Kinetics of CO substitution in Co₄(CO)₁₂ and Rh₄(CO)₁₂

The kinetics of rapid CO substitution by PPh₃ in Co₄(CO)₁₂ and Rh₄(CO)₁₂ have been examined by stopped-flow and low temperature FT-IR methods. In Co₄(CO)₁₂ rapid (k_obs ∼ 1.8 s^-1) substitution of CO occurs after a 1-15 s induction period at 28 °C in C₆H₅Cl solvent by a catalytic process. Addition of PPh₃ to Rh₄(CO)₁₂ yields Rh₄(CO)₁₁(PPh₃) according to a predominantly second order rate law k₁[Rh₄- (CO)₁₂] + k₂[Rh₄(CO)₁₂][PPh₃] with k₁ = 25 ± 11 s^-1 and k₂ = 2.97 ± 0.27 X 10⁴ M^-1 s^-1 at 28 °C. Substitution of a second CO ligand also occurs rapidly with k₁ = 0.15 ± 0.09 s^-1 and k₂ = 6.54 ± 0.07 X 10² M^-1 s^-1 at 28 °C. The reactivity of Rh₄(CO)₁₂ toward associative substitution is 10⁴- 10¹¹ faster than for the Co and Ir analogues, In Rh₄(CO)₁₁(PPh₃) the increase in CO substitution rates over Co and Rh analogues is 10²-10⁷. The ordering of associative substitution rates Co << Rh >>> Ir in these clusters exaggerates the trend seen in mononuclear metal complexes.