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 metha crylate (mma) to produce complexes of the type Mo(CO)5fmma) (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)s(mma) with CCL, CCl3COOEt and CBr4 have been determined and the radical yield (J) for the first of these reactions. The rate coefficient (k)) for Ute 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 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 CCL < CCl3COOEt < CBr4, but the range of reactivities with Mo(CO)5(mma) is much narrower. For the primary oxidation with CCl4. f->l at low [Mo(CO)6], consistent with the formation of Mo1 (although formation of Mom cannot be ex cluded). At higher [Mo(CO)fi],/assumes lower values and it is suggested that this effect is a reflection of the self-inhibition observed in these systems.