Redox reactions between halides and reducing metal complexes in non-aqueous solution. Part 2.—Derivatives of molybdenum carbonyl

Abstract

Molybdenum carbonyl undergoes ligand exchange with vinyl monomers such as methyl methacrylate (mma) to produce complexes of the type Mo(CO)₅(mma)(I) which readily enter into redox processes with organic halides to generate free radicals. The absolute rate coefficients (k₃) for the redox reactions of Mo(CO)₅(mma) with CCl₄, CCl₃COOEt and CBr₄ have been determined and the radical yield (ƒ) for the first of these reactions. The rate coefficient (k′₂) for the spontaneous decay of (I) has also been measured.

Activation entropies of the redox reactions are more positive than expected for a bimolecular reaction with a polar transition state in a polar medium and it is suggested that weakening of Mo—CO bonds in the transition state is responsible for this. Values of the ratios of rate coefficients k′₂/k₃ are compared with those derived from polymerization kinetics. As found in Part 1 for reactions of Ni(tp)₄(mma) the halide activity increases in the order CCl₄<CCl₃COOEt<CBr₄, but the range of reactivities with Mo(CO)₅(mma) is much narrower. For the primary oxidation with CCl₄. ƒ→1 at low [Mo(CO)₆], consistent with the formation of Mo^I(although formation of Mo^III cannot be excluded). At higher [Mo(CO)₆], f assumes lower values and it is suggested that this effect is a reflection of the self-inhibition observed in these systems.