Kinetic and mechanistic studies of carbon monoxide substitution processes in the tetranuclear metal clusters Co4-(CO)12-n[P(OCH3)3]n (n = 0, 1, 2) are reported. Entering ligands employed in these investigations were trimethyl phosphite, triphenylphosphine, or 13CO. The CO substitutional rates in Co4(CO)12 with P(OCH3)3 or P(C6H5)3 were too rapid to be studied by conventional techniques and were unaffected by the addition of the radical scavenger 2, 6-di-tert-butylphenol; nevertheless, the rate of 13CO incorporation into Co4(CO)12 occurs at only a moderate velocity. The exchange reaction between 13C-labeled carbon monoxide and Co4(CO)12 has allowed for a detailed analysis of the v(CO) stretching vibrations for the bridging carbonyl ligands. Although the rate of reaction of Co4(CO)11[P(OCH3)3] with P(OCH3)3 to afford Co4(CO)10[P(OCH3)3]2 has been shown to be strongly dependent on the concentration of trimethyl phosphite, the comparable rate for formation of the mixed-ligand complex Co4(CO)10[P(OCH3)3][P(C6H5)3] from Co4(CO)11[P(OCH3)3] and triphenylphosphine follows a first-order dissociative path. Rate constants and activation parameters for dissociative CO loss in Co4(CO)12-n[P(OCH3)3]n (n = 0, 1, 2) were found not to be significantly dependent upon the value of n. 1980, American Chemical Society. All rights reserved.
