QALE analysis of CO dissociative kinetics of Ru(CO)4L (L = P-donor ligands): accelerating effects of hydrogen in PHnR(3 - n) ligands (n = 1-2)

Studies of CO-dissociative substitution reactions of the complexes Ru(CO)4L (L = a wide variety of P-donor ligands) have been extended and analysis of the results by the QALE methodology has been refined (QALE = quantitative analysis of ligand effects). Rates increase substantially with increasing size of L, mainly as a consequence of increasingly favourable activation entropies. These can be associated with increasing Ru-CO bond breaking that is compensated enthalpically by increasing Ru-P bond making allowed by release of steric strain. Explicit allowance for pi-acidity shows that these effects are just significant while sigma-donor and aryl effects are negligible. However, pendent hydrogen atoms, attached directly to the phosphorus atoms, have a pronounced and unique positive effect on the rates, with significant kinetic isotope effects (KIE). This is associated with the novel occurrence of direct Ru-H or incipient Ru-(eta2-P-H) agostic bond making as the CO ligand departs.