STABILIZING INTERACTIONS AND COORDINATION IN CATIONIC ZIRCONOCENE COMPLEXES - CP2ZRL+ - A MO THEORETICAL-STUDY

The results of ab initio RHF and RMP2 geometry optimization on the model compounds of the postulated intermediates in Ziegler-Natta catalysis Cl2ZrCH2CH3+ (3) and Cl2ZrCH=CH2+ (4) and for comparison on three neutral complexes Cl2Zr(X)CH=CH2, X = H (5), CH3 (6), Cl (7), show that these species are stabilized through a three-center 2-electron Zr-H(beta)-C interaction. In 5-7 this interaction is regiospecific and takes place either cis (5a) or trans (6b, 7b) to the Zr-X bond, depending on the steric repulsion between X and H(beta). The examination of the potential energy surface on the basis of extended Huckel calculations on substituted species Cp2ZrC(R1)=CH(R2)+, R1 = SiMe3 (8), CMe3 (9) and R2 = H (a), Me (b), SiH3 (c), CMe3 (d), SiMe3 (e), Ph (f), reveals that 8 and 9 can be stabilized through either gamma-Si (C)-C or beta-CH agostic interactions, depending on the subtle balance between the electronic and steric effects on R1 and R2.