Probing the validity of the -CH₂SiMe₃ group as a model of the growing chain in mechanistic studies of olefin polymerization with group 4 catalysts

Using a DFT approach we investigated the geometry and the stability of cationic bisindenyl zirconocenes paired up with a counterion and of general formula [rac-Me₂Si(1-Ind)₂ZrR]⁺/[MeB(C ₆F₅)₃]^- and [rac-Me ₂Si(1-Ind)₂ZrR]⁺/[B(C₆F ₅)₄]^-, R = -CH₂SiMe₃ or -CH₂CHMe₂ groups. The two R groups are differently agostic-bonded to the metal. The calculations evidence rather good similarity when systems bearing the two different R groups are compared in terms of geometry, relative energies, and the ion-pair separation energies. They validate to a large extent the -CH₂SiMe₃ group as a model of the growing chain in catalytic olefin polymerizations. However, they also underline some geometric differences that should be considered when mechanistic schemes are developed from experiments.