Conformational preferences of racemic ethylene-bridged bis(indenyl)-type zirconocenes: An ab initio Hartree-Fock study

The ab initio Hartree-Fock method has been utilized to rationalize the conf ormational preferences of racemic ethylene-bridged bis (indenyl) -type zirc onocenes in dichloride and cationic methyl forms. This group of catalysts i s characterized by conformational isomerism due to fluxionality of the ethy lene bridge resulting in two distinct orientations of the indenyl ligands, indenyl-forward (II) and indenyl-backward (Y), A systematic investigation o f the influence of methyl and trialkylsiloxy substitution on the relative s tabilities of Pi and Y conformations was performed. For the dichloride cata lyst precursors the stabilities and orientations of the equilibrium structu res are determined on the basis of repulsive interactions within the ligand framework, In the cationic methyl intermediates of the olefin polymerizati on catalytic cycle, the electron-rich indenyl ligand appears to reduce the electron deficiency of the cation by tilting towards the metal.