Effects of Mixed-Metal Composition upon the Kinetics and Thermodynamics of Cluster Reactions. Preparation of Hru_3-N Osn(μ-Come)(Co)₁₀and H₃Ru_3-nOs_n(μ₃-COMe)(CO)₉ (n = 1 or 2) and Their Relative Reactivities toward Ligand Substitution and Reductive Elimination of Hydrogen

The new clusters HRu₂Os(μ-COMe)(CO)₁₀ H₃Ru₂Os(μ₃-COMe)(CO)₉, HRuOs₂(μ-COMe)(CO)₁₀, and H₃RuOs₂(μ₃-COMe)(CO)₉ have been synthesized and characterized by spectroscopic methods. Retention of the mixed-metal core during hydrogenation of HM₂M'(μ-COMe)(CO)₁₀ and carbonylation of H₃M₂M'(μ₃-COMe)(CO)₉ proves that cluster fragmentation does not occur during these reactions. Competition experiments were used to determine relative rates of hydrogenation of HM₃(μ-COMe)(CO)₁₀ to H₃M₃(μ₃-COMe)(CO)₉, attributed to the relative rates of CO dissociation, as M₃= Ru₃ (4500) > Ru₂Os (1100) > Ruos₂(220) ≫ Os₃(1) at 343 K. Direct measurements of the rate constants for carbonylation of H₃M₃(μ₃-COMe)(CO)₉ to HM₃(μ-COMe)(CO)₁₀ give relative rates: M₃= Ru₃ (210) > Ru2Os (72) > RuOs₂(38) ≫ Os₃(1) at 353 K. The similarity of the rates of carbonylation for all clusters having at least one Ru atom, but a greatly reduced rate for the Os₃ cluster, suggests that reductive elimination of dihydrogen occurs from a single metal atom in the transition state, even though the hydride ligands bridge two metal atoms in the ground state. Equilibrium constants for hydrogenation and for isomerization of HM₂M'-(μ-COMe)(CO)₁₀ were used to obtain estimates of the Ru-H-Ru (408 kJ/mol) and Os-H-Os (425 kJ/mol) bond energies and for the differences in bond energies {E(RuHOs)-E(RuOs)} (299 kJ/mol), |E(Os2(μ-C))-E(Ru₂(μ-C))| (42 kJ/mol), and {E(Os2(μ-C))-E(Ruos(M-C)))(24 kJ/mol), based upon the assumptions that {E(M2(μ-C))-E(M3(μ3-C))|= 0 (M = Ru or Os), {E(Os3(μ3-C))-E(RuOs₂(μ₃-C))} = (1/3){E(Os3(μ3-C))-E(Ru3(μ3-C))}and {E(Os3(μ3-C))-E(Ru2Os(M3-C))} = (2/3){E(Os3(M3-C))-E(Ru₃(μ₃-C))}.