By the use of Nicholson's method, the heterogeneous electron-transfer rate constants (k(s)) for the oxidation of a series of M-2(O2CR)(4) complexes have been determined in benzonitrile, where the metal M = Mo, W, Ru, or Rh and R = alkyl or aryl. For R = Bu-t, the values of k(s) follow the order M = Mo > W > Ru > Rh. No simple influence of R on k(s) was observed, although added ligands that are known to reversibly bind to the dinuclear center were shown to influence the E-1/2 values in order of their basicity and to suppress the rate of electron transfer. The reported data are compared with those obtained for Cp2Fe0/+, Cp-2*Fe0/+, and Ru(bpy)(2)(2+/3+) and with earlier work on dirhenium multiply bonded compounds.