Reaction mechanisms of metal–metal bonded carbonyls. Part I. Some reactions of manganese decacarbonyl

Abstract

The substitution reaction of manganese decacarbonyl with triphenylphosphine, its reaction with iodine to form iodopentacarbonylmanganese, and its decomposition in the presence or absence of oxygen, have been studied over a wide temperature range in inert solvents. The kinetic behaviour is, in almost all cases, very good. Partial inhibition of the substitution by carbon monoxide suggests that it can proceed in part (about 27%) by a dissociative mechanism, but the similarity of the residual reaction to the oxidation reactions shows that another mechanism is more important. The form of the kinetic results is in excellent agreement with this mechanism involving rate-determining homolytic fission of the Mn–Mn bond with an Arrhenius activation energy of 37·0 ± 0·27 kcal./mole, and this value can be reconciled with recent mass spectrometric results. A value of ca. 27 kcal./mole for the activation energy of recombination of Mn(CO)₅ radicals is derived from the data but, although this can be understood in terms of a relatively stable bipyramidal structure for the radicals, it leads to an exceptionally high value of the pre-exponential term for this reaction. Alternative mechanisms are discussed and the value of 37 kcal./mole is assigned as a lower limit for the energy needed to produce homolytic fission. The reaction with iodine follows a pseudo-first-order rate law: k_obs=k_a+k_b[I₂]. k_a is equal to the rate constant for decomposition in oxygenated solution, and the activation energy for the bimolecular path is 31 kcal./mole.