Structure and Bonding in Metallocenes and Oligomers

Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 102, No. 3, June 1990, pp. 379-393. 9 Printedin India.

Structure and bonding in metallocenes and oligomers

ELUVATHINGAL D JEMMIS* and A CHANDRASEKHAR REDDY School of Chemistry, University of Hyderabad, Central University PO, Hyderabad 500 134, India

Abstract. The structural variations observed as a function of the number of valenceelectrons, the metals and the rings present in tripledecker and tetradecker sandwiches have been studied by the extended Huckel method. In tripledecker complexes the electron count varies from 26 electrons to 34 electrons depending on the magnitude of the interactions 8(_~-e_). With the ~ckel complex, CpNiCpNiCp § 1, the e'~ antibonding interaction, 8(_~), is not very high so that the presence of electrons in these orbitals do not affect the stability of the complex. With the vanadium complex, CpVC6H6VCp, the e'~, interaction 8(.8r is substantial so that 30-valence electron complexes are preferred with the transition metals on the left side of the periodic table. With P6, Ps and Ass complexes the a~, 8(.~), also becomes very strong. This leads to a 28-valence electron closed shell structure. The distortion of the Ass ring in CpMoAssMoCp is attributed to the presence of three electrons in the degenerate HOMOs. In tetradecker complexes46-valence electron complexesare found to have highly antibonding degenerate HOMOs which lead to a distorted structure where the two end CpMCp units slip away from the central metal in a direction perpendicular to the original metal-metal axis (7_). The extent of antibonding interactions is maximum when the M' is a late transition metal and M is an early transition metal of the periodic table. With appropriate metals it is possible to have a symmetric 46-valence electron tetradecker complex. Calculations on a series of CpCo(C3B2Hs)M'(C3B2Hs)CoCp, where M' = Cr, Mn, Fe, Co, Ni, Cu and Zn reproduced thb experimental structural trends. The variation in total energies has been traced predominantly to the energy difference between the valence orbitals of the metal atoms and the n and cr MOs of the ring. The bent geometry of CpCo(CaB2Hs)Sn(CaB2Hs)CoCp is caused by factors similar to those which operate in making CpSnCp a bent metallocene.

Keywords. Metallocenes; tripledecker, tetradeeker, pentadecker sandwich compounds; one-dimensional sandwich compounds; molecular orbitals; Jahn-Teller distortion; slip distortion.

1. Introduction

The discovery of ferrocene spearheaded the development of transition metal organometallic chemistry. The sandwich structure was demonstrated for a wide variety of metal and ring combinations (Wilkinson et al 1982). Many theoretical studies on these metallocenes (1) (Veillard 1986) have been known. Addition of one metal and ring to 1 leads to the tripledecker sandwich compounds (_2). A variety of tripledeeker complexes with C,H,, P6, Ps, P4 and Ass as the middle rings have been synthesized. A list of the experimentally characterised tripledecker complexes (Werner and Salzer 1972; Jemmis and Reddy 1988) with the associated number of valence electrons and

* For correspondence

379 380 Eluvathinyal D Jemmis and A Chandrasekhar Reddy

Table 1. Currently known tripledecker sandwich compounds with metal-metal distances, number of valence electrons and references.

No. of M-M valence Compound distance~ electrons Referencesb

(1) CpNiCpNiCp* 3"580 34 1 (2) CpCoC6R6CoC p 34 3 (3) CpVC6R6VC p 3"400 26 2 (4) C6H6CrC6H6CrC6H 6 3.338 30 4 (_5) CpFe(C2B2SR4)FeCp 3"236 30 6 (6) (C2 B2 SR4)Co(C2B2SR4)Co(C2 B2SR4) 30 7 (7) CpM(C3B2Rs)M'Cp (a) M = Ni, M' = Ni 3"416 33 (b) M = Fe, M' = Co 3'337 32 8 (c) M = Fe, M' = Co 3.204 30 (d) M =Co, M'=Co 31 (8) CpCo(C2B3Rs)CoCp 3-140 30 9 (9_) CpRh(C4B2R~)RhCp +2 3.440 30 10 CpM(C3B2Rs)M'(C3B2Rs) (a) M = Co, M' = Pt 30 (b) M = Ni, M' = Pt 31 11 (c) M = Co, M' = Ni 30 (LlJ (C6HsR)Fe(C3B2Hs)Ni(C3B2Hs) 30 12, 13 CpMP~MCpM = Mo, V,W, Ti 2"648 28 14~'d CpMA% MCpM = Mo, Cr 2"762 27 16 e CpCrPsCrCp 2'728 27 15 CpMP4MCpM = Rh, Co 3"324 32 f.s CpCoC2B3HsMC6H6M = Ru, Os 3'297 30 h C6H6RuC2B3HsMC6H6M = Ru, Os 3'478 30 h

"Distances in Angstroms; b Numbers correspond to the reference numbers found in Jemmis and Reddy (1988); ~ Scherer et al (1988a); o Seherer et al (1987); Scherer et al (1989); f Scherer (1987); s Scherer et al (1988b); h Davis et al (1989). the M-M distances have been given in table 1. With P6, P5 and As 5 middle rings a short metal-metal distance is observed leading to direct metal-metal bonding. The Ass ring in CpMoAs~MoCp has a distorted structure of As2 and As3 fragments as shown in _3 (Rheingold et aI 1982). The electron count in these complexes varies from 24 to 34. In most of the complexes with electron counts ranging from 26 to 34, the central ring is planar. In the recently synthesised 24-valence electron CpTiP6TiCp (Scherer and Swarowsky 1987) complex, the P6 ring is puckered. In two other complexes with 32-valence electrons with central P4 rings (Scherer et al 1988), the P4 ring is found to be rectangular. Addition of one metal and ring to _2 leads to the tetradecker complexes, (_4). A variety of tetradecker complexes with C2B2SR4, CaB2Rs, C2B3Rs, C5H5 rings and various transition metals have been synthesized (table 2) (Jemmis et al 1990). The electron counts in these complexes vary from 40 to 46. Tetradecker complexes with main group elements are known only with one example i.e., (CpCo(C3B2Hs))2Sn (Wadepohl et al 1982). In this complex the two metallocences, (CpCo(C3B2Rs))2, are bent at an angle of 112 ~ around the central metal Sn (_5). The most important Structure and bonding in metallocenes and olioomers 381

C=>

0 0 2.570 O 14 O 1.755 l.'/55 O 14 As As C~ C~ 2.,90"~,,./2 .,,0 AS M

1 ? 3 4

O M M O M O _5

structural feature of the tetradecker complexes is the slip-distortion (_6) which varies with the central metal ranging from 0.1 to 0"5 A. There is no linear relationship between the number of Valence electrons and the extent of slip distortion. For example a large amount of slipping is observed in the Zn complex, (CpCo(C3BzHs))2Zn, whereas the (CpNi(C3BzHs))2Pt complex with the same electron count has no slipping at all. 382 Eluvathingal D Jemmis and A Chandrasekhar Reddy

Table 2. Geometric parameters and number of valence electrons of the tetradecker sandwich compounds characterised by diffraction methods.

Cp-M M-M Number of Extent of distance distance valence slip Compound (/~) (A,) electrons distortion References~

(1) [(C2B2SR4)Co(C2 B2 SR4)]2 Fe 1-69 3'53 42 -- 4 (II) [(CpFe(C2 B2 SR4)] 2M (a) M' = Fe 1.67 3'27 42 -- 11 (b) M' = Co 1'69 3-86 43 -- 11 (III) (CpCo(CaB2Rs))2M 8-10 (a) M' = Cr 1.66 3-47 40 0'10 (b) M' = Mn 1.66 3"53 41 0'20 (c) M' = Fe 1.66 3.42 42 0.22 (d) M' = Co 1-66 3.38 43 0"20 (e) M' = Ni 1.66 3.28 44 0"10 (f) M' = Cu 1.66 3.41 45 0.50 (g) M' = Zn 1.66 3.46 46 0-45 (IV) [CpNi(CaB~Rs)]2Ni -- -- 46 -- 14 (V) [CpM(C3B2Rs)]2Pt (a) M = Fe 1-66 3-40 42 0-00 13 (b) M =.Ni 1.66 3'59 46 0.00 13 (VI) [CpCo(C2 BaRs)]2Co-H -- -- 42 0"00 7 (VII) I-CpCo(C3B2Rs)]2Sn* -- 1'66 48 -- 6

* The two [CpCoC3B2Rs)] units are bent by 112~ Numbers correspond to the reference numbers found in Jemmis and Reddy (1990).

Further addition of one metal and ring leads to the pentadecker complexes, (_7). There are only three such complexes (Siebert 1985) known to date. None of them has been characterized crystallographically. Among these three complexes two of them have 57-valence electrons and the third has 58-valence electrons with two bridging-CO groups instead of a cyclic polyene ring in the middle. As we go on adding metal and ring combinations a one-dimensional polymer results. So far there is only one one-dimensional polymer known with C3B2RsNi as the unit cell (Kuhlmann et al 1985). In this paper, we review theoretical studies on one-dimensional sandwich compounds with special reference to the following. First, in tripledecker complexes, (a) the requirements for M-M bonding and its consequences, (b) reasons for the distortion of As s as a middle ring and (c) the number of valence electrons required for stability in tripledecker complexes; in tetradecker complexes, the (d) electronic factors responsible for the slip distortion observed in the tetradecker complexes having 40-46 valence electrons and (e) the reasons for the observed bending of the 48-valence electron complex, (CpCo(CaB2Hs))2Sn. Extended Huckel calculations have been carried out to explain the above structural variations (Hoffmann and Lipscomb 1962; Hoffmann 1963; Hoffmann et al 1973; Fujimoto and Hoffmann 1974).

2. Results and discussion

2.1 Electronic structure of tripledecker sandwich complexes with CnHn, P6, P5 and Ass middle rings The electronic structure of a model tripledecker complex (CnH~MC~HnMC~H~) can Structure and bondin9 in metallocenes and oligomers 383

-8.0 z /

-10.0 8cW.. ~e

-1 2-0 8b Z,e* ~,.~v~

-14.0

A B C D E F G H I

Figure 1. Correlation of the energy levels of the tripledecker sandwich compounds A, CpNiCpNiCp+, B. CpCoCnH6CoCp, C. CpCoCTHvCoCp+5, D. CpVC6H6VCp, E. C6H6CrC~H6CrC6H6, F. CpMoP6MoCp, G. CpCrPsCrCp, H. CpMoAssMoCp (symmetric),and I. CpMoAssMoCp(distorted), with electron count ranging from 34 to 26. be obtained by interacting the orbitals of C,H,M... MC, H, with those of CnH n. The interaction diagram between CsHsNi--NiCsH5 and C5H5 had shown that the major binding of the middle ring with the metals takes place through the e~ orbitals (Lauher et al 1976). The lower lying d orbitals (remnants of the t2o set) are not affected in this considerably. Since tripledecker complexes are known with various combinations of metals and rings a correlation of the frontier orbitals of CpNiCpNiCp +1 with CpCoC6H6CoCp, CpCoC7HTCoCp, CpVC6H6VCp and C6H6CrC6H6CrC6Hg is given in figure 1 to know the effect of metals and rings. The 30- and 34-electron count found for Ni complexes remain valid for the cobalt complexes except for the fact that the el, (8c), orbitals go up in energy, as the middle ring becomes C7H7. Thus there is less chance of a 34-electron count with CpCoC7HTCoC p system. The situation is very different in CsHsVC6H6VCsH5 whose electronic structure is similar to C6H6CrC6H6CrC6H 6. Figure 2 gives an interaction diagram between the fragments CaHsV--VCsH5 and C6H6. The high energy and greater diffusivity of the vanadium orbitals provide strong interactions as given in 8b-_e in addition to / 384 EluvathingalD Jemmis and A Chandrasekhar Red@

8o b c d e

-8.0

-10.C

-I 2.0 eV I'

-14.C

Figure 2. Construction of the orbitals of CpVC6H6VCp from the fragments CpV...VC p and C6H 6.

8__aa. The e't MOs of C6H6 are stabilised by the interaction with the e~ MOs of the CpV--VCp fragment resulting in the donation of four ~ electrons to the metal (8_r With these C6H6 can be considered as a 10-electron donor. This is also observed in the chromium compound. The e~ orbitals of CpV... VCp are stabilised by the e[ orbitals of C6H6. The a't and a[ orbitals of the t2g set are slightly pushed up. Catching up with this are the e[ MOs of the fragment pushed up by the corresponding e~ MOs of the C6H6 ring. The result is a close bunch of four frontier MOs, with the 26 valence electrons (30 valence electrons, including tr electrons) in CpVC6H6VC p a quintet state corresponding to the four unpaired electrons. The bonding in C6H6CrC6H6CrC6H6 is along the same lines. This is found to be true experimentally. There are four electrons Structure and bondin9 in metallocenes and oligomers 385

more in this system, teading to a closed shell configuration of 30 electrons, filling all the valence MOs (figure 1) (or 34 electrons assuming benzene to be a 10-electron donor). The concept of donation of the a electrons is not new in transition metal organometallic chemistry. The initial process in the activation of C-H and C--C bonds is indeed donation of tr electrons to the metal. However the unique symmetry conditions present in the benzene complex provides the possibility of donation of the tr C-C bonding electrons of a hexahapto benzene ring to the metal. Metal-metal distances are traditionally expected to be of the order of 3.4 A in complexes with central Cp rings. In complexes with P6, P5 and As 5 middle rings the metal-metal distance is found to be in the bonding range. We have calculated the metal-metal distances for representative ttipledecker complexes with C~Hn, P~ and Asn middle rings using average metal-ligand distance available in the literature. These distances show that only for n > 7 CnHn middle tings and for n > 4 P, and Ash middle rings, metal-metal distances fall within the bonding range. Figure 1 indicates a 28-electron cognt for closed shell species with phosphorus and arsenic rings. The crucial difference that leads to a 28-electron count is the result of the a[ being pushed up by interaction with the low-lying a[ orbital of P6, (8_r This again is a result of the shorter M-M distance and the more diffuse nature of the P atoms and consequent large overlap between the FMOs. A 32-valence electron count may be obtained for the P6 complex, CpMoP6MoCp, if P6, similar to C6H6, is taken as a ten-electron donor. The P5 and As 5 complexes would then have 31 valence electrons. The extent of overlap between e'~ MOs in these compounds is significant. The general level pattern remains the same for the As5 and P5 complexes with one major difference. There is one electron less in both P5 and As 5 complexes. The three electrons in the degenerate MO of CpMoAssMoCp leads to a Jahn-Teller distortion of the central ring which is clearly seen in the X-ray structure of the compound, CsHsMoAssMoCsH s. A Walsh diagram, figure 1, extreme tight, indicates that among the many variations in energy levels, the prominent one is the splitting of the degenerate HOMO, one going up in energy and the other down. There is a net gain because there is only one electron to occupy the higher level. The corresponding P5 compound, CsHsCrPsCrCsH 5, may be free of a similar distortion because the HOMO is a non-degenerate singly occupied orbital. This, however, could not be confirmed from the X-ray analysis available as there was high disorder in the crystal. If there are 28 electrons no distortion is expected as seen in CpMoP6MoC p. In most of these tripledecker complexes with electron count ranging from 26 to 34 the structure of the middle ring is planar. In the recently synthesized Ti complex, CpTiP6TiCp, the P6 ring is puckered. This is due to the large stabilisation of the metal-metal bonding orbital by the n* orbital of puckered P6 ring. Detailed electronic structure studies are in progress. Addition of one metal and ring to _2 leads to tetradecker complexes, 4. These are studied in the next section.

2.2 Electronic structure of the model tetradecker sandwich complex CpCoCpCoCpCoCp +1 CpCoCpCoCpCoCp with 46 valence electrons was selected as a prototype for the tetradecker sandwiches. The electronic structure of the model is constructed by interacting a central CpCoCp -1 unit with CpCo +1 groups from each side. The important MOs of the CpCoCp- 1 fragment which are involved in the tetradecker 386 Eluvathingal D Jemmis and A Chandrasekhar Reddy

3elg

-10.0

evT

2elg -12.0 alg 2el u lu e2g ""2g alg elg e2u e2u '~lllmn n -14.0 ,.,g '2.g _= a2u lelu lelcj -2 ~ ~ Cp.... Cp CpCoCp Co

Figure 3. Constructionof the orbitals of CpCoCp-1 from the fragments Cp...Cp -2 and Co +1. Symmetrylabels are for Dsd point group. sandwich formation are 2elg, 2el, and 3etg (figure 3). The interaction diagram involving CpCo... CoCp § 2 and CpCoCp-1 is given in figure 4 (middle). Several weak two-orbital four-electron interactions involving a~g, a2u and e2g MOs of _4 are not shown. The interaction leading to two e~, pairs (le~,, 2e~,) are low in energy and filled. The antibonding combination, 3e~, is vacant. The elg MOs also lead to two low energy pairs, however the third one, 3e~g, which is antibonding between the central metal and the adjacent rings, lies below 3e~, and is filled in the 46 valence electron systems. The antibonding nature of 3e~g is seen in 9_. Therefore, tetradecker systems with 42 valence electrons where the antibonding MOs are empty will be preferred. However, table 2 shows that compounds with higher electron counts are known. Three complexes are reported with 46 valence electrons. Two of these have been structurally characterized. Surprisingly one of them, CpCo(C3B2Hs)Zn(C3B2Hs)CoCp has a distorted structure, _6, with a 0.45/~ slip of the terminal metallocene units with respect to the central metal while the other, CpNi(C3B2Hs)Pt(C3B2Hs)NiCp, with the same electron count is almost symmetric along the metal-metal axis. Structure and bonding in metallocenes and oligomers 387

-8-0 3elu 3elg evT 3e

-t0.0 2elg zel----= :p.:~ 2 elu

-12.0 a2u alg 2el u 2el u a i ,,-, ID, mild lID 2e2g ,., ---- g'__. 2elg Ir lu ~ letu

-14.0 lelg 92 "1 CpCo "1 CpCo----CoCp CpCoCpCoCpCoCp CpCo Cp-1

Figure 4. Construction of the orbitals of CpCoCpCoCpCoCp +1 from the fragments CpCo...CoCp +2 and CpCoCp-'. Several 4e-.2 orbital interactions are neglected in this.

~o Z

t =u •

9a b 388 EluvathingalD Jemmis and A Chandrasekhar Reddy

Total Energy Curves

- 208343

-2084.0 42 VE

- 2122-0

46 VE -212~0

3elg

-10.0

~ ~ -12.0 2e2g~ ~'''~'~P--

| I I i 04) 0-2 ~4 O~ Slip Distortion , Figure 5. Walsh diagram for the slip distortion as observed in the 46-valence electron tetradecker sandwich compounds, 6, with the model CpCoCpCoCpCoCp+ 1 (below). Sum of one-electronenergies of 42VE (valence electrons) and 46VE complexesare given on the top half of the diagram. What are the reasons for the distortion in one particular complex and not in the other, even though both have the same electron count? A potential energy surface is constructed for the distortion of the 46-valence electron CpCoCpCoCpCoCp complex by moving the two terminal CpCoCp units away from each other along an axis perpendicular to the initial M-M axis, (figure 5, top). Figure 5 (bottom) shows the Walsh diagram for the above process. The degenerate HOMO, 3e10, which is Structure and bonding in metallocenes and oligomers 389

L lelg 2e19

,~ . ~ o~ % §

3elg 2elg 2elg CpCr Cp CpNi Cp 2e1r ,

a3 cz 3elg Scheme I. antibonding between the central metal and the adjacent rings, comes down in energy with distortion. In this model compound the variations in the 3elg MOs almost parallel the behaviour of the sum of one-electron energy. Scheme 1 shows how the 390 Eluvathingal D Jemmis and A Chandrasekhar Reddy

3e10 MO of CpCoCpCoCpCoCp § is obtained from the 2elo and 3elg MOs of CpM'CP and 2elg of CpM...MCp. The extent of ring character of 3elg MOs of 4 controls the extent of slipping in these complexes. This depends on the magnitude and nature of the 2e~o and 3elg MOs of CpM'Cp that constitute it. This is again controlled by the energy difference between the e~g MOs of metal and the le~g and 2elg MOs of Cp...Cp (scheme 1). The extremes where the Cp contribution changes from n to tr are shown in scheme 1. To explain these we have calculated potential energy surfaces for slipping in nine combinations of terminal and central metals in _4, always keeping a 46-valence electron count. The variations in the sum of one electron energies as a function of distortion are given in figure 6. The extent of slipping from the symmetric structure is maximum with M = early transition metal and M'= late transition metal (a3). Possibility of a slipped structure is minimum when the situation is reversed (c~). 2.0

c I

c 2 1.0

.v T bl 0.0

c 3

b 2

- ~0 I b 3

a 2 a 3

-2~ ' i I i O0 0.2 0-4 0.6 Slip Distortion D Figure 6. Plot of the sum of one-electron energies as a function of slip distortion for 46-valence electron complexes of the type CpMCpM'CpMCp +. (a~) M = Cr, M' -- Cr; (a2) M = Cr, M' = Fe; (a3) M = Cr, M' = Ni; (bt) M = Fe, M' = Cr; (b2) M = Fe, M' - Fe; (b3) M = Fe, M' = Ni; (c0 M = Ni, M' = Cr; (c2) M = Ni, M' = Fe; (c~) M = Ni, M' -- Ni. Structure and bonding in metallocenes and oligomers 391

Most of the experimentally known tetradecker complexes have C3B2H 5 and its derivatives as middle rings. A direct comparison between CpCoCpCoCpCoCp + 1 and CpCo(C3 B2 H5)Co(C3 B2 H 5)CoCp- 3 with regard to the variation in total energy for slipping shows that the decrease in the sum of one-electron energies as a function of slipping is less for the all-Cp compounds. This is due to the high energy of the C 3BzH 5 n-orbitals which give more n-character to the 3elg MOs of the tetradecker sandwich complexes which are responsible for slipping. Similar calculations were also carried out for the series of CpCo(CaB2Hs)M' (C3B2Hs)CoCp (M'= Cr- Zn) with different charges so that 46 valence electrons are maintained. The energy of the 3elg orbitals of (CaB2Hs)M'(C3B2Hs) decreases gradually and becomes comparable to that of the 2e~g MOs of CpM...MCp at M' = Ni. Further decrease in energy of M' (as in Cu and Zn) increases this energy difference so that the ring-M' interactions are not as strong. Correspondingly, the 3exg orbital is less antibonding, resulting in less distortions. The foregoing discussions can be directlyused in the two 46-valence electron count tetradecker complexes IIIg and Vb of table 2. The terminal metal M has gone from Co in IIIg to Ni in Vb which leads to less Pz character on the C3B2H 5 ligands in the 3elg oritals. Similarly the change of the central metal from Zn to Pt also decreases the tendency for slipping according to the discussion above. The variation of energy with slip distortion for the NiPtNi combination shows that the energy increases though not very steeply with distortion. Thus the general conclusions about slipping hold good. This also helps us to predict the structure of CpNi(C3B2Hs)Ni(C3B2Hs)NiCp which is known experimentally but no structural studies are available. The difference between Vb and the NiNiNi complex is that the central metal is changed from Pt to Ni. Thus the tendency for distortion should be very low in this complex. The most important conclusion to be drawn from the above discussion is that all 46-valence electron tetradecker complexes need not have a slipped structure. Qualitative trends can be obtained for the relation between M and M' and the extent of distortion. In tetradecker complexes even with 46 valence electrons the extent of distortion should vary considerably. Simultaneous changes in the number of valence electrons and the central metal make predictions difficult in the series IIIa-g (table 2). However, calculations on a series of complexes with the general formula CpCo(C3B2Hs)M(C3B2Hs)CoCp (M = Cr- Zn), reproduce the experimental trends correctly. Small distortions have been previously noted in transition metal complexes (Albright and Hoffmann 1978) with unsymmetrical zr-ligands. The only example of a bent tetradecker complex, _5, is found with the central metal Sn. The electronic structure of (CpCo(C3B2Hs))2Sn differs from the transition metal tetradecker complexes in the following way. The 4d orbitals of Sn are too low in energy to interact with the MOs of (CpCo(CaB2Hs))2. The alg, a2u and 3elu orbitals of (CpCoC3B2Hs)2 have the right symmetry for interaction with the 5s, 5pz and 5px, py orbitals of Sn respectively. These interactions are very weak with M' = transitipn metal. When M'= Sn, these are the only interactions that bind the metallocene to Sn. We constructed a Walsh diagram for the bending of (CpCoC3B2Hs)2Sn so as to give the structure _5. The decrease in the energy of the 2alg MO is mainly responsible for the observed bending. The sum of one-electron energies gave a minimum around 120 ~ of bending. The reasons for bending are the same as that found for bending in Cp2Sn (Jutzi et al 1980) and are not discussed further. Further addition of ring and metal to 4 leads to the pentadecker complexes, _7. 392 Eluvathingal D Jemmis and A Chandrasekhar Red@

Only three pentadecker complexes are known to date. Among these three complexes two have 57 valence electrons and the other one has 58 valence electrons. Calculations show that complexes with 54 valence electrons should give rise to a closed shell electronic structure. Electrons enter into the antibonding orbitals in complexes having more than 54 valence electrons. The nature of these antibonding orbitals varies with variation in the metals and cyclic polyene rings present in the system. It again depends on the metals and rings present in the system. The electronic structure details of these compounds (7) will be discussed in a separate paper.

3. Conclusions

Molecular orbital studies using extended Huckel calculations on the tripledecker and tetradecker sandwich compounds reveal the following. The bonding between C~HnM .... MC~H, and the middle ring is controlled by a combination of individual interactions, (8a--_e), each of which contributes differently in individual examples. The strength of each interaction, (8a-e_) depends on the metal and the size of the middle ring and leads to the observed electron counts of 34, 30, 28 and26 valence electrons. The three electrons in the degenerate HOMO of CpMoAs 5 MoCp in the symmetric structure lead to a Jahn-Teller distortion in the As5 ring. Such a distortion in the P5 ring is absent in CpCrPsCrC p because the HOMO is nondegenerate. In tetradecker sandwich compounds a 46-valence electron count demands that four electrons have to occupy antiboding MOs. These complexes distort from high symmetry to reduce the antibonding interactions. The nature of the metal and the ligands decide the extent of antibonding interaction in the HOMO (3e19) of the 46-electron systems. Calculations on the 46-valence electron systems of the type CpMCpM'CpMCp show that the extent of slipping from the symmetrical structure is maximum with M = early transition metal and M' = late transition metal (figure 6, a3). Possibility of a slipped structure is minimum when the situation is reversed (figure 6, cl). A comparison between CpCoCpCoCpCoCp § 1 and CpCo(C 3B 2Hs)Co(C 3B 2Hs)COCp- 3 with regard to the variation in total energy for slipping shows that the decrease in the sum of one-electron energies as a function of slip distortion is less for the complex CpCoCpCoCpCoCp +1. There is no slipping of the metallocene units arround the central metal in the tetradecker complexes Cp4M3 with 42 or less valence electrons. Small amounts of slip distortions found in the complexes with C3B2H 5 as middle rings having 42 or less valence electron systems is attributed to the unsymmetrical nature of these rings. The decrease in energy of the alg orbital is responsible for the observed bending in CpCo(C3B2Hs)Sn(C3B2Hs)CoCp.

Acknowledgments

We thank the Computational facilities of the National Informatics Centre and of the University of Hyderabad for computations. ACR thanks the Council of Scientific and Industrial Research, New Delhi, for financial assistance. Structure and bonding in metallocenes and oligomers 393

Appendix

Atomic parameters for all the atoms used in the extended Huckel calculations are taken from previous studies. Geometrical parameters are taken from the experimental structures wherever known (table l, table 2). All C-H, P-P, C-C, C-B, B-H and Cp ring centre-M distances used in the calculations are 1.08, 2.17, 1.40, 1-55, 1-21 and 1.66/~ respectively. For the symmetrical As5 geometry an As-As distance of 2.509 is used.

References

Albright T A and Hoffmann R 1978 Chem. Bet. 11 1578 Davis J H, Sinn E Jr and Grimes R N 1989 J. Am. Chem. Soc. 111 4776 Fujimoto H and Hoffmann R 1974 J. Phys. Chem. 78 1167 Hoffmann R 1963 J'. Chem. Phys. 39 1397 Hoffmann R and Lipscomb W N 1962 J. Chem. Phys. 36 3179, 3489 Hoffmann R, Swenson J R and Wan C-C 1973 J. Am. Chem. Soc. 95 7644 Jemmis E D and Reddy A C 1988 Organometallics 7 1561, and references therein Jemmis E D and Reddy A C 1990 J. Am. Chem. Soc. (in press), and references therein Jutzi P, Kohl F, Hoffmann P, Kruger C and Tsay Y-H 1980 Chem. Ber. 113 757 Kuhlmann T, Roth S, Roziere J and Siebcrt W 1985 Angew. Chem. 97 88 Lauher J W, Elian M, Summerville R H and Hoffmann R 1976 J. Am. Chem. S'oc. 98 3219 Rheingold A L, Foley M J and Sullivan P J ~982 J. Am. Chem. Soc. 104 4727 Scherer O J 1987 Nachr. Chem. Tech. Lab 35 1140 Scherer O J, Schwalb J, Swarowsky H, Wolmershauser G, Kaim W and Gross R 1988a Chem. Bet. 121 443 Scherer O J and Swarowsky H 1987 Angew. Chem. Int. Ed. Engl. 26 1153 Scherer O J, Swarowsky M and Wolmershauser G 1988b Angew. Chem. Int. Ed. Engl. 27 405 Scherer O J, Wiedemann W and Wolmershauser G 1989 J. Organomet. Chem. 361 Cl1-C14 Siebert W 1985 Angew. Chem., Int. Ed. Engl. 24 943 Veillard A 1986 Quantum chemistry: The challenge of transition metals and coordination chemistry (New York: D Reidel) Werner H and Salzer A 1972 Inoro. Met-Org. Chem. 2 239 Wadepohl H, Pritzkow H and Siebcrt W 1982 Organometallics 2 1899 Wilkinson G, Stone F G A and Abel E W 1982 Comprehensive organometallic chemistry (Oxford: Pergamon)