The kinetics of the reactions of copper(II) with ferrocene (Fc) and 1,1'-dimethylferrocene (Dmfc) have been studied at 25 degreesC in aqueous acetonitrile (AN) containing 50-97.5 vol % AN. With increasing % AN, the rate constant increases along with the driving-force for the reaction. The results are analyzed in terms of Marcus theory to estimate the Cu(II/I) electron self-exchange rate constant (k(11)) for the system. Over the solvent range studied, the calculated k(11) changes from 1.1 x 10(-9) to 17 x 10(-9) M-1 s(-1), with an average value of 5 x 10(-9). In addition, the structures of the trifluoromethanesulfonate salts of [Cu(AN)(4)](+), [Cu(OH2)(2)(AN)(2)](2+), and [Cu(AN)(4)](2+) are reported. It is found that the Cu-NCCH3 bond-length difference between the Cu(I) and Cu(II) oxidation states is only similar to0.02 Angstrom.