Kinetics and Mechanism of Reversible Oxidative Addition of Hydrogen across the Metal−Metal Bond of (μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2. Steric Promotion of Metal−Metal Bond Cleavage But a CO Dissociative Mechanism

The kinetics of the reaction (μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2 + H2 ⇄ (μ-H)2Ru3(CO)8(H)2(μ-P(t-Bu)2)2 have been studied. The reaction of (μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2 with H2 has a rate law which is first-order in cluster concentration and in hydrogen pressure and inverse order in CO pressure; on the basis of the rate law, activation parameters, and deuterium kinetic isotope effect, hydrogen addition is proposed to involve rapid, reversible dissociation of a carbonyl ligand, followed by rate-determining oxidative addition of hydrogen through a three-center transition state at a single metal atom. Loss of hydrogen from (μ-H)2Ru3(H)2(CO)8(μ-P(t-Bu)2)2 also involves reversible loss of a carbonyl, followed by rate-determining reductive elimination of molecular hydrogen. The reaction is highly sensitive to the steric bulk of the phosphido substituents, as (μ-H)2Ru3(CO)8(μ-PR2)2, R = cyclohexyl and phenyl, do not react with hydrogen. In addition, the rate of exchange with 13CO is much faster for R = t-Bu than for R = cyclohexyl. Based upon the temperature dependence of the equilibrium constant for hydrogenation, the energy for the unbridged Ru−Ru bond of (μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2 is estimated to be 47−59 kJ/mol, the low value being attributed to steric strain.