Mechanism of the Substitution Reactions of the Nineteen-Electron Co(CO)3L2 Complex

Article abstract of DOI:10.1021/ja00183a023

The substitution reactions of the Co(CO)3L2 complex with phosphines, phosphites, and iodide were studied.This 19-electron complex (perhaps better described as an 18-electron complex with a reduced ligand) was chosen as a model to study the substitution reactions of more reactive 19-electron complexes.Although a dissociatively activated substitution pathway seemd likely, the molecule has a square-pyramidal geometry and the possibility of associatively activated reactions was also anticipated.The complex reacted with entering ligands at room temperature in CH2Cl2 to form Co(CO)2L2L' .The derivative with L'=PPh3 was structurally characterized by a single-crystal X-ray analysis.The substitution reactions followed first-order kinetics, and the rates were independent of the entering ligand concentration and the nature of the entering ligand.A dissociatively activated pathway involving loss of CO (not chelate dissociation) is proposed.Rate constants as a function of temperature yielded the following activation parameters: ΔH(excit.)=23.8+/-0.6 kcal mol^-1 and ΔS(excit.)=11.1+/-2.2 cal mol^-1 K^-1.For comparison purposes, the substitution reactions of the 18-electron Co(CO)3L2(+) complex were studied.No substitution reactions of this complex took place under the same conditions used for Co(CO)3L2.Thus, the lability of the latter complex can be attributed to the extra electron.ESR studies of Co(13CO)3L2 showed a slight delocalization of the extra electron into (13)CO orbitals (orbitals that are likely Co-CO antibonding); electronic occupation of these orbitals will weaken the Co-CO bond, and we propose this is the origin of the lability.