STRUCTURE AND BONDING IN GROUP-4 METALLOCENE ACETYLIDE AND METALLACYCLOPENTADIENE COMPLEXES

Ab initio methods (STO-3G basis set) have been used to obtain structures for CP2Ti(C=CR)2 (R = Me, H) and Cp2TiCH=CHCH=CH. Both ab initio (3-21G and 6-31G basis sets) and semiempirical (ZINDO) calculations were carried out on the optimized structures to examine the energies and atomic orbital compositions of the complexes' molecular orbitals. Evidence for extensive mixing between alkynyl pi and "Cp2Ti" bonding orbitals is seen in these calculations. The proposed mechanism for this mixing involves interactions between filled acetylide pi and Cp-Ti bonding orbitals. Very little evidence is seen for pi donation from filled acetylide orbitals to vacant metal-based orbitals. In addition very little mixing is observed between the ligand pi orbitals and the Cp-Ti bonding orbitals Of (CP2TiCH=CHCH=CH). The lack of mixing in both of these situations arises from poor energy matching of the interacting orbitals, since the degree of orbital overlap in both cases is similar to that observed for CP2Ti(C=CR)2.