SYNTHESIS AND FRAGMENTATION KINETICS OF TETRAKIS(TRIPHENYLPHOSPHINE)OCTACARBONYLTETRACOBALT

Article abstract of DOI:10.1016/S0022-328X(00)83410-5

Reaction of Co4(CO)12 with triphenylphosphine is shown to proceed at room temperature in n-heptane or 1,2-dichloroethane (DCE) to form Co4(CO)8(PPh3)4 which has been isolated and characterized by analysis and IR spectroscopy.The complex is rather unstable to air and light, especially in solution.Its IR spectra show that Co4(CO)8(PPh3)4 probably exists as an equilibrium mixture of forms containing symmetrically bridging and semibridging CO groups, the form with semibridging carbonyls being favored by steric effects and by DCE as a solvent as compared to n-heptane.The kinetics of fragmentation in DCE to form Co2(CO)6(PPh3)2 have been studied and shown to proceed by three paths that involve, in order of decreasing ease, rate-determining PPh3 dissociation, CO dissociation, and spontaneous activation without loss or gain of ligands in forming the transition state.The activation parameters for spontaneous fragmentation suggest that the degree of weakening of the bonding within the Co4 cluster is quite high in the transition state and that the Co4 cluster is weaker than the Ru3 cluster in Ru3(CO)9(PPh3)3.The complexes Co4(CO)84, Co4(CO)8(dppm)2 (dppm = Ph2PCH2PPh2), and probably Co4(CO)8(P-n-Bu3)4 are shown to be much more stable to fragmentation than Co4(CO)8(PPh3)4 and the instability of the latter is ascribed to the steric effect of its large substituents.