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Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc

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Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc Metal-Metal (MM) Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc Richard H. Duncan Lyngdoh*,a, Henry F. Schaefer III*,b and R. Bruce King*,b a Department of Chemistry, North-Eastern Hill University, Shillong 793022, India B Centre for Computational Quantum Chemistry, University of Georgia, Athens GA 30602 ABSTRACT: This survey of metal-metal (MM) bond distances in binuclear complexes of the first row 3d-block elements reviews experimental and computational research on a wide range of such systems. The metals surveyed are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc, representing the only comprehensive presentation of such results to date. Factors impacting MM bond lengths that are discussed here include (a) n+ the formal MM bond order, (b) size of the metal ion present in the bimetallic core (M2) , (c) the metal oxidation state, (d) effects of ligand basicity, coordination mode and number, and (e) steric effects of bulky ligands. Correlations between experimental and computational findings are examined wherever possible, often yielding good agreement for MM bond lengths. The formal bond order provides a key basis for assessing experimental and computationally derived MM bond lengths. The effects of change in the metal upon MM bond length ranges in binuclear complexes suggest trends for single, double, triple, and quadruple MM bonds which are related to the available information on metal atomic radii. It emerges that while specific factors for a limited range of complexes are found to have their expected impact in many cases, the assessment of the net effect of these factors is challenging. The combination of experimental and computational results lead us to propose for the first time the ranges and “best” estimates for MM bond distances of all types (Ti-Ti through Zn-Zn, single through quintuple). n1 CONTENTS 1. Introduction 1.1 Metal-Metal (MM) Bonds in Binuclear Complexes 1.2 Scope of this Survey 1.3 Some Critical Observations 1.4 Some Challenges 1.5 Objectives of this Review 2. Factors Influencing Metal-Metal Bond Distances 2.1 Size of the Metal Atom/Ion 2.2 Metal Oxidation States 2.3 Influences of Ligands 2.4 Other Effects 2.5 Different Influences in Tandem 3. Theoretical Descriptions of Metal-Metal Bonds 3.1 Theories of Metal-Metal Bonding 3.2 Metal-Metal Covalent Bonds of Various Orders 3.3 DFT Methods for Binuclear Carbonyl Complexes 3.4 Assessment of Theoretical Methods 4. Titanium-Titanium Bonds 4.1 Titanium Dimer 4.2 Experimentally Known Dititanium Complexes 4.3 Binuclear Titanium Carbonyl Complexes 4.4 Ti-Ti Bond Length Ranges 5. Vanadium-Vanadium Bonds 5.1 Vanadium Dimer n2 5.2 Binuclear Vanadium Complexes with Bidentate and Carbonyl Ligands 5.3 Binuclear Vanadium Complexes with Nitrogen and Carbocyclic Ligands 5.4 Divanadaboranes and Complexes with Chalcogen Ligands 5.5 Computational Studies on V-V Bonded Complexes 5.6 V-V Bond Length Ranges 6. Chromium-Chromium Bonds 6.1 Chromium Dimer 6.2 Quadruply-Bonded Dichromium Tetracarboxylates 6.3 Other Dichromium Paddlewheel Complexes 6.4 Quintuply Bonded Dichromium Complexes 6.5 Theoretical Analysis of Dichromium Quadruple and Quintuple Bonds 6.6 Binuclear Chromium Carbonyl Complexes 6.7 Cr-Cr Bond Length Ranges 7. Manganese-Manganese Bonds 7.1 Manganese Dimer 7.2 Experimentally Known Binuclear Manganese Complexes 7.3 Homoleptic Binuclear Manganese Carbonyl Complexes 7.4 Binuclear Manganese Carbonyl Complexes with Hydride, BF and C4H4P Ligands 7.5 Binuclear Manganese Carbonyl Complexes with Carbocyclic Ligands 7.6 Mn-Mn Bond Length Ranges 8. Iron-Iron Bonds 8.1 Iron Dimer 8.2 Experimentally Known Binuclear Iron Complexes (Non-Carbonyl) 8.3 Binuclear Iron Carbonyl Complexes (Homoleptic and with C2H2 Ligands) 8.4 Binuclear Iron Carbonyl Complexes with C4H4 and C4H6 Ligands n3 8.5 Binuclear Iron Carbonyl Complexes with Cyclopentadienyl and C5F6 Ligands 8.6 Binuclear Iron Carbonyl Complexes with Fused Carbocyclic Ligands 8.7 Binuclear Iron Carbonyl Complexes with Boron-Based Ligands 8.8 Fe-Fe Bond Distances and Bond Orders 9. Cobalt-Cobalt Bonds 9.1 Cobalt Dimer 9.2 Experimentally Known Binuclear Cobalt Complexes 9.3 Binculear Cobalt Carbonyls and Cobalt Cluster Carbonyls 9.4 Homoleptic Binuclear Cobalt Complexes 9.5 Binuclear Cobalt Carbonyl Complexes with Inorganic Ligands 9.6 Binuclear Cobalt Carbonyl Complexes with Carbon Ligands 9.7 Co-Co Bond Length Ranges 10. Nickel-Nickel Bonds 10.1 Nickel Dimer 10.2 Binuclear Nickel Complexes with Ni(0) Centers 10.3 Binuclear Nickel Complexes with Ni(I) Centers 10.4 Binuclear Nickel Complexes with Ni(II) and Ni(III) Centers 10.5 Computational Studies on Binuclear Nickel Complexes 10.6 Ni-Ni Bond Length Ranges 11. Copper-Copper Bonds 11.1 Copper Dimer 11.2 Some Cu(I)-Cu(I) Complexes 11.3 Cupric Acetate Dimer and Mixed Valent Dicopper Complexes 11.4 Homoleptic Binuclear Copper Carbonyls 11.5 Cu-Cu Bond Length Ranges n4 12. Zinc-Zinc Bonds 12.1 Zinc Dimer and Zn2X2 Molecules 12.2 Binuclear Zinc Complexes with Organometallic and Nitrogen Ligands 12.3 Zn-Zn Bond Length Ranges 13. Various Metal-Metal Bond Lengths Compared 13.1 Binuclear Homoleptic Metal Carbonyls 13.2 Binuclear Cyclopentadienylmetal Carbonyls 13.3 Single, Double and Triple MM Bond Length Ranges 13.4 Quadruple and Quintuple MM Bond Length Ranges 13.5 Effects of Change in Metal Upon MM Bond Length Ranges 14. Bold or Foolhardy Final Estimates of Metal-Metal Bond Distances 15. Summary and Conclusions Author Information Acknowledgments Abbreviations References n5 1. INTRODUCTION Transition metal (TM) complex chemistry began with Werner’s coordination theory treating primarily a ligand array around a single metal center.1,2 Such complexes can contain two or more metal atoms but no direct metal-metal bonds. In 1957, evidence of the first metal-metal 3 (MM) bonds was seen in the binuclear metal carbonyls M2(CO)10 (M = Mn, Re). This was the year that one of the authors (RBK) of this review started his doctoral work at Harvard University. In the only slightly more than six decades from the definitive identification of the first MM bond between d-block metals, this field has mushroomed into a very rich area of chemistry with well characterized binuclear d-block metal molecules having all possible integral formal bond orders between 1 and 5. The mid-60’s saw the first recognition of MM multiple bonds in the quadruple Re-Re bond 2– 4,5 of the (Re2Cl8) anion. Prior to this, many TM complexes had been synthesised which only later were shown to have MM multiple bonds, including Cr-Cr,6 Mo-Mo,7 W-W,8 Pt-Pt,9 Nb- 10 11 2– Nb, and Ta-Ta bonds. The anion Re3Cl12 was the first instance of metal-metal multiple 12,13 bond recognition by MO analysis, in the Re3 triangle with three Re=Re double bonds. Since then, the chemistry of MM bonded TM complexes has been documented in many monographs.14,15,16,17,18 Thousands of TM complexes with MM bonds are now known and experimentally characterized over the three TM periods, spanning elements from groups 4 to 12. Figure 1 depicts structures of a mononuclear complex A, of a bimetallic complex B without an MM bond, and of a binuclear complex C with a metal-metal covalent bond. In general, this review is focused on complexes of the type C with metal-metal bonds. n6 2- 3+ N N Cl Cl NH3 N Cl N Cl Cl H3N NH3 V V Co N N Re Re Cl NH H3N 3 N N Cl Cl NH 3 Cl Cl A B C Figure 1. Mononuclear, non-MM-bonded binuclear, and MM-bonded binuclear complexes 1.1. Metal-Metal (MM) Bonds in Binuclear Complexes Metal-metal distances are key experimentally measurable features of binuclear or polynuclear metal complexes. This survey of MM bond distances in homobinuclear metal complexes of the first row 3d-block elements derives much from experimental structure determinations, mostly X-ray diffraction studies. Heteronuclear MM bonds are also well known19 but are not dealt with here. Cluster complexes with three or more metal atoms are also not discussed in detail here. Extended metal atom chain complexes with up to an infinite number of metal centers also fall outside the scope of this review. Computational (mainly density functional) studies furnish a wealth of information on MM bond lengths predicted for a wide variety of binuclear and other complexes. Theory often corroborates experiment and provides reasonable predictions for species not yet synthesized. Unequivocal experimental studies on the geometrical structure of a complex provide a benchmark against which the reliability of computational findings may be gauged. Metal-metal interactions in binuclear complexes include (a) covalent MM bonds involving overlap between metal orbitals, (b) weak interactions supported mainly by bridging ligands, (c) metal-to-metal dative bonds, (d) antiferromagnetic coupling between metal centers, (f) three-center two-electron bonds via ligand atoms not invoking direct metal-metal bonding, n7 and (f) no discernible metal-metal interaction at all in systems with widely separated metal centers. This survey focuses on MM interactions with predominantly covalent bonding. Various structural and electronic factors influence MM covalent bond distances including the following: (a) the atomic and covalent radii of the metal, (b) the oxidation state of the n+ metal atoms and the formal charge on the (M2) core (n ranging from 0 to 8), (c) the formal MM bond order, (d) the basicity and electron-releasing capacity of the ligands, (e) the spatial n+ arrangement of the ligands around the (M2) core, (f) the effects of steric bulk and crowding, (g) stereoelectronic factors like the “bite” angle of bidentate ligands, (h) the mode of metal- ligand bonding which includes a number of distinct types, (i) the spin state of the complex, and (j) crystal packing forces, discussed in more detail later.
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