Molecular orbital study of stereoselective addition of ligands to the hydride, alkyl, and halide complexes of iridium and zirconium. The question of central versus lateral attack

Abstract

Electronic structures of Ir(N(SiH 3) 2)(PH 3) 2H 2, Cp 2ZrMe 2, Cp 2ZrH 2, and Cp 2ZrCl 2 complexes and stereoselectivity of CO addition to each of them are studied by the nonparametrized Fenske-Hall method. A new measure of the interaction between reactants, designated Δ E, is defined and used to calculate energy profiles of the carbonylation reactions. Stereoselectivity seems to be controlled by the composition and localization of the frontier orbitals. The experimental observations are explained successfully: the first two complexes undergo lateral attack, the third one undergoes central attack, and the fourth one is unreactive. The unreactivity of the chloro complex is attributed to the repulsion between the lone pairs of the Cl and CO ligands. The nature of the frontier orbitals, and consequently the preferred approach of an incoming ligand, depends markedly on the LZrL angle: the LUMO is localized laterally when the angle is acute and centrally when it is obtuse. Experiments are proposed that may test the correlation between the facility of ligand-addition reactions and the localization of the frontier orbitals.