On the Mechanism of Reduction of Cobalt (III) and Ruthenium (III) Hexaammine Complexes by Several Aliphatic Radicals

The specific rates of reaction of several aliphatic radicals with Co(NH₃)₆³⁺ and Ru(NH₃)₆³⁺ were determined. Radicals on the α carbon relative to OH or NH₂ groups were shown to be stronger reducing agents than radicals on the α carbon to a carboxylic or an amide group. The reactivity of the radicals ĊH₂OH, CH₃-ĊHOH, (CH₃)₂ĊOH, CH₃ĊOHCOO^–, and CH₃COĊ(O^–)CH₃, toward Ru(NH₃)₆³⁺ increases along this series, whereas the reversed order of reactivities is found for the reactions with Co(NH₃)₆³⁺ and Cu_aq²⁺. This order is different from the order of reactivities toward nitrobenzene and 2,3-butanedione where the reactivity increases along the series CH₃ĊOHCO₂^–, ĊH₂OH, CH₃ĊHOH, and (CH₃)₂ĊOH. The latter order is identical with that of increasing pK of the OH group. The results are interpreted as suggesting that the contribution of π character to the orbital containing the unpaired electron in the radical significantly affects the specific rates of reactions. The implication of the results in radiobiology is discussed.