THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS .4. OXIDATIVE ADDITION OF DIHYDROGEN TO D8 SQUARE-PLANAR IRIDIUM COMPLEXES WITH TRANS PHOSPHINES

The oxidative addition of H-2 to Vaska-type complexes, trans-IrX(CO)(PR3)2, is investigated with ab initio quantum chemical calculations. The direction of addition in these complexes is controlled by how the ligands in the plane of addition react to a close encounter with concentrations of charge density around the metal center as the complex evolves from a four-coordinate to a six-coordinate species. Strong electron-donating ligands destabilize the five-coordinate transition state while electron-withdrawing ligands stabilize the transition state. When X is a weak electron donor ligand such as Cl-, H-2 adds in the Cl-Ir-CO plane. When X = H- or Ph-, however, H-2 adds in the PR3-Ir-PR3 plane; the destabilizing influence of these strong electron-donating ligands on the transition state outweigh those of the PR3 ligands. The electronic contribution to the relative stabilities of the six-coordinate final products can be predicted based on the relative orientations of the strongest trans-influence ligands. The isomers in which these ligands are facial are lower in energy than those in which they are meridional.