THE ROLE OF AGOSTIC H -] ZR INTERACTIONS IN THE INSERTION OF OLEFINS WITH METALLOCENE CATALYSTS - AN MO STUDY

The possible role of an alpha-agostic interaction in the course of an olefin insertion at a Ziegler-Natta catalytic centre in [Cp2Zr(C2H4)CH3]+ has been studied by Extended Huckel calculations. It is shown that in a cationic zirconocene catalyst an alpha-agostic H half arrow pointing right Zr contact plays no role in a ground state species but becomes increasingly important along the reaction coordinate for the C-C bond-forming insertion reaction. This can be traced to the weakening of the Zr-C(alkyl) bond and the simultaneous transformation to a more electron deficient zirconium species. In contrast, a beta-agostic interaction in [CP2Zr(C2H4)CH2CH3]+ leads to a stable ground state beta-H half arrow pointing right Zr contact. The agostic interaction of one hydrogen on the alpha-carbon is also compared to an agostic contact of two alpha-hydrogens. The behaviour of orbital energies and the H half arrow pointing right Zr overlap population has been examined as a function of the angles of methyl tilt and olefin shift.