Systematic substitution kinetics of the clusters Ru3(CO)11L and Ru3(CO)10L2 (L = P-donor ligands): The unimolecular path

Abstract

The kinetics have been studied of the substitution reactions of Ru3(CO)11L and Ru3(CO)10L2 (L = PCy3 (Cy = C6H11), P-n-Bu3, P(p-MeOC6H4)3, P(p-ClC6H4)3, P(OPh)3 and etpb (P(OCH2)3CEt) with AsPh3 or various P-donor nucleophiles, L′, in hexane. The reactions proceed through both [L′]- independent (CO dissociative) and [L′]-dependent (associative) paths. The values of the rate constants, k1, for the dissociative path for Ru3(CO)11L can be combined with other published data and are found to fit well to the equation: log k1 = α + βLδL + γLθL (βL and γL are constants that define how sensitive the values of log k1 are to Bodner's electronic parameter, δL, and to Tolman's steric parameter, θL, respectively.) Values of (βL and γL are both positive, increasing electron donicity and increasing size of L both leading to increased rates. The steric effect is more important and accounts for ca. 60% of the decrease in the free energies of activation. The effects, on the fit and on the values of βL and γL, of using different electronic (Bartik's χ values) and steric (Brown's ER values) parameters are examined and found to be small, and the question of which goodness-of-fit parameters should be used is considered. The kinetic effects of the substituents are only a small (ca. 20%) perturbation on the average free energies of activation, and the steric effect shows that use is made of only a few percent of the available ligand–ligand repulsion energy in accelerating the reactions. The second-order reactions of M3(CO)11L with Mc3NO (M = Ru or Os) also fit an equation analogous to that given above and the values of βL and γL obtained for these assisted CO displacement reactions are compared with those for the unimolecular, unassisted, CO dissociation. Unassisted reactions of the Ru3(CO)10L2 clusters show values of βL and γL that are closely similar to those for Ru3(CO)11L, but published data for CO dissociative reactions of some Ru3(CO)9L3 clusters lead to much larger positive values of βL and a large inverse steric effect, i.e., rates actually decrease substantially with increasing substituent size after electronic effects are allowed for. This effect is accounted for in terms of the different natures of the Ru3(CO)8L3 intermediates. Keywords: metal carbonyls, ruthenium, clusters, kinetics, substituent effects.