Metal and ancillary ligand structural effects on ethylene insertion processes at cationic group 4 centers. A systematic, comparative quantum chemicalinvestigation at various ab initio levels

Ethylene insertion into the metal-methyl bonds of group 4 (Ti, Zr) (C5H5)(2 )MCH3+ and H2Si(C5H4)((BuN)-Bu-t)MCH3+ catalyst cations has been investigat ed at the ab initio level, employing DZV- and DZP-quality basis sets togeth er with Moller-Plesset perturbative and coupled-cluster single-double excit ation wave function expansions. All reactions are found to proceed from rea ctants to products via intermediate.,pi -complexes and subsequent Cossee-Ar lman four-center transition state structures. Enthalpic barriers for the in sertion step strongly depend on the nature of the ancillary ligand and meta l, with DeltaH(double dagger) increasing in the order (C5H5)(2)TiCH3+ < (C5 H5)(2)ZrCH3+ approximate to H2Si(C5H4)((BuN)-Bu-t)TiCH3+ < H2Si(C5H4)((BuN) -Bu-t)ZrCH3+. Furthermore, metallocene H2Si < bridging has the effect of in creasing the electrophilicity toward ethylene. The observed ethylene activa tion/insertion structural and energetic trends may be rationalized using qu alitative electronic structure arguments, ancillary ligand steric hindrance , and metal ionic radius. Electron correlation effects are found in all cas es to play a crucial role in predicting reaction energetics. Reasonable, in cremental convergence in computed energies is obtained for Zr systems and f or the H2Si(C5H4)((BuN)-Bu-t)TiCH3+ cation upon increasing the calculationa l level (MP2 --> MP3 --> MP4-SDQ --> CCSD). In contrast, fluctuations in re sults are found for the (C5H5)(2)TiCH3+ cations, indicating the desirabilit y of high-level calculations.