47-Electron organometallic clusters derived by chemical and electrochemical oxidation of trihydrido(alkylidyne)triruthenium clusters: II. Disproportionation mechanism for decomposition

The decomposition of 47-electron clusters, generated by chemical or electrochemical oxidations of 48-electron H3Ru3(μ3-CX)(CO)9−nLn (X=OMe; L=PPh3; n=0–3: X=OMe, SEt; L=dppm; L=PPh3; n=3: X=SEt, NMeBz; L=PR3, SbPh3; n=2,3) occurs by disproportionation back to the 48-electron precursor and very unstable 46-electron species. Both 47/48- and 46/47-e redox potentials display similar ligand additivity trends, which are correlated with HOMO energies determined by Fenske–Hall MO calculations. Decomposition of [H3Ru3(μ3-COMe)(CO)6(PPh3)3]1+ in the presence of added PPh3 forms 48-e H3Ru3(μ3-COMe)(CO)6(PPh3)3, 46-e [H3Ru3(CO)7(PPh3)3]1+, and [MePPh3]1+; the rate law is second order with respect to concentration of the 47-e cluster and displays a small dependence on PPh3 concentration. Decompositions of [H3Ru3(μ3-COMe)(CO)7(PPh3)2]1+, [H3Ru3(μ3-CSEt)(CO)7(dppm)]1+, and [H3Ru3(μ3-CSEt)(CO)6(PPh3)3]1+ are also second order, although only the 48-e product could be characterized. The mechanism is proposed to involve rate-limiting outer-sphere electron transfer, the slow rate of disproportionation is a consequence of the fact that the reaction involves transfer of an electron from one half-filled bonding orbital to another half-filled bonding orbital, thus requiring considerable reorganization energy. Electrochemical studies of the 2-e oxidations of H3Ru3(μ3-COMe)(CO)9−n(PPh3)n, n=0 and 1, indicate that the initial 46-e cluster rearranges very rapidly to a new 46-e species, which decomposes rapidly to electrochemically inactive products.