Two heterobimetallic complexes of [Fc(4-Py)Ru(NH3)(5)](PF6)(2) (1) and [Fc(3-Py)Ru(NH3)(5)](PF6)(2) (2) (Fc(4-Py) = 4-ferrocenylpyridine, Fc(3-Py) = 3-ferrocenylpyridine) have been synthesized and characterized for the purpose of investigating optical and thermal electron transfer. The mixed-valence species generated in situ using ferrocenium hexafluorophosphate as the oxidant show Robin and Day class II behavior, and the oxidized sites are ruthenium centered, Delta E(1/2)degrees, E(1/2)degrees(Fe-III/Fe-II) - E(1/2)degrees(Ru-III/Ru-II), an estimate of Delta E-o that is an energetic difference between the donor and acceptor sites, changes sharply with variation of solvents. Good linear relationship exists between Delta E(1/2)degrees and Gutmann solvent donor number (DN) and Delta E(1/2)degrees versus DN plots yield a slope of 20.4 +/- 1.6 mV/DN for [Fc(4-Py)Ru(NH3)(5)](2+/3+/4+) and a slope of 21.1 +/- 2.2 mV/DN for [Fc(3-Py)Ru(NH3)(5)](2+/3+/4+). The solvent-dependent IT bands were found to vary almost exclusively with Delta E(1/2)degrees. The continuum dielectric approximation is found to be adequate, and the (E-op - Delta E-o) versus (1/epsilon(op) - 1/epsilon(s)) plot yields a straight line with a slope of 3848 +/- 1444 and an intercept of 4265 +/- 227 cm(-1) for [Fc(4-Py)Ru(NH3)(5)](3+). The corresponding values for [Fc(3-Py)Ru(NH3)(5)](3+) are 2328 +/- 1560 and 6712 +/- 100 cm(-1). The thermal electron transfer (the reverse of the optical process, viz., Ru-II --> Fe-III electron transfer) is adiabatic for [FC(4-Py)Ru(NH3)(5)](3+) but somewhat nonadiabatic (kappa approximate to 0.6) for [Fc(3-Py)Ru(NH3)(5)](3+). The thermal transfer rate constants of both complexes decrease exponentially with increasing solvent donor number and show close magnitude in all solvents despite [Fc(3-Py)Ru(NH3)(5)](3+) having apparently a shorter through-space distance and lower activation energies. However, these calculated k(th) values should be used with caution because no experimental data as measured by flash photolysis techniques are available.