Kinetics and mechanism of reductive elimination of C-H bonds from (μ-H)₃Ru₃(μ₃-CX)(CO)₉ revisited: CO associative, CO independent, or CO dissociative?

The kinetics of the isomerizations of (μ-H)₃Ru₃(μ₃-CCO₂Me)(CO) ₉ to (μ-H)₂Ru₃(μ₃-η²-CHCO ₂-Me)(CO)₉ and of (μ-H)₃Ru₃(μ₃-CSEt)(CO)₉ to (μ-H)Ru₃(μ₃-η²-CH ₂SEt)(CO)₉, including the measurements of activation volumes, are reported. An earlier study of reductive elimination of CH₃X from H₃Ru₃(μ₃-CX)(CO)₉ found that the reaction was promoted by CO for X = Ph, Cl, and Et and was unaffected by CO pressure for X = CO₂Me. The activation volume ΔV^‡ for the intramolecular isomerization of (μ-H)₃Ru₃(μ₃-CCO₂Me)(CO) ₉ to (μ-H)₂Ru₃(μ₃-η²-CHCO ₂-Me)(CO)₉ was determined to be -0.3(0.7) cm³/mol at 57.0 °C. For the isomerization of (μ-H)₃Ru₃(μ₃-CSEt)(CO)₉ to (μ-H)Ru₃(μ₃-η²-CH ₂SEt)(CO)₉, the rate is inhibited by CO; activation parameters (ΔH^‡=121(3) kJ/mol, ΔS^‡= +36(10) J/(mol K), ΔV^‡= +22.0(1.4) cm³/mol) are consistent with a mechanism involving reversible CO dissociation prior to the rate-determining step but following an intramolecular rearrangement. The change in mechanism of reductive elimination of a C-H bond from CO associative to CO independent to CO dissociative is due to anchimeric assistance by the methylidyne substituent. These results may have relevance to C-H formation occurring on metal surfaces.