QALE analysis of CO dissociative kinetics of Ru(CO)4L (L = P-donor ligands): accelerating effects of hydrogen in PHnR3 − n ligands (n = 1–2)

Abstract

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 π-acidity shows that these effects are just significant while σ-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–(η²-P–H) agostic bond making as the CO ligand departs.