AtDouble-layer effects on electrochemical kinetics in nonaqueous media Dependence reactant charge for some metallocene redox couples

Abstract

The effect of altering the double-layer composition on standard rate constants, k ob, for metallocene and related redox couples has been examined at mercury electrodes in dimethylformamide and acetonitrile containing tetraalkylammonium supporting electrolytes. For cationic metallocene couples, M(Cp) +/0 2, where M = Fe, Mn, or Co, and Cp = cyclopentadienyl or pentamethylcyclopentadienyl, k ob is virtually unaffected by changes in the double-layer structure brought about via alterations in M, as well as by variations in the ionic strength and the nature of the supporting electrolyte cation. In contrast, rate constants for the structurally related anionic couple Co(Cp) 0/− 2, along with those for several other anionic metallocene and organic couples, display the usual sensitivity to such changes in double-layer structure even though equal and opposite double-layer effects for M(Cp) +/0 2 and M(Cp) 0/− 2 couples are predicted from conventional treatments. The behavioral disparities between cationic and anionic metallocene couples are rationalized in terms of the charge distribution between the Cp rings and the metal core. These findings illustrate the limitations of employing the usual Frumkin model to estimate double-layer corrections for such redox couples in nonaqueous media.