A theoretical investigation of Ziegler-type catalysis. Part I. Soluble catalyst systems

A theoretical investigation of the Cossee mechanism for Ziegler-type catalysis has been made by use of the all valence electron self-consistent field method. The electronic structure of a titanium–aluminium–ethylene complex was studied, as well as the sequence of steps in which ethylene inserts into a titanium–methyl bond. The study of the olefin complex reveals that (a) the titanium–olefin bond has no retrodative character and the antibonding π* orbital of ethylene acquires little stability as a result of interaction with the titanium d orbitals; (b) the titanium–methyl σ-bond is localised almost completely in the highest bonding level of the olefin complex and the contribution to it from the metal atom is almost pure d in character; (c) the > AIR2 group in a molecular catalyst seems merely to function as a substrate which maintains a high co-ordination number at the titanium site. Calculations on the reaction co-ordinate show that the methyl group can migrate from its site to join the ethylene molecule without becoming detached from the titanium atom and that the dyz orbital functions as a ‘transfer agent’. This, and a study of the total energy changes over the co-ordinate, lead to a new rationale of the mechanism of catalysis and the existence of vacant sites.