Fast charge-transfer reactions of binuclear complexes undergoing large changes in metal---metal bond lengths

Abstract

Heterogeneous electron-transfer rates have been measured for three binuclear complexes, Cp2Fe2(CO)2(μ-SMe)2, 1, Cp2Fe2(CO)2(μ-PPh2)2, 2, and Cp2Mo2(μ-SMe)4, 3. Dahl and co-workers had shown previously that the cations of the two iron complexes display significant shortening of the metal---metal distance (47 pm and 36 pm, respectively, for 1 and 2) due to removal of an electron from an antibonding M---M orbital from the neutral compounds. By comparison, the structure of 3 is virtually unchanged upon one-electron oxidation. Surprisingly, the greater structural changes of 1 and 2 do not lead to lower heterogeneous electron-transfer rates than 3. In acetonitrile at Pt or mercury electrodes all three complexes have ks values, measured by cyclic voltammetry, of ca. 0.2 cm s−1. Several possible reasons for this behavior are discussed.