Extensive inhibition by ion pairing in a bimolecular, outer-sphere electron transfer reaction, reduction of a cobalt clathrochelate by ferrocene in methylene chloride

The kinetics of the reduction of [Co(dimethylglyoximate)3(BF)2]BF4 by ferrocene has been studied by the stopped-flow method in methylene chloride as a function of reactant concentration, temperature, and the added electrolytes Bu4NX (where X− is BF4 −, Cl−, Br− and NO3 −). The data were fit to a rate law with separate paths for the free ion and the ion paired Co complex. At −21°C ion pairing causes inhibition by a factor of 60 for Br− and Cl−, but 150 for BF4 − and NO3 −. Factors of 280 for Br− and 450 for BF4 − were estimated when the data were extrapolated to 25°C. Such large effects have been seen before only for faster, intramolecular processes involving organic systems and for two other reactions of the Co oxidant. The possible origins of this behavior are discussed.