Labilization of CO Dissociation from Metal Clusters: An Investigation of the Substitution Reactions of Monosubstituted Derivatives of Tetrairidium, Ir₄(CO)₁₁L (L = PBu₃, PPh₃, AsPh₃, and P(OPh)₃)

The reactions of Ir₄(CO)₁₁L with additional ligand have been investigated for L = PPh₃, PBu₃, AsPh₃, and P(OPh)₃. Good first-order kinetics are seen for each complex, except when PBu₃ is the entering ligand. The PBu₃ reactions apparently proceed by ligand attack. Activation parameters are evaluated for each reaction. The predominant reaction mode is CO dissociation. Examination of the rates of CO dissociation from the series of compounds Ir₄(CO)₁₁L (L = CO, PBu₃, PPh₃, P(OPh)₃, and AsPh₃) shows a remarkable labilization in the order CO < P(OPh)₃ < AsPh₃ < PPh₃ < PBu₃. These ligands span two orders of magnitude in the CO dissociation rate. It is suggested that this labilization of dissociation by the ligand L occurs at the substituted metal with bridging carbonyls transferring the coordinative unsaturation to a different iridium. This kinetic labilization probably arises from a transition-state stabilization.