Labilization of CO Dissociation from Metal Clusters. 2. An Investigation of Substitution Reactions of Bissubstituted Derivatives of Tetrairidium Dodecacarbonyl, Ir₄(CO)₁₀L₂ (L = PPh₃, P(OPh)₃, PBu₃, and AsPh₃)

The reactions of Ir₄(CO)₁₀L₂ with an additional ligand have been studied for L = PPh₃, P(OPh)₃, AsPh₃, and PBu₃. Good pseudo-first-order kinetics were observed for each cluster, and the activation parameters were evaluated for each reaction. The primary reaction pathway involves CO dissociation from the cluster in the rate-determining step. Examination of the rates of CO loss from the series of complexes Ir₄(CO)₁₀L₂, L = PPh₃, P(OPh)₃, AsPh₃, PBu₃, and CO, reveals a dramatic labilization in the order CO < P(OPh)₃ < PBu₃ < PPh₃ ≤ AsPh₃. These ligands span 3 orders of magnitude in the CO dissociation rate. It is proposed that CO dissociation occurs at a substituted metal center with bridging carbonyls transferring the coordinative unsaturation to a second unsubstituted center. The kinetic labilization by the presence of the substituent ligand L arises from a stabilization of the transition state.