Exceptional substitutional lability of the ‘raft’ cluster Os6(CO)20NCMe1Dedicated to Professor Brian Johnson on the occasion of his 60th birthday in recognition of his outstanding contribution to organometallic and inorganic chemistry.1

The kinetics of reactions of the planar ‘raft’ cluster Os6(CO)20(NCMe) with a wide variety of P-donor ligands, L, in toluene have been studied. Replacement of NCMe by L proceeds via initial, very rapid and reversible formation of the adducts Os6(CO)20(NCMe)L in a pre-equilibrium step which is followed by relatively slow disssociation of the NCMe ligand. The equilibrium and rate constants vary systematically with the electronic and steric natures of the ligands, L, in a way that suggests that bond making by the P-donors in the adducts is comparable with the increase of bond making during the dissociation of NCMe, and that the bonding in the adducts is therefore quite weak. The susceptibility of Os6(CO)20(NCMe) to nucleophilic attack by the P-donors is at least 105 times greater than that of Os3(CO)11(NCMe) while the dissociative lability of the NCMe is less by at least a factor of 100.