Kinetics of CO substitution in Co 4(CO) 12 and Rh 4(CO) 12

Abstract

The kinetics of rapid CO substitution by PPh 3 in Co 4(CO) 12 and Rh 4(CO) 12 have been examined by stopped-flow and low temperature FT-IR methods. In Co 4(CO) 12 rapid ( k obs ∼ 1.8 s −1) substitution of CO occurs after a 1–15 s induction period at 28 °C in C 6H 5Cl solvent by a catalytic process. Addition of PPh 3 to Rh 4(CO) 12 yields Rh 4(CO) 11(PPh 3) according to a predominantly second order rate law k 1[Rh 4- (CO) 12] + k 2[Rh 4(CO) 12][PPh 3] with k 1 = 25 ± 11 s −1 and k 2 = 2.97 ± 0.27 X 10 4 M −1 s −1 at 28 °C. Substitution of a second CO ligand also occurs rapidly with k 1 = 0.15 ± 0.09 s −1 and k 2 = 6.54 ± 0.07 X 10 2 M −1 s −1 at 28 °C. The reactivity of Rh 4(CO) 12 toward associative substitution is 10 4– 10 11 faster than for the Co and Ir analogues, In Rh 4(CO) 11(PPh 3) the increase in CO substitution rates over Co and Rh analogues is 10 2–10 7. The ordering of associative substitution rates Co << Rh >>> Ir in these clusters exaggerates the trend seen in mononuclear metal complexes.