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 H2 to Vaska-type complexes, transIrX(CO)(PR3)2, are investigated with ab initio quantum chemical calculations. The direction of addition in these complexes is controlled by how the ligands in the plane od 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-, H2 adds in the Cl-Ir-Co plane. When X=Hor Ph-, however, H2 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.