Addition and elimination of hydrogen and ligand substitution on triosmium clusters. Kinetics and mechanism

The kinetics of the reversible reaction HOs₃(μ-COMe)(CO)₁₀ + H₂ ⇌ H₃Os₃(μ³-COMe)(CO)₉ + CO has been investigated. The reaction of HOs₃(μ-COMe)(CO)₁₀ with hydrogen involves dissociation of a CO ligand prior to the rate-determining step, which is proposed to be the oxidative addition of molecular hydrogen. The reaction of H₃Os₃(μ₃-COMe)(CO)₉ with CO involves rate-limiting hydrogen loss. The equilibrium constant and the competition ratio for hydrogen and CO for the unsaturated intermediate were determined. The mechanism of substitution by AsPh₃ on HOs₃(μ-COMe)(CO)₁₀ also involves a CO dissociative mechanism. Based upon relative rate constants for CO, AsPh₃, and hydrogen addition to HOs₃(COMe)(CO)₉, CO dissociation and hydrogen addition are proposed to occur at different metal sites.