A homogeneous series of heterobimetallic complexes of [R-Fc(4-py)Ru(NH3)(5)](PF6)(2) (R = II, Et, Br, acetyl; Fc(4-py) = 4-ferrocenylpyridine) have been prepared and characterized. The mixed-valence species generated in situ using ferrocenium hexafluorophosphate as the oxidant show class II behavior, and the oxidized sites are ruthenium centered. Delta E-1/2, E-1/2(Fe-III/Fe-II) - E-1/2(Ru-III/Ru-II), an upper limit for Delta G degrees that is an energetic difference between the donor and acceptor sites, changes sharply and linearly with Gutmann solvent donor number (DN) and Hammett substituent constants (sigma). The solvent-dependent and substituent-dependent intervalence transfer bands were found to vary almost exclusively with Delta E-1/2. The activation energy for the optical electron transfer versus Delta E-1/2 plot yields a common nuclear reorganization energy (lambda) of 0.74 +/- 0.04 eV for this series. The equation that allows one to incorporate the effect of both solvent donicity and substituents on optical electron transfer is E-op = lambda + Delta G degrees, where Delta G degrees = (Delta G degrees)(intrinsic) + (Delta G degrees)(solvent donicity) + (Delta G degrees)(substituent effect) (Delta G degrees)(intrinsic) with a numerical value of 0.083 +/- 0.045 eV was obtained from the intercept of the Delta E-1/2 of [H-Fc(4-py)Ru(NH3)(5)](2+,3+,4+) versus DN plot. (Delta G degrees)(solvent donicity) was obtained from the average slopes of the Delta E-1/2 of [H-Fc(4-py)Ru(NH3)(5)](2+,3+,4+) versus DN plot, and (Delta G degrees)(substituent effect) was Obtained from the average slopes of the corresponding Delta E-1/2 versus a plot. The empirical equation allows one to finely tune E-op of this series to E-op = 0.82 + 0.019(DN) + 0.44 sigma eV at 298 K, and the discrepancy between the calculated and experimental data is less than 6%.