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 conformational preferences of racemic ethylene-bridged bis (indenyl) -type zirconocenes in dichloride and cationic methyl forms. This group of catalysts is characterized by conformational isomerism due to fluxionality of the ethylene bridge resulting in two distinct orientations of the indenyl ligands, indenyl-forward (II) and indenyl-backward (Y), A systematic investigation of the influence of methyl and trialkylsiloxy substitution on the relative stabilities of Pi and Y conformations was performed. For the dichloride catalyst precursors the stabilities and orientations of the equilibrium structures are determined on the basis of repulsive interactions within the ligand framework, In the cationic methyl intermediates of the olefin polymerization catalytic cycle, the electron-rich indenyl ligand appears to reduce the electron deficiency of the cation by tilting towards the metal.