Creutz-Taube complex, [(NH3)(5)Ru-pyrazine-Ru(NH3)(5)](5+) (1), and its analogues, [(NH3)(5)Os-pyrazine-Os(NH3)(5)](5+) (2), [(NH3)(5)Ru(4,4'-bipyridine)Ru(NH3)(5)](5+) (3), and [(NH3)(5)Os(4,4'-bipyridine)Os(NH3)(5)](5+) (4), were theoretically investigated by the combination of a two-state model and the dielectric continuum model. Their electronic structures are very sensitive to the metal, ligand, and solvent. In the gas phase, the electronic structures of 1-4 would be completely delocalized. In aqueous solution, that of 3 becomes localized because the polar solvent stabilizes the localized electronic structure with the large dipole moment. However, 1 and 2 are still delocalized in aqueous solution. In 4, the electronic structure would be localized when the dihedral angle between two pyridyl rings is 80 degrees, while it would become delocalized when the angle is small. The origins of the difference are the smaller overlap integral and larger energy difference between two diabatic states, of which electronic structure is almost localized on each metal center.