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-nL_n (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(PPh ₃)₃]¹⁺ in the presence of added PPh₃ forms 48-e H₃Ru₃(μ₃-COMe)(CO)₆(PPh ₃)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(PPh ₃)₂]¹⁺, [H₃Ru₃(μ₃-CSEt)(CO)₇(dppm)] ¹⁺, and [H₃Ru₃(μ₃-CSEt)(CO)₆(PPh ₃)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(PPh ₃)_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.