Reaction mechanisms of metal–metal bonded carbonyls. Part II. The insertion reaction of stannous halides with hexacarbonylbis-(tri-n-butylphosphine)dicobalt

Abstract

The kinetics of the thermal and photochemical reactions between SnX₂(X = Cl and Br) and the metal–metal bonded dimer [Co(CO)₃PBu₃]₂ to form the complex [Co(CO)₃PBu₃]₂SnX₂ have been studied. Both types of reaction proceed through reactive intermediates but these are not the same in each case. The formation of the intermediate in the thermal reactions requires an activation enthalpy of 26·4 ± 0·7 kcal./mole, and an activation entropy of +5 ± 2 cal. mole^–1 deg.^–1. This value of ΔH‡ can be taken as a lower limit for the enthalpy necessary to break the Co^–Co bond since the kinetics are in agreement with the reactive intermediates' being a pair of geminate Co(CO)₃PBu₃ radicals trapped together in a solvent cage, although other formulations are possible.

The thermal reaction with stannous bromide proceeds mainly by a bimolecular mechanism with kinetic parameters, ΔH‡ and ΔS‡, being 21·6 ± 0·6 kcal./mole and –2·5 ± 1·7 cal. mole^–1 deg.^–1, respectively.