Reaction mechanisms of metal–metal-bonded carbonyls. Part 19. Homolytic fission of bis[tetracarbonyl(triphenylphosphine)manganese]-(Mn–Mn) as a path for thermal substitution

Abstract

The complex [{Mn(CO)₄(PPh₃)}₂] reacts with P(OPh)₃ in cyclohexane at 40–50 °C to form [Mn₂(CO)₈(PPh₃)-{P(OPh)₃}]. A detailed study of the dependence of the rate on the concentrations of complex, P(OPh)₃, and PPh₃ shows that the kinetics are fully consistent with a mechanism involving initial, reversible, homolytic fission. This is followed by reversible bimolecular substitution of PPh₃ in [Mn(CO)₄(PPh₃)] by P(OPh)₃ before final formation of the product by combination of two unlike radicals. This is the first clearly demonstrated example of a mechanism of this type operating in a simple thermal-substitution reaction.