The relationships between the intervalence energy (E-IT) and the free energy difference (Delta G) that exists between the minima of redox isomers (Fe-II-Ru-III/Fe-III-Ru-II) for various heterobimetallic complexes [(R-Fcpy)Ru(NH3)(5)](2+/3+) (R = H, ethyl, Br, actyl; Fcpy = (4-pyridyl)ferrocenyl; Ru(NH3)(5) = pentaam(m)ineruthenium) were examined. The changes in Delta G for the complexes in various solvents were due to the effects of both solvent donicity and the substituents, The intervalence energy versus Delta G, Delta G approximate to F Delta E-1/2 (Delta E-1/2 = E-1/2(Fe-III/II) - E-1/2(Ru-III/II)), plots for the complexes in various solvents suggest a nuclear reorganization energy (lambda) of 6000 cm(-1) (Chen et al. Inorg. Chem. 2000, 39, 189). For [(R-Fcpy)Ru(NH3)(5)](2+) and [(et-Fcpy)Ru(NH3)(4) (py)](2+) (Ru(NH3)(4) = trans-tetraam(m)ineruthenium; py = pyridine) in various solvents, the E-1/2(Ru-III/II) of rutheniumam(m)ine typically was less than the E-1/2(Fe-III/II) of the ferrocenyl moiety. However, the low-clonicity solvents resulted in relatively large values of E-1/2(Ru-III/II) for [(et-Fcpy)Ru(NH3)(4)(py)](2+/3+/4+). Under our unique solvent conditions, a dramatic end-to-end interaction was observed for the trimetal cation, [(et-FcpY)(2)Ru(NH3)(4)](4+), in which the [(et-FcPY)(2)Ru(NH3)(4)](4+) included a central trans-tetraam(m)ineruthenium(III) and a terminal Fe-II/Fe-III pair. In general, results of electrochemical studies of [(etFcpY)(2)Ru(NH3)(4)](2+) indicated both solvent-tunable E-1/2(Ru-III/II) (1 e(-)) and solvent- in sensitive E-1/2(Fe-III/(II)) (2 e(-)) redox centers. However, in nitriles, two E-1/2(Fe-III/II) peaks were found with Delta E-1/2(Fe-III/II - Fe-III/II) ranging between 83 and 108 mV at a terminal metal-to-metal distance of up to 15.6 angstrom. Furthermore, the bridging d pi orbital of the ruthenium center mediated efficient end-to-end interaction between the combinations of the terminal Fe-II-Fe-III/Fe-III-Fe-II pair. To our knowledge, this is the first example of solvent-tunable end-to-end interactions in multimetal complexes.