Behind the Scenes of Group 4 Metallocene Catalysis: Examination of the Metal–Carbon Bond

Abstract

This contribution provides the first detailed analysis of the nature of the M–C σ-bond of three alkylated, isostructural group 4 (M = Ti, Zr, Hf) metallocenes, thereby elucidating individual peculiarities of each metal center in the catalytic conversion of olefins. Therefore, the subtle electronic differences of the individual M–C σ-bonds, which are considered crucial for several subprocesses in the coordinative polymerization of olefins, were examined by detailed experimental charge density studies. These studies provided measures of the increasing ionic character of the M–C bonds along the group 4 elements (Ti–C < Zr–C < Hf–C). These results are further supported by high-pressure diffraction studies showing that the predominantly ionic Hf–C bond is more compressible than the more covalent Zr–C bond in line with a smaller degree of electron localization in the valence shell of the hafnium relative to the zirconium atom along the M–C bond directions. The Ti–C bond displays the largest degree of electron l...