Kinetics and mechanisms of halide ion catalysis of CO substitution reactions in Os₃(CO)₁₂ and Ru₃(CO)₁₂ metal carbonyl clusters

Abstract

The results of kinetic studies on ligand substitution in [M₃(CO)₁₁X]⁻ complexes (M = Ru, Os; X = Cl, Br, I) are summarized. The [Os₃(CO)₁₁X]⁻ complexes react with PPh₃ under mild conditions to initially yield monosubstituted products [Os₃(CO)₁₀(PPh₃)X]⁻. The rate of CO substitution obeys a first-order equation with respect to the concentration of the complex and does not depend on the ligand concentration. The rates of the reactions decrease in the order Cl > Br > I withΔH^≠ values increasing from 15 to 18 kcal mol⁻¹ and ΔS ^≠ values varying from −19 to −13 cal mol⁻¹ K⁻¹. The enhanced reactivities of these complexes as well as the low activation energies and negative activation entropies are discussed in terms of the effects of μ-X bridge formation on the transition state of the reaction. Reactions of PPN[Ru₃(CO)_11−x (Cl)] (PPN is the bis(triphenylphosphine)iminium cation;x=0, 1) and PPN[Ru₃(CO)₉(μ₃-I)] with alkynes are also reported. The reactivities of alkynes follow the order BuC≡CH ≥ PhC≡CH ≥ EtC≡CEt ≥ PhC≡CPh. The higher rates of the reactions of monosubstituted acetylenes compared with those of their disubstituted analogs are explained by agostic interaction between the metal atom and the C-H bond in the reaction transition state and by steric effects. The results obtained attest that the reaction with alkynes occursvia intermediates containing halide bridges and that μ₃-halide complexes are more reactive than μ₂-halide complexes.