STRENGTH OF THE METAL-OLEFIN BOND IN TITANIUM COMPLEXES RELATED TO ZIEGLER-NATTA CATALYSIS - A THEORETICAL-MODEL STUDY OF A SQUARE-PYRAMIDAL ACTIVE-CENTER POSTULATED TO BE FOUND IN TITANIUM HALIDE BASED CATALYSTS

Calculations, generally including correlation effects of all valence electrons and using extended atomic basis sets, were performed on square-pyramidal (SP) models of the active center in a heterogeneous titanium halide based Ziegler-Natta catalyst. All our calculations reveal a weak complexation between ethylene and titanium. Coordinating ethylene to the SP active center, TiX5n- (X = Cl, F, H; n = 1, 2), gives binding energies between 7 and 11 kcal/mol and a titanium-ethylene bond distance of 2.80 angstrom at the MCPF level. Allowing for relaxation of the SP active center to a trigonal-bipyramidal geometry makes ethylene coordination unfavorable. Only minor differences were found when comparing the ethylene binding energies in Ti(IV) and Ti(III) model complexes. Formaldehyde, included to represent carbonyl-type moderators, has 15-20 kcal/mol higher binding energy than ethylene in complexes such as TiF2H2(CH2O)(C2H4). Our results are compared to recent calculations on various model homogeneous catalysts, and special attention is paid to the charge and rigidity of the active centers.