Ligand substitution processes in tetranuclear carbonyl clusters. 7. Molecular structure and carbon monoxide exchange processes of Co4(CO)9(tripod), tripod = 1,1,1-tris(diphenylphosphino)methane or HC(PPh2)3

Article abstract of DOI:10.1021/om00086a012

A kinetic investigation of intermolecular carbon monoxide exchange reactions of Co₄(CO)₉(tripod) (tripod = 1,1,1-tris(diphenylphosphino)methane) with ¹³C-labeled carbon monoxide is reported. The effect of incoming CO ligand concentration was examined by performing reactions under a pressure of carbon monoxide (up to 160 psi of ¹²CO) employing Co₄(¹³CO)₉(tripod) as substrate molecule. No dependence of the rate of CO incorporation into the tetranuclear cluster with carbon monoxide pressure was noted. Carbon monoxide substitutional parameters (rate constants and activation parameters) for CO displacement in the Co₄(CO)₉(tripod) species were found to be quite similar to those previously observed in the parent Co₄(CO)₁₂ and its monoligated phosphorus derivatives. These observations taken in toto imply a common mechanistic pathway for CO substitution in all these derivatives, i.e., a CO dissociative process. Reactions carried out utilizing phosphines and phosphites as incoming ligands were found to be complicated by ligand-dependent pathways for CO substitution. Phosphorus ligand substitution occurs stereoselectively at the apical cobalt sites. The crystal and molecular structure of Co₄(CO)₉(tripod) has been determined and is discussed in terms of the influence of structural factors on the reactivity of this species as well as in terms of the matching of the tripod ligand framework with the triangular face of the Co₄(CO)₁₂ moiety. The title compound forms monoclinic crystals in space group I2/a with a = 21.943 (6) ?, b = 17.289 (7) ?, c = 26.137 (4) ?, β = 105.14 (2)°, V = 9585 (9) ?³, and Z = 8. The discrepancy indices were R₁ = 0.052 and R₂ = 0.051 for 3001 reflections with I > 3σ(I).