 72 2 26 0 93 1 2au -9.41 8 96 4 0 0 92 0 8blu -9.55 10 31 7 59 3 64 26 5b3u -9.88 5 18 16 66 0 47 48 9as -9.93 9 52 9 38 1 89 2 5b3g -9.97 16 92 0 8 0 80 4 6b2u -10.05 15 93 1 6 0 85 0 2blg -10.22 12 98 2 0 0 32 56 7blu -10.32 16 74 22 3 1 78 6 K -10.37 15 71 28 1 0 85 0 ^b2g -10.46 10 1 87 12 0 90 0 -10.63 12 27 73 0 0 88 0 6blu -10.68 19 71 16 4 9 32 49 lbi8 -10.80 10 1 99 0 0 66 24 *b3g -10.96 24 77 14 9 0 14 62 4b3u -11.10 10 2 97 1 0 50 40 221 the oxygen atoms In Mo-O-R systems. A simple orbital counting basis has been shown that provides the reasoning for the differences between the two metals. In the case of the molybdenum compounds, a maximum of four metal orbitals are present in an ML5 fragment with which to form M-L 7r interactions in contrast to twelve in the case of uranium. This drastic difference in number of available orbitals arises from the availability of 5f orbitals in addition to 7s, 7p, and 6d. In the case of the D2h compounds of general formulation M2(OR)10, a total of six M-OR a interactions are removed from the picture, leaving eleven atomic orbitals on uranium to interact with the ligands in a ir sense. The lack of a metal-metal bond in the uranium alkoxides can be explained by two factors: 1) The U-OR interactions destabilize the metal 5f and 6d orbitals such that the most favorable metal-metal interactions are removed from the lowest part of the metal-metal manifold leaving weakly metal-metal bonding and antibonding levels as the HOMO and LUMO in the U(V) case. 2) The ML5 units adopt a "pseudo- octahedral" environment about each uranium atom, which serves to reduce the metal-metal interaction by directing a ligand based hybridization. The preference of the D2h geometry in the U2(OR)10 molecules can be explained on the basis of the lack of a metal-metal bond and the pseudo-octahedral environment about the metal atoms. By adopting the geometry, the octahedral environment about the metal atoms is further enhanced and the reduction in symmetry allows more of the alkoxide rc levels to interact with the metal center. CHAPTER VIII CONCLUSIONS To date the only type of metal-metal Interactions present in discrete actinide molecules are dative type interactions between an anionic transition metal fragment and a cationic actinide fragment. The studies on the heterobimetallie systems presented here have shown: 1) The metal-metal bonding character located on the Th, and U atoms is principally 6d in character with a smaller amount of 5f character. 2) The decompositions of these orbitals show very little difference between Cp3An, Cp2AnX, and Cp2ZrX type fragments. 3) The decompositions of the RuCp(C0)2 fragments in these compounds show that they are very similar, as supported by the experimentally determined t'co values. 4) The RuCp(C0)2 fragment behaves as a "pseudo-halide" and, as such, presents a large amount of chemistry that can yet be achieved. 5) When the Cp3U case is examined, the last two electrons begin to fill a U 5f level. This type of a compound not only resembles the early transition metal chemistry but also closely resembles lanthanide chemistry. For these reasons, it is likely that a diverse collection of dative heterobimetallic compounds which contain a cationic Cp3An, Cp2AnX, CpAnX2, or AnX3 fragment will be synthesized in the near future. 222 223 In the subsequent sections the homobimetallic chemistry of hypothetical and known compounds were probed. To date, no compounds of this type are known. In the comparison of U2Me6 and Mo2Me6 molecules two major similarities were noted: 1) The metal-metal bonding of a and it type levels is found to be similar to that in Mo2Me6. 2) The U-L interactions destabilized metal based orbitals similar to those in Mo2Me6. While the XJ2 molecules were found to be very similar to the Mo2 molecules, the U2Me6 molecules were shown to have catastrophic differences compared to the Mo2Me6 molecule. The dissimilarities were: 1) The potential for metal-metal bonding in the actinide molecule is increased for the a, it, and more importantly the S levels. 2) The positioning of the 5ffi level is catastrophic to the formation of U2Me6. 3) There is a small HOMO/LUMO gap in U2Me6 compared to Mo2Mes. 4) 5f orbitals play an important role in M-L and M-M bond formation. As a result of this study, it is easy to see why alkylation of the UC14 molecule would result in reduction to uranium metal and formation of organic compounds and hydrogen. Recently, a bulky UR3 compound was prepared. The stability of this compound can be explained by either an agostic interaction or by the prevention of close U-U contact by the bulky groups. This portion of the study suggests that the position of the f- orbitals relative to the M-L bonding levels is very important for two reasons: 1) the M-M bonding interactions can end up lower in energy than the M-L bonding interactions. 2) the HOMO/LUMO gap can be very small and partial occupation of orbitals can present problems. The first of these can be overcome by choosing a ligand that can provide a 224 much lower metal-ligand orbital energy. This is generally achieved by forming a more covalent interaction between the metal and the ligand. The second condition can be overcome by choosing the proper number of ligands and oxidation state such that the HOMO/LUMO is maximized, or by minimizing the problems associated with a small gap. A way to make sure that the M-L bonding interactions lie well below the HOMO is to make sure all the M-M bonding levels are occupied. In a simple scheme employing acetylene, Mo2H6, and U2H6, it can be shown that the U-U case is not so similar to the other two cases. In each of these three molecules we would have predicted a triple bond between the central atoms.In these three cases we would have 4 (s + 3p), 6 (s + 5d), and 8 (s + 7f) total orbitals on each central atom. Of these orbitals a total of 1, 3, and 3 would be used for M-L bonding in each molecule. This would leave 3, 3, and 5 available for M-M bonding in the three cases. Clearly the U2Me6 molecule does not fit this scheme and shows why the 8 interactions arise due to the presence of 5f levels. The studies presented in Chapter IV demonstrate that 5f and 6d mixing occur and make the picture more complex. According to this simple scheme the f-element analog would be Pu2H10 with 8 total orbitals, 5 used by the ligands, and 3 left over for Pu-Pu bonding. The three orbitals between the two centers should consist of a a and two n/6 levels. Also, the amount of 8 character should increase in going from p to f orbitals if the geometries for the three (HCCH, Mo2H6, and Pu2H10) cases are linear, D3d bipyramidal, and D4h bioctahedral. 225 While this scheme would predict a metal-metal interaction for Pu2Me10, it should be pointed out that the extension of the 5f levels decreases in going from Th to Pu. Another approach may be to take advantage of the similarities between Th and Zr. The studies presented comparing the M2Me6 molecules where M - Zr, Mo, Ru or Th, U, Pu, have presented good evidence of the variability of the importance of 6d and 5f contribution to the electronic structures of the molecules. The Zr, Mo, and Ru series shows a a 2 single, o2jr4 triple, and c^ttV*4 single bond configuration between the metal centers. These simple molecules are seen to follow a straightforward metal-metal manifold description. The actinide series is shown to be very different from this in that 6d and 5f importance changes with the actinide metal. The observations presented are supportive of the notion that thorium has 6d orbitals lower in energy than the 5f orbitals while plutonium is the opposite. This demonstrates that thorium behaves like a transition metal with 5f orbitals available at higher energy that can help to stabilize some ligand orbitals and aid to a small degree with M-L binding. They also will provide a smaller HOMO/LUMO gap. Plutonium on the other hand uses its 5f orbitals preferentially to the 6d levels. These two cases lead to a single and quintuple bond order in the Th and Pu cases, respectively. The uranium case is seen to be the real oddity in this set, since it can readily form 6d and 5f hybrids of a and n type orbitals that can interact better with the ligands or increase metal- metal bonding overlap. 226 The studies of the Cp6An2 (An - Th, U) molecules do not show any clear reason why dimers of this nature have not been prepared. It may be possible that a preference of planar over a bent structure and an ionic repulsion between the two metal centers may preclude the formation of a metal-metal bond. It seems possible that the preparation of either a UCp3 analogue where the Cp type ligands are tied back may allow a metal-metal contact. Another approach may be to use fulvalene or a related ligand to essentially hold the two actinides in close proximity. The ideal situation may be to encase two actinide atoms inside three fulvalene ligands, protecting the metals from outside influences as well as holding the atoms in close proximity. The preparation of this compound could be beset with difficulties such as polymerization. The electronic structures of [Cp2ML]2 (M - Zr, Th, or U, and L = Met, yii-He, or /i-Cl) have also been studied. These molecules have demonstrated some results that tend to support the experimental observations and others that stand in contrast. The electronic structures of the chloro bridged cases are very similar and by themselves yield no reasons why the actinides would not adopt the chloro bridged structure. The three metals studied are shown to behave very similarly. In the case of thorium, the 5f atomic orbitals are much less important than the metal 6d atomic orbitals, and in uranium the metal 5f and 6d mix effectively to form hybrid orbitals. The study involving the Met or fi-Me molecules of zirconium tends to favor the formation of a metal-metal bond and a Met configuration. This is in agreement with the experimental work performed by Cuenca 227 and Royo as well as the recent work by Casey et al. The electronic structure also indicates a metal-metal bonding situation that does not appear very stable. The studies on the thorium analogues of this system seem to not favor either situation strongly and a prudent choice of ligands may be able to force the formation of one conformation over the other. This, of course, depends on whether the molecules can be made at all. Th(III) is a very difficult oxidation state to achieve and probably would undergo disproportionation. It seems likely that if it could be prepared, the fulvalene bridged dimer would behave similarly to the analogous zirconium compound. It would have the same stability problems relating to the metal-metal bond. The studies of the uranium analogues are the ones that stand in direct contradiction to the experimental work of Anderson et al. It is possible that the bulkiness of their Cp' ligands may force the bridging U'‘’U case over the terminal U-U case. This is another situation where some work with bridging fulvalenes may be able to force a metal-metal bonding situation. The calculations on the hypothetical alkoxide dimers of U(V) yield some interesting results. 1) The near>linearity of the U-OR angle compared to the bent Mo-O-R can be explained in terms of hybridization of the oxygen atoms and a simple orbital counting scheme. 2) The lack of a metal-metal bond is partially due to the destabilization of all but an M-L 8 type orbital on the uranium atoms. 3) Further, the near octahedral environment about each uranium atom provides for a ligand directed hybridization of the metal atomic 228 orbitals, which serves to reduce the potential for metal-metal a and 7r interactions. 4) On adopting a D2h geometry, the molecule even further adopts a local octahedral environment and allows two additional alkoxide orbitals to interact with the metals, a situation that favors this geometry over the D4h case. For these reasons, the molybdenum and the uranium alkoxides are structurally different from each other. These differences can therefore be explained in terms of 5f interactions under a ligand produced octahedral environment. Also, the relative importance of ligand ir to the formation of the octahedral environment is much greater than the a interactions. The jt interactions are able to interact effectively with the metal 5f orbitals and destabilize them by a sizeable amount. This type of effect has not been observed to this extent in any of the other molecules studied. The only case that comes close, is the Cp6U2 case where one of the level of the ligand set. In conclusion, a great deal has been learned about the potential for interaction between two actinide centers. These studies have demonstrated that the metal-metal interactions of f-elements may be stronger than for their transition metal analogues, owing to their additional z dependance in f atomic orbitals. For this reason, a prudent choice of ligands may allow the formation of a metal-metal bond. The studies have shown that U2Me6 should not exist as a stable molecule and why U2(OR)10 molecules do not fora a metal-metal bond. The considerations that must be included in the potential for forming a metal-metal bond are: 1) the ligands must not be too bulky or they will cause steric problems. 2) The oxidation state must be low or the metal orbitals will contract and prevent the formation of a metal-metal bond. 3) The earlier actinides should present the best overlap. 4) The metal-ligand levels must be low enough in energy to prevent decomposition. These studies have provided three potential methods for producing a metal-metal interaction: 1) It is apparent that dimers of Th(III) should be very similar to those of Zr(III) and may be able to form a weak metal-metal a interaction. 2) A Cp3 ligand set which are tied back may allow Th-Th or U-U bonding. 3) A template such as fulvalene may force two actinides into close proximity and allow bonding. Some basic research that would allow more accurate assessment of these potentials would include 1) publication of the spectroscopic studies of U2 including the U-U distance, and 2) the application of a better molecular orbital method. Such a method may be the DV-Xa method. Early indications from studies using this method indicate that the importance of 5f levels in M-L interactions may be overemphasized in the Xa-SW method. APPENDIX A DATA RELEVANT TO CHAPTER III 230 a Values H 0.77725 C 0.75928 0 0.74447 I 0.70000 Zr 0.70424 Mo 0.70341 Ru 0.70253 Th 0.69200 U 0.69200 Pu 0.69200 232 Formula: Cp2Zr(I)RuCp(C0)2 Library Name: ZRRU Symmetry: Cs Sphere Radii in A: Out - 5.27175 Zr - 1.67046 Ru - 1.24231 I - 1.78073 C2 - 0.87913 C3 - 0.87911 C4 - 0.87913 C5 - 0.87912 C6 - 0.87454 C7 - 0.87430 C8 - 0.87386 C - 0.86158 0 - 0.87732 HI - 0.64600 H2 - 0.64598 H3 - 0.64557 H4 - 0.64583 H5 - 0.64602 H6 - 0.64556 H7 - 0.64518 H8 - 0.64489 Bond Lengths in A: Zr-Ru - 2.910 Zr-Cp - 2.274 Zr-I - 2.851 C-H - 1.00 C-C - 1.35 C-0 - 1.153 Ru-Cp - 1.943 Ru-CO - 1.84 Bond Angles in Degrees: Zr-Ru-CO - 81.4 Zr-Ru-Cp - 123.5 Cp-Zr-Cp - 125.8 Cp-Zr-I - 107.8 I-Zr-Ru - 98.6 233 Formula: Cp2Th(I)RuCp(C0)2 Library Name: THRU Symmetry: Cs Sphere Radii in A: Out — 5.4287 Th - 1.7400 Ru - 1.2526 I - 1.7769 Cl - 0.8940 C2 - 0.8940 C3 - 0.8939 C4 - 0.8938 C5 - 0.8937 C6 - 0.8916 C7 - 0.8924 C8 - 0.8925 C 0.9106 0 - 0.9239 HI - 0.6458 H2 - 0.6459 H3 - 0.6456 H4 - 0.6442 H5 - 0.6441 H6 - 0.6441 H7 - 0.6465 H8 - 0.6466 Bond Lengths in A: Th-Ru - 3.03 Th-C - 2.82 Th-I - 3.04 C-H - 1.00 C-C - 1.40 Ru-C - 2.29 Ru-CO - 1.86 C-0 - 1.27 Bond Angles in Degrees: Th-Ru-CO - 85.5 Th-Ru-Cp - 118.4 Cp-Th-Ru - 107 I-Th-Ru - 95 CO-Ru-CO - 88.3 234 Formula: Cp3ThRuCp(CO)2 Library Name: TTHRU Symmetry: CB Sphere Radii in A: Out - 5.1045 Th - 1.6834 Ru - 1.2508 Cl m m 0.8900 C2 - 0.8898 C3 - 0.8897 C4 - 0.8900 C5 - 0.8900 C6 - 0.8900 C7 - 0.8891 C8 - 0.8891 CA - 0.8910 CB - 0.8924 CC - 0.8927 C - 0.9095 0 - 0.9227 HI - 0.6453 H2 HI 0.6450 H3 - 0.6448 H4 - 0.6454 H5 - 0.6454 H6 - 0.6454 H7 - 0.6418 H8 - 0.6421 HA - 0.6415 Hfi - 0.6465 HC - 0.6462 Bond Lengths in A: Th-Ru - 2.98 1 Th-( - 2.79 C-H - 1.00 C-C - 1.39 Ru-C - 2.305 Ru-CO - 1.85 C-0 - 1.27 C-C - 1.40 C-H - 1.00 Bond Angles in Degrees: Th-Ru-CO - 85.5 Th-Ru-Cp - 118.4 Cp-Th-Ru - 100 235 Formula: Cp3URuCp(CO)2 Library Name: TURU Symmetry: Cs Sphere Radii in A: Out - 4.3648 U mm 1.6469 Ru - 1.2456 Cl - 0.8905 C2 - 0.8902 C3 - 0.8898 C4 - 0.8906 C5 - 0.8906 C6 mm 0.8906 C7 - 0.8890 C8 - 0.8889 CA - 0.8897 CB - 0.8920 CC - 0.8920 C - 0.9083 0 - 0.9213 HI - 0.6453 H2 - 0.6447 H3 - 0.6442 H4 - 0.6455 H5 - 0.6455 H6 - 0.6455 H7 - 0.6404 H8 - 0.6407 HA - 0.6384 HB - 0.6467 HC - 0.6462 Bond Lengths in A: U-Ru - 2.75 U-C - 2.79 C-H - 1.00 C-C - 1.39 Ru-C - 2.30 Ru-CO - 1.85 C-0 - 1.27 C-C - 1.40 C-H - 1.00 Bond Angles in Degrees: U-Ru-CO - 85.5 U-Ru-Cp - 118.4 Cp-U-Ru - 100 APPENDIX B DATA RELEVANT TO CHAPTER IV 236 237 Formula: U2Me6 Library Name: U2M2 Symmetry: D3d Sphere Radii in A: OUT - 4.4554 U - 1.6032 C - 0.9650 HI - 0.6772 H2 - 0.6779 Bond Lengths in A: U-U - 2.40 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 238 Formula: U2Me6 Library Name: U2M2 Symmetry: D3d Sphere Radii in A: OUT - 4.4554 U - 1.6032 C - 0.9650 HI - 0.6772 H2 - 0.6779 Bond Lengths in A: U-U - 2.40 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 239 Formula: U2Me6 Library Name: U2M6 Symmetry: D3d Sphere Radii in A: OUT - 4.5213 U - 1.6860 C - 0.9653 HI - 0.6774 H2 - 0.6780 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 240 Formula: U2Me6 Library Name: U2M690 Symmetry: D3d Sphere Radii in A: OUT - 4.2587 U - 1.6836 C - 0.9641 HI - 0.6757 H2 - 0.6778 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 90 U-C-H - 109.5 241 Formula: U2Me6 Library Name: U2M6B Symmetry: D3d Sphere Radii in A: OUT - 4.9057 U - 1.6866 C - 0.9653 HI - 0.6779 H2 - 0.6770 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 135 U-C-H - 109.5 Formula: U2Me4 Library Name: U2M4A Symmetry: D2d Sphere Radii in A: OUT - 4.2587 U - 1.7129 C - 0.9642 HI - 0.6759 H2 - 0.6778 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 90 U-C-H - 109.5 243 Formula: U2Me4 Library Name: U2M4C Symmetry: D2d Sphere Radii in A: OUT - 4.9067 U - 1.7153 C - 0.9659 HI - 0.6779 H2 - 0.6780 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 135 U-C-H - 109.5 244 Formula: U2Me4 Library Name: U2M4D Symmetry: D2a Sphere Radii in A: OUT - 4.9065 U - 1.7525 C - 0.9648 HI - 0.6778 H2 - 0.6778 Bond Lengths in A: U-U - 2.6 U-C - 2.6 C-H - 1.08 Bond Angles in Degrees: U-U-C - 135 U-C-H - 109.5 245 Formula: U2Me8 Library Name: U2M8 Symmetry: D4h Sphere Radii in A: OUT - 4.5211 U - 1.6611 C - 0.9651 HI - 0.6767 H2 - 0.6778 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 246 Formula: U2Me8 Library Name: U2ME8 Symmetry: D4h Sphere Radii in A: OUT - 4.4552 U - 1.5874 C - 0.9647 HI - 0.6757 H2 - 0.6777 Bond Lengths in A: U-U - 2.40 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 APPENDIX C DATA RELEVANT TO CHAPTER V 247 248 Formula: Zr2Me6 Library Name: ZR2ME6 Symmetry: D3d Sphere Radii in A: OUT - 4.4655 Zr - 1.6218 C - 0.9677 HI - 0.6823 H2 - 0.6826 Bond Lengths in A: Zr-Zr - 2.90 Zr-C - 2.30 C-H - 1.08 Bond Angles in Degrees: Zr-Zr-C - 105.0 Zr-C-H - 109.5 249 Formula: Mo2Me6 Library Name: Mo2ME6 Symmetry: D3d Sphere Radii in A: OUT - 3.8835 Mo - 1.3377 C - 0.9270 HI - 0.6414 H2 - 0.6422 Bond Lengths in A: Mo-Mo - 2.17 Mo-C - 2.13 C-H - 1.08 Bond Angles in Degrees: Mo-Mo-C - 105 Mo-C-H - 109.5 Formula: Ru2Me6 Library Name: Ru2ME6 Symmetry ’ D3d Sphere Radii in A: OUT- 4.0140 Ru - 1.3074 C - 0.9537 HI - 0.6819 H2 - 0.6822 Bond Lengths in A: Ru-Ru - 2.31 Ru-C - 1.83 C-H - 1.08 Bond Angles in Degrees Ru-Ru-C - 109.3 Ru-C-H - 109.5 251 Formula: Th2Me6 Library Name: Th2ME6 Symmetry : D3d Sphere Radii in A: OUT - 2.8395 Th - 1.8373 C - 0.9641 HI - 0.6777 H2 - 0.6779 Bond Lengths in A: Th-Th - 3.30 Th-C - 2.60 C-H - 1.08 Bond Angles in Degrees: Th-Th-C - 105 Th-C-H - 109.5 252 Formula: U2Me6 Library Name: U2ME6 Symmetry : ®3d Sphere Radii in A: OUT - 4.5213 U - 1.6860 C - 0.9653 HI - 0.6774 H2 - 0.6780 Bond Lengths in A: U-U - 2.60 U-C - 2.60 C-H - 1.08 Bond Angles in Degrees: U-U-C - 105 U-C-H - 109.5 Formula: Pu2Me6 Library Name: Pu2ME6 Symmetry: D3d Sphere Radii in A: OUT - 4.5215 Pu - 1.6727 C - 0.9674 HI - 0.6776 H2 - 0.6782 Bond Lengths in A: Pu-Pu - 2.60 Pu-G - 2.60 C-H - 1.08 Bond Angles in Degrees Pu-Pu-C - 105 Pu-C-H - 109.5 APPENDIX D DATA RELEVANT TO CHAPTER VI 254 Formula: Th2Cp6 Library Name: TTH2 Symmetry: D3d Sphere Radii in A: OUT - 5.2353 Th - 1.6812 Cl - 0.8893 C2 - 0.8900 C3 - 0.8884 HI - 0.6432 H2 - 0.6454 H3 - 0.6386 Bond Lengths in A: Th-Th - 3.30 Th-C - 2.79 C-C - 1.39 C-H - 1.00 Bond Angles in Degrees Th-Th-Cp - 100 Formula: U2Cp6 Library Name: TU2 Symmetry: D3d Sphere Radii in A: OUT - 5.0458 U - 1.6310 Cl - 0.8886 C2 - 0.8905 C3 - 0.8868 HI - 0.6389 H2 - 0.6454 H3 - 0.6294 Bond Lengths in A: U-U - 2.84 U-C - 2.79 C-C - 1.39 C-H - 1.00 Bond Angles in Degrees; U-U-Cp - 100 Formula: U2Cp6 Library Name: TU26 Symmetry: D3d Sphere Radii in A: OUT - 4.9455 U - 1.5894 Cl - 0.8873 C2 - 0.8904 C3 - 0.8846 HI - 0.6345 H2 - 0.6454 H3 - 0.6222 Bond Lengths in A: U-U - 2.60 U-C - 2.79 C-C - 1.39 C-H - 1.00 Bond Angles in Degrees U-U-Cp - 100 APPENDIX E DATA RELEVANT TO CHAPTER VII 258 259 Formula: U2H10 Library Name: UHA Symmetry: DAh Sphere Radii in A: OUT - 4.0327 U - 1.5705 HI - 0.8127 H2 - 0.8087 Bond Lengths in A: U-U - 2.84 U-H - 1.90 Bond Angles in Degrees: U-U-H2 - 90 U-U-Hl - 180 260 Formula: U2H10 Library Name: UHB Symmetry : D2d Sphere Radii in A: OUT - 4.2273 U - 1.6161 HI - 0.8133 H2 - 0.8125 H3 - 0.8377 Bond Lengths in A: U-U - 3.79 U-Hl - 1.90 U-H2 - 1.90 U-H3 - 2.10 Bond Angles in Degrees: U-U-H2 - 90 U-H3-U - 129 U-U-Hl - 135 Formula: U2H10 Library Name: UHC Symmetry : °2d Sphere Radii in A: OUT - 3.7909 U - 1.5760 HI - 0.8125 H2 - 0.8087 H3 - 0.8450 Bond Lengths in A: U-U - 2.84 U-Hl - 1.90 U-H2 - 1.90 U-H3 - 1.90 Bond Angles in Degrees: U-U-H2 - 90 U-H3-U - 85.1 U-U-Hl - 135 Formula: U2OH10 Library Name: U20H6 Symmetry: D*h Sphere Radii in A: OUT - 5.0603 U - 1.4084 01 - 0.9438 02 - 0.9470 HI - 0.6068 H2 - 0.6072 Bond Lengths in A: U-U - 2.84 U-0 - 2.03 O-H - 1.00 Bond Angles in Degrees U-U-02 - 90 U-U-01 - 180 U-O-H - 180 Formula: U2OH10 Library Name: U20H10B Symmetry: D2h Sphere Radii in A: OUT - 5.1798 U - 1.4299 01 - 0.9467 02 - 0.9473 03 - 0.9476 HI - 0.6199 H2 - 0.6071 H3 - 0.6047 Bond Lengths in A: U-U - 3.79 U-01 - 2.03 U-02 - 2.03 U-03 - 2.28 0-H - 1.00 Bond Angles in Degrees: U-U-01 - 90 U-U-02 - 135 U-03-U - 112 U-01-H - 180 U-02-H - 180 U-03-H - 124 264 Formula: U2OH10 Library Name: U20H10A Symmetry: D2d Sphere Radii in A: OUT - 4.8096 U - 1.4211 01 - 0.9444 02 - 0.9468 03 - 0.9482 HI - 0.6069 H2 - 0.6071 H3 - 0.6057 Bond Lengths in A: U-U - 2.84 U-01 - 2.03 U-02 - 2.03 U-03 - 2.28 0-H - 1.00 Bond Angles in Degrees: U-U-01 - 90 U-U-02 - 135 U-03-U - 80.1 U-01-H - 180 U-02-H - 180 U-03-H - 140 LIST OF REFERENCES 1. Freeman, A. J.; Keller, C., Eds.; Handbook on the Physics and Chemistry of the Actinides; Vol. 3, Amsterdam: North-Holland, 1985. 2. Katz, J. J.; Seaborg, G. T.; Morss, L. R . , Eds.; The Chemistry of the Actinide Elements; 2nd ed., Vol. 1, London: Chapman and Hall Ltd., 1986. 3. Katz, J. J . ; Seaborg, G. T . ; Morss, L. R . , Eds.; The Chemistry of the Actinide Elements; 2nd ed., Vol. 2, London: Chapman and Hall Ltd., 1986. 4. Fournier, J. M . ; Manes, L. "Actinide Solids: 5f Dependence of Physical Properties" In Structure and Bonding: Actinides - Chemistry and Physical Properties; Manes, L . , Ed.; Berlin: Springer-Verlag, 1985, Vols. 59-60, p 1. 5. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 4th ed., New York: John Wiley & Sons, 1980. 6. Manes, L.; Benedict, U. "Structural and Thermodynamic Properties of Actinide Solids and Their Relation to Bonding" In Structure and Bonding: Actinides - Chemistry and Physical Properties; Manes, L . , Ed.; Berlin: Springer-Verlag, 1985, Vols. 59-60, p 75. 7. Fournier, J. M. "Magnetic Properties of Actinide Solids" In Structure and Bonding: Actinides - Chemistry and Physical Properties; Manes, L . , Ed.; Berlin: Springer-Verlag, 1985, Vols. 59-60, p 127. 8. Fuggle, J.C.; Hillebrecht, F. U . ; Zolnierek, Z.; Bennett, P. A.; Freiberg, Ch. Phys. Rev. B 1983, 27, 2145. 9. Baer, Y. ; Lang, J. K. Phys. Rev. B 1980, 21, 2060. 10. Skriver, H. L.; Jan, J.-P. Phys. Rev. B 1980, 21, 1489. 11. Naegele, J. R . ; Ghijsen, J. "Localization and Hybridization of 5 f States in the Metallic and Ionic Bond as Investigated by Photoelectron Spectroscopy" In Structure and Bonding: Actinides - Chemistry and Physical Properties; Manes, L . , Ed.; Berlin: Springer-Verlag, 1985, Vols. 59-60, p 197. 265 266 12. Grohs, H.; Hoechst, H . ; Steiner, P.; Huefner, S.; Buschow, K.H.J. Solid State Commun. 1980, 33, 573. 13. Greuter, F . ; Hauser, E.; Oelhafen, P.; Guentherodt, H.J.; Reihl, B.; Vogt, 0. Physica 1980, 102B, 117. 14. Iwan, M . ; Koch, E. E.; Himpsel, F.-J. Phys. Rev. B 1981, 24, 613. 15. Brooks, M. S. S. "The Theory of 5 f Bonding in Actinide Solids" In Structure and Bonding: Actinides - Chemistry and Physical Properties; Manes, L . , Ed.; Berlin: Springer-Verlag, 1985, Vols. 59-60, p 263. 16. Ernst, R. D.; Marks, T. J. J. Organomet. Chem. 318, 29. 17. Marks, T. J.; Ernst, R. D. "Scandium, Ytrrium and the Lanthanides and Actinides", in Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G.; Abel, E. W . , Eds.; Pergamon Press, Oxford, England, 1982. 18. Lyle, S. J. Annual Reports on the Progress of Chemistry 1981, 78, 299. 19. Lyle, S. J. Annual Reports on the Progress of Chemistry 1982, 79, 359. 20. Marks, T. J. J. Organomet. Chem. 1982, 227, 317. 21. Miller, J. D. Annual Reports on the Progress of Chemistry 1984, 81, 325. 22. Miller, J. D. Annual Reports on the Progress of Chemistry 1985, 82, 347. 23. Cotton, F. A.; Schwotzer, W. Organometallics 1985, 4, 942. 24. Sheline, R. K . ; Slater, J. L. Angew. Chem. Intl. Ed. Engl. 1975, 14, 309. 25. Brennan, J. G.; Andersen, R. A.; Robbins, J. L. J. Am. Chem. Soc. 1986, 108, 335. 26. Bursten, B. E.; Strittmatter, R. J. J. Am. Chem. Soc. 1987, 109, 6606. 27. Kanellakopulos, B . ; Fischer, E 0.; Dornberger, E.; Baumgartner, F. J. Organomet. Chem. 1970, 24, 507. 28. Kanellakopulos, B.; Dornberger, E.; Baumgartner, F. Inorg. Nucl. Chem. Lett. 1974, 10, 155. 267 29. Zanella, P.; Rossetto, G.; De Paoli, G.; Traverso, 0. Inorg. Chim. Acta 1980, 44, 1,155. 30. Laubereau, P.G.; Burns, J.H. Inorg. Chem. 1970, 9, 1091. 31. Laubereau, P.G.; Burns, J. Inorg. Nucl. Chem. Lett. 1970, 6, 59. 32. Baumgartner, F . ; Fischer, E.O.; Laubereau, P. Radiochim. Acta 1967, 7, 188. 33. Fischer, E.O.; Fischer, H. J. Organomet. Chem. 1966, 6, 141. 34. Fischer, E.O.; Fischer, H. Angew. Chem. 1964, 76, 52. 35. Baumgartner, F.; Fischer, E. 0.; Kanellakopulos, B . ; Laubereau, P. Angew. Chem. 1965, 77, 866. 36. Baumgartner, F.; Fischer, E. 0.; Kanellakopulos, B.; Laubereau, P. Angew. Chem. Int. Ed. Engl. 1966, 5, 134. 37. Baumgartner, F.; Fischer, E.O.; Billich, H . ; Dornberger, E.; Kanellakopulos, B.; Roth, W . ; Stieglitz L. J. Organomet. Chem. 1970, 22, C17. 38. Nugent, L. J.; Laubereau, P.G.; Werner, G.K.; Vander Sluis, K.L. J. Organomet. Chem. 1971, 27, 365. 39. Blake, P. C.; Lappert, M. F.; Atwood, J. L . ; Zhang, H. J. Chem. Soc., Chem. Commtm. 1986, 1148. 40. Van Der Sluys, W. G . ; Burns, C. J.; Sattelberger, A. P. Organometallics 1989, 8, 855. 41. Burns, J. H.; Baldwin, W. H. J. Organomet. Chem. 1976, 120, 361. 42. Maginn, R.E.; Manastyrsky j, S.; Dubeck, M.J. Am. Chem. Soc. 1963, 85, 672. 43. Laubereau, P. G. Inorg. Nucl. Chem. Lett. 1970, 6, 611. 44. Dornberger, E.; Klenze, R.; Kanellakopulos, B. Inorg. Nucl. Chem. Lett. 1978, 14, 319. 45. Manriquez, J.M.; Fagan, P.J.; Marks, T.J.; Vollmer, S.H.; Day, C.S.; Day, V.W. J. Am. Chem. Soc. 1979, 101, 5075. 46. Fagan, P.J.; Manriquez, J.M.; Marks, T.J.; Day, C.S.; Wollmer, S.H.; Day, V.W. Organometallics 1982, 1, 170. 47. Blake, P. C.; Lappert, M. F.; Taylor, R. G . ; Atwood, J. L . ; Hunter, W. E . ; Zhang, H. J. Chem. Soc., Chem. Commun. 1986, 1394. 268 48. Anderson, R. A., personal communication. 49. Manastyrsky j, S.; Maginn, R E.; Dubeck, M. Inorg. Chem. 1963, 2, 904. 50. Day, C.S.; Day, V. W . ; Ernst, R. D.; Vollmer, S. H. Organometallics 1982, 17, 998. 51. Mares, F.; Hodgson, K . ; Streitwieser, A. Jr. J. Organomet. Chem. 1970, 24, C68. 52. Hodgson, K. 0.; Mares, F.; Starks, D. F . ; Streitwieser, A. Jr. J. Am. Chem. Soc. 1973, 95, 8650. 53. Streitwieser, A. Jr.; Miiller-Westerhoff, U. J. Am. Chem Soc. 1968, 90, 7364. 54. Streitwieser, A. Jr. in Organometallics of the f-Elements; T. J. Marks; R. D. Fischer, Eds.; Reidel, Dordrecht, 1979, chap. 5. 55. Streitwieser, A. Jr.; Yoshlda, N. J. Am. Chem. Soc. 1969, 91, 7528. 56. Westerhoff, A.; De Liefde Meijer, H. J. J. Organomet. Chem. 1976, 116, 319. 57. Hayes, R.G.; Edelstein, N. J. Am. Chem. Soc. 1972, 94, 8688. 58. Warren, K. D. Struct. Bonding (Berlin) 1977, 33, 97. 59. Rdsch, N . ; Streitwieser, A. Jr. J. Organomet. Chem. 1978, 145, 195. 60. Warren, K. D. Inorg. Chem. 1975, 14, 3095. 61. Clark, J. P.; Green, J. C. J. C. S. Dalton 1977, 505. 62. Fragala, I.; Condorelli, G.; Zanella, P.; Tondello, E. J. Organomet. Chem. 1976, 122, 357. 63. Kalsotra, B. L . ; Multani, R. K . ; Jain, B. D. Isr. J. Chem. 1971, 9, 569. 64. Kalsotra, B. L.; Anand, S. P.; Multani, R. K.; Jain, B. D. J. Organomet. Chem. 1971, 28, 87. 65. Kalsotra, B. L.; Multani, R. K . ; Jain, B. D. J. Inorg. Nucl. Chem. 1972, 34, 2679. 66. Fischer, E. 0.; Treiber, A. Z. Naturforsch., Teil B 1962, 17, 276. 269 67. Fischer, E. 0.; Hristidu, Y. Z. Naturforsch, Teil B 1962, 27, 275. 68. Kdhler, E . ; Bruser, W . ; Thiele, K.-H. J. Organomet. Chem. 1974, 76, 235. 69. Reynolds, L. T.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 246. 70. Green, J. C.; Kelly, M. R . ; Long, J. A.; Kanellakopulos, B . ; Yarrow, P. I. W. J. Organomet. Chem. 1981, 212, 329. 71. Fragala, I. Ciliberto, E . ; Fischer, R. D . ; Sienel, G. R . ; Zanella, P. J. Organomet. Chem. 1976, 120, C9. 72. Bursten, B. E.; Ozin, G. A. Inorg. Chem. 1984, 23, 2910. 73. Schumann, H . ,' MGller, J. Angew. Chem., Int. Ed. Engl. 1978, 27, 276. 74. Marks, T.J.; Seyam, A.M. J. Organomet. Chem. 1974, 67, 61. 75. Sigurdson, E.R.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1977, 812. 76. Cramer, R. E.; Mori, A. L . ; Maynard, R. B . ; Gilje, J. W . ; Tatsumi, K . ; Nakamura, A. J. Am. Chem. Soc. 1984, 106, 5920. 77. Lauke, H . ; Swepston, P. J.; Marks, T. J. J. Am. Chem. Soc. 1984, 6841. 78. Cotton, F. A. Acc. Chem. Res. 1978, 22, 225. 79. Cotton, F. A.; Marler, D. 0.; Schwotzer, W. Inorg. Chim. Acta 1984, 85, L31. 80. Cotton, F. A.; Marler, D. 0.; Schwotzer, W. Inorg. Chem. 1984, 23, 4211. 81. Reid, A. F . ; Wailes, P. C. Inorg. Chem. 1966, 5, 1213. 82. Berton, A.; Porchin, M . ; Rossetto, G.; Zanella, P. J. Organometallic Chem. 1986, 302, 351. 83. Paolucci, G . ; Zanella, P.; Berton, A. J. Organomet. Chem. 1985, 295, 317. 84. Manriquez, J. M . ; Fagan, P. J.; Marks, T. J. J. Am. Chem. Soc. 1978, 100, 3939. 270 85. Fagan, P. J.; Manriquez, J. M . ; Marks, T. J. in Organometallics of the f-Elements; Marks, T. J.; Fischer, R. D. Eds.; Reidel: Dordrecht, 1979, chap. 4. 86. Fagan, P. J.; Manriquez, J. M . ; Maatta, E. A.; Seyam, A. M . ; Marks, T. J.J. Am. Chem. Soc. 1981, 103, 6650. 87. Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 23, 121. 88. Cooper, R. L.; Green, M. L. H. J. Chem. Soc. A 1967, 1155. 89. Casey, C. P.; Jordon, R. F.; Rheingold, A. L. J. Am. Chem. Soc. 1983, 105, 665. 90. Casey, C. P.; Jordon, R. F . ; Rheingold, A. L. Organometallics 1984, 3, 504. 91. Sternal, R. S.; Brock, C. P.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8270. 92. Mattausch, H j .; Hendricks, J. B.; Eger, R . ; Corbett, J. D.; Simon, A. Inorg. Chem. 1980, 19, 2128. 93. Marianelli, R. S.; Durney. M. T. J. Organomet. Chem. 1971, 32, C41. 94. Crease, A. E.; Legzdins, P. J. Chem. Soc., Chem. Commun. 1972, 268. 95. Crease, A. E . ; Legzdins, P. J. Chem. Soc., Dalton Trans. 1973, 1501. 96. Tilley, T. D . ; Andersen, R. A. J. Am. Chem. Soc. 1982, 104, 1772. 97. Bennett, R.L., Bruce, M.I.; Stone, F.G.A. J. Organomet. Chem. 1971, 26, 355. 98. Ritchey, J. M . ; Zozulin, A. J . ; Wrobleski, D. A.; Ryan, R. R. ; Wasserman, H. J . ; Moody, D. C . ; Paine, R. T. J. Am. Chem. Soc. 1985, 107, 501. 99. Hay, P. J.; Ryan, R. R . ; Salazar, K. V.; Wrobleski, D. A.; Sattelberger, A. P. J. Am. Chem. Soc. 1986, 108, 313-315. 100. Bursten, B. E.; Novo-Gradac, K. J. J. Am. Chem. Soc. 1987, 109, 904. 101. Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Wiley-Interscience: New York, 1982. 271 102. Bursten, B. E.; Cotton, F. A.; Hall, M. B. J. Am. Chem. Soc. 1980, 102, 6348. 103. Riley, S., Argonne National Laboratory, private communication. 104. Cuenca, T . ; Royo, P. Journal of Organometallic Chemistry 1985, 295, 159. 105. Pitzer, K. S. Acc. Chem. Res. 1979, 12, 271. 106. Pyykkd, P.; Desclaux, J.-P. Acc. Chem. Res. 1979, 12, 276. 107. Hohl, D . ; Rdsch, N. Inorg. Chem. 1986, 25, 2711. 108. Slater, J. C. Quantum Theory of Molecules and Solids. The Self Consistant Field for Molecules and Solids; New York: McGraw-Hill, 1974; Vol. 4. 109. Johnson, K. H. Ann. Rev. Phys. Chem. 1975, 26, 39. 110. Case, D. A. Ann. Rev. Phys. Chem. 1982, 33, 151. 111. Slater, J. C. Solid State and Molecular Theory: A Scientific Biography; New York: Wiley, 1975. 112. Wood, J. H . ; Boring, A. M. Phys, Rev. B 1978, 18, 2701. 113. Yang, C. Y. J. Chem. Phys. 1978, 68, 2626. 114. Yang, C. Y . ; Case, D. A. in Local Density Approximations in Quantum Chemistry and Solid State Physics; Dahl, J. P.; Avery, J. Eds.; New York: Plenum Press, 1984, p. 684. 115. Case, D. A.; Yang, C. Y. J. Chem. Phys. 1980, 72, 3443. 116. Amberger, H.-D.; Fischer, R. D . ; Kanellakopulos, B. Z. Naturf. B 1976, 31, 12. 117. Hohl, D.; Ellis, D. E . ; Rdsch, N. Inorganica Chimica Acta 1987, 127, 195. 118. Tatsumi, K.; Nakamura, A. J. Organomet. Chem. 1984, 272, 141. 119. Pyykkd P.; Lohr, L. L. Jr. Inorg. Chem. 1981, 20, 1950. 120. Wadt, W. R . ; Hay, P. J. J. Am. Chem. Soc. 1979, 101, 5198. 121. Beach, D. B.; Bomben, K. D . ; Edelstein, N. M . ; Eisenberg, D C . ; Jolly, W. L . ; Shinomoto, R . ; Streitwieser, A. Jr. Inorg. Chem. 1986, 25, 1735. 272 122. Spitsyn, V. I.; Ionova, G. V. Russian Chemical Reviews 1984, 53, 725. 123. Boring, M.; Hoskowltz, J. W. Chem. Phys. Lett. 1976, 38, 185. 124. Dyke, J. M . ; Fayad, N. K . ; Morris, A.; Trickle, I. R. ; Allen, G. C. J. Chem. Phys. 1980, 72, 3822. 125. Maylotte, D. H . ; St. Peters, R. L . ; Messmer, R. P. Chem. Phys. Lett. 1976, 38, 181. 126. Bursten, B. E.; Fang, A. J. Am. Chem. Soc. 1983, 105, 6495. 127. Ciliberto, E.; Condorelli, G . ; Fagan, P. J.; Manriquez, J. M . ; FragalA, I.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 4755. 128. Bursten, B. E.; Fang, A. Inorg. Chim. Acta 1985, 110, 153. 129. Bursten, B. E.; Casarin, M . ; DiBella, S.; Fang, A.; FragalA, I. L. Inorg. Chem. 1985, 24, 2169. 130. Kot, W. K . , Shalimoff, G. V.; Edelstein, N. M . ; Edelman, M. A.; Lappert, M. F. J. Am. Chem. Soc. 1988, 110, 986. 131. Bursten, B. E.; Strittmatter, R. J. unpublished results. 132. Bursten, B. E.; Fang, A.; Wilson, B. A. unpublished results. 133. Calabro, D. C.; Hubbard, J. L . ; Blevins, C. H . ; Campbell, A. C.; Lictenberger, D. L. J. Am. Chem. Soc. 1981, 103, 6839. 134. Herman, F.; Skillman, S. Atomic Structure Calculations; Prentice- Hall: Englewood Cliffs, NJ, 1963. 135. Schwartz, K. Phys. Rev, B 1972, 5, 2466. 136. Norman, J. G. Jr. Mol. Phys. 1976, 31, 1191. 137. Sternal, R. S.; Marks, T. J. Organometallics 1987, 6, 2621. 138. Ortiz, J. V. J. Am. Chem. Soc. 1986, 108, 550. 139. Sattelberger, A. P., Los Alamos National Laboratories, personal communication. 140. Schore, N. E . ; Hope, H. J. Am. Chem. Soc. 1980, 102, 4251. 141. Schore, N. E. J. Am. Chem. Soc. 1979, 101, 7410. 142. Threlkel, R. S.; Bercaw, J. E. J. Am. Chem. Soc. 1981, 103, 2650. 273 143. Marsella, J. A.; Folting, K . ; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc. 1981, 103, 5596. 144. Marsella, J. A.; Caulton, K. G. J. Am. Chem. Soc.1980, 102, 1747. 145. Marsella, J. A.; Huffman, J. C . ; Caulton, K. G.; Longato, B . ; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 6360. 146. Schmid, G . ; Stutte, B . ; Boese, R. Chem. Ber. 1978, 111, 1239. 147. Berry, D. H . ; Bercaw, J. E.; Jircitano, A. J.; Mertes, K. B. J. Am. Chem. Soc. 1982, 104, 4712. 148. Barger, P. T.; Bercaw, J. E. J. Organomet. Chem. 1980, 201, C39. 149. Longato, B . ; Norton, J. R . ; Huffman, J. C.; Marsella, J. A.; Caulton, K. G. J. Am. Chem. Soc. 1981, 103, 209. 150. Casey, C. P.; Palermo, R. E.; Jordon, R. F . ; Rheingold, A. L. J. Am. Chem. Soc. 1985, 107, 4597. 151. McKee, S. D. ; Bursten, B. E . , unpublished results. 152. Bush, M. A.; Sim, G. A. J. Chem Soc. A 1971, 2225. 153. Cotton, F. A. Polyhedron 1987, 6, 667. 154. Emfremov, Y. M, Samoilova, A. N . ; Kozhirkhovsky, V. B , ; Gurvick, L. V. J. Mol. Spectrosc. 1978, 73, 430-440. 155. KlotzbQcher, W . ; Ozin, G. A. Inorg. Chem. 1977, 5, 984. 156. Cotton, F. A.; Ilsley, W. H . ; Kaim, W. Inorg. Chem. 1979, 18, 2717. 157. Norman, J. G. Jr.; Kolari, H. J.; Gray, H. B . ; Trogler, W. C. Inorg. Chem. 1977, 16, 987-993. 158. Bursten, B. E.; Fenske, R. F. J. Chem. Phys. 1977, 67, 3138-3145. 159. Bursten, B. E.; Cotton, F. A. Symp. Faraday Soc. 1980, 14, ISO- 93. 160. Beck, H. P.; Strobel, C. Angew. Chem. Intl. Ed. Engl. 1982, 21, 525. 161. Bursten, B. E.; Cotton, F. A.; Green, J. C.; Seddon, E. A.; Stanley, G. G. J. Am. Chem. Soc. 1980, 102, 4579. 162. Gingerich, K.A. Symp. Faraday Soc. 1980, No. 14, 109. 274 163. Cotton, F. A.; Stanley, G. G . ; Kalbacher, B. J.; Green, J. C.; Seddon, E.; Chisholm, M. H. Proc. Natl. Acad. Sci. USA 1977, 74, 3109. 164. Chisholm, M. H . ; Cotton, F. A.; Frenz, B. A.; Reichert, W. W . ; Shive, L. W . ; Stults, B. R. J. Am. Chem. Soc. 1976, 98, 4469. 165. Ziegler, T. J. Am. Chem. Soc. 1983, 105, 7543. 166. Tsutsui, M, Ely, N . ; Dubois, R. Acc. Chem. Res. 1976, 9, 217- 222. 167. Edwards, P. G.; Andersen, R. A.; Zalkin, A. Organometallics 1984, 3, 293. 168. Huq, F . ; Mowat, W . ; Shorthand, A.; Skapski, A. C.; Wilkinson, G. J. Chem. Soc., Chem. Commun. 1971, 1079. 169. Cramer, R. E . ; Maynard, R. B . ; Paw, J. C.; Gilje, J. W. Organometallics 1983, 2, 1336. 170. Perego, G, Cesari, M . ; Farina, F . ; Lugli, G. Acta Crystallog. B. 1976, B32, 3034. 171. Cramer, R. E.; Mori, A. L . ; Maynard, R. B . ; Gilje, J. W. unpublished results. 172. Feedback from the Central Regional ACS meeting in Columbus, 1987. 173. Marks, T. J., personal communication. 174. Tooze, R. P.; Wilkinson, G.; Motevalli, M . ; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1986, 2711. 175. Cotton, F. A.; Diebold, M. P.; Kibala, P. A. Inorg. Chem. 1988, 27, 799. 176. Gambarotta, S.; Chiang, M. Y. Organometallics 1987, 6, 897. 177. Fochi, G.; Guidi, G . ; Floriani, C. J. Chem. Soc., Dalton Trans. 1984, 6, 1253. 178. Hunter, W. E . ; Hrncir, D. C.; Bynum, R. V.; Pentilla, R. A.; Atwood, J. L. Organometallics 1983, 2, 750. 179. Wailes, P. C . ; Weigold, H . ; Bell, A. P. J. Organomet. Chem. 1971, 33, 181. 180. Marks, T. J.;Wachter, W. A. J. Am. Chem. Soc. 1976, 98, 703. 181. Bruno, J. W . ; Kalina, D. G . ; Mintz, E. A.; Marks, T. J. J. Am. Chem. Soc. 1982, 104, 1860. 275 182. Duttera, M. R . ; Fagan, P. J.; Harks, T. J.; Day, V. W. J. Am. Chem. Soc. 1982, 104, 865, 183. Wasserman, H. J.; Zozulin, A. J.; Moody, D. C . ; Ryan, R. R . ; Salazar, K. V. J. Organomet. Chem. 1983, 254, 305. 184. Meunier-Piret, J.; Declercq, J. P.; Germain, G.; Van Meerssche, M. Bull. Soc. Chim. Belg. 1980, 89, 121. 185. Soulid, E. J.; Folcher, G.; Marquet-Ellis, H. Can. J. Chem. 1982, 60, 1751. 186. Lauher, J.W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729. 187. Smart, J. C.; Curtis, C. J. J. Am. Chem. Soc. 1977, 99, 3518. 188. McKinney, R. J. J. Chem. Soc., Chem. Commun. 1980, 603. 189. Davison, A.; Smart, J. C. J. Organomet. Chem. 1973, 49, C43. 190. MClller-Westerhoff, U. T.; Eilbracht, P. J. Am. Chem. Soc. 1972, 94, 9272. 191. Smart, J. C.; Pinsky, B. L. J. Am. Chem. Soc. 1977, 99, 956. 192. Kohler, F. H . ; Doll, K. H . ; Prossdorf, W . ; Muller, J. Angew. Chem. Intl. Ed. Engl. 1982, 21, 151. 193. Smart, J. C.; Pinsky, B. L.; Fredrich, M. F . ; Day, V. W. J. Am. Chem. Soc. 1979, 101, 4371. 194. Bell, W. L . ; Curtis, C. J.; Eigenbrot, C. W. Jr.; Pierpont, C. G.; Robbins, J. L . ; Smart, J. C. Organometallics 1987, 6, 266. 195. Bell, W. L. ; Curtis, C. J.; Miedaner, A.; Eigenbrot, C. W. Jr.; Haltiwanger, R. C.; Pierpont, C. G.; Smart, J. C. Organometallics 1988, 7, 691. 196. Zozulin, A. J.; Moody, D. C.; Ryan, R. R. Inorg. Chem. 1982, 21, 3083. 197. Gilman, H . ; Jones, R. G.j.Karmas, G . ; Martin, G. A. J. Am. Chem. Soc. 1956, 78, 4285. 276 198. Bradley, D. C.; Chakravarti, B. N.; Chatterjee, A. K. J. Inorg. Nucl. Chem. 1957, 3, 367. 199. Bradley, D. C.; Chatterjee, A. K. J. Inorg. Nucl. Chem. 1957, 4, 279. 200. Bursten, B. E.; Casarln, M . ; Ellis, D. E.; FragalA, I.; Marks, T. J. Inorg. Chem. 1986, 25, 1257.