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 e lectrons and using extended atomic basis sets, were performed on squar e-pyramidal (SP) models of the active center in a heterogeneous titani um 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 bindi ng energies between 7 and 11 kcal/mol and a titanium-ethylene bond dis tance of 2.80 angstrom at the MCPF level. Allowing for relaxation of t he SP active center to a trigonal-bipyramidal geometry makes ethylene coordination unfavorable. Only minor differences were found when compa ring the ethylene binding energies in Ti(IV) and Ti(III) model complex es. Formaldehyde, included to represent carbonyl-type moderators, has 15-20 kcal/mol higher binding energy than ethylene in complexes such a s 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.