THE ZIEGLER NATTA OLEFIN INSERTION REACTION INTO A METAL ALKYL BOND FOR 2ND-ROW TRANSITION-METAL ATOMS

The Ziegler-Natta olefin insertion reaction into a metal-alkyl bond has been studied for the second-row transition metal atoms, without additional ligands and with two hydride ligands added. Correlation effects of all valence electrons are included in the calculations using reasonably large basis sets. The results are compared to the corresponding insertion into a metal-hydride bond which has been studied previously. For most metals the barrier height for the olefin insertion into the metal-alkyl bond is about 20 kcal/mol higher than for insertion into the metal-hydride bond. The origin of this difference is the directional character of the methyl bond. The hydride can continuously change the direction of its bond from the metal to the olefin, but for the methyl group a large part of the metal-methyl bond has to be broken before the methyl group can start to bind towards the olefin. For the reaction mechanism, the d character of the bond at the transition state and the importance of repulsion between non-bonding electrons on the metal and the electrons on the olefin is emphasized.