Zirconocene catalysts: Ion-pairs, zwitterions, or weakly bound molecules?

The bonding between electron deficient zirconocene complexes such as [(eta( 5)-C5Me5)(2)ZrR](+) and the counterions [(C6F5)(3)BR](-) (R = CH3, 1, R = H , 2) has been investigated by DFT and extended Huckel calculations. The mol ecular bonding analysis has been carried out in the light of the structural information on the inter-ion interactions available for these and other si milar electron deficient coordinatively unsaturated zirconocene complexes. It has been shown that the "anion-cation" interactions in 1 and 2 are diffe rent manifestations of the same need to provide electron density to the met al centre. In 1 this is achieved via the C-H sigma-bonds and has been descr ibed as a pseudo-agostic interaction, whereas in 2 electron donation occurs via the phenyl F-atoms. The study of the covalent component of the bonding between the ionic components has shown that the weak electron donation, th ough not sufficient to saturate the metal centre, stabilises the unsaturate d species and allows it to have a longer life time. It has been inferred th at in the homogenous Ziegler-Natta catalytic process the approach of a stro ngly coordinating molecule, such as an olefin in the polymerisation, destro ys the weak interaction and allows the reaction to proceed whilst the ion-p airing maintains the counterion in proximity of the complex.