The compounds [Ru(NH3)(5)(dtdp)](TFMS)(3), [Os(NH3)(5)(dtdp)](TFMS)(3), [(NH3)(5)Os(dtdp)Os(NH3)(5)](TFMS)(6), [(NH3)(5)Os-(dtdp)Ru(NH3)(5)](TFMS)(3)(PF6)(2), and [(NH3)(5)Os(dtdp)Fe(CN)(5)] (dtdp = 4,4'-dithiodipyridine, TFMS = trifluoromethane-sulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and H-1 NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os-(NH3)(5)(dtdp)](3+) as a function of the solution pH enable us to calculate the pK(a) for the [Os(NH3)(5)(dtdpH)](4+) and [Os(NH3)(5)(dtdpH)](3+) acids as 3.5 and 5.5, respectively. The comparison of the above pK(a) data with that for the free ligand (pK(1) = 4.8) provides evidence for the -S-S- bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH3)(5)Os(dtdp)Os(NH3)(5)](6+), is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the -S-S- bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin-spin interaction parameter of 150-200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the pi orbitals of the dftp ligand relative to that found in [Os(NH3)(5)(dtdp)](3+). The comproportionation constant, K-c = 2.0 x 10(5), for the equilibrium equation [(OsOsII)-Os-II] + [(OsOsIII)-Os-III] reversible arrow 2[(OsOsIII)-Os-II] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH3)(5)Os(dtdp)Os(NH3)(5)](5+) (lambda(MMCT) = 1665 nm, epsilon = 3.5 x 10(3) M-1 cm(-1), Delta(ν) over bar (1/2) = 3.7 x 10(3) cm(-1), alpha = 0.13, and H-AB = 7.8 x 10(2) cm(-1)), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH3)(5)Os-III(dtdp)Ru-II(NH3)(5)](5+) (lambda(MMCT) = 965 nm, epsilon = 2.2 x 10(2) M-1 cm(-1), Delta(ν) over bar (1/2) = 3.0 x 10(3) cm(-1), and H-AB = 2.2 x 10(2) cm(-1)) and [(NH3)(5)Os-III(dtdp)Fe-II(CN)(5)] (lambda(MMCT) = 790 nm, epsilon = 7.5 x 10 M-1 cm(-1), Delta(ν) over bar (1/2) = 5.4 X 103 cm(-1), and HAB = 2.0 x 102 cm(-1)) also suggest a considerable electron delocalization through the S-S bridge. As indicated by a comparison of K, and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order: Fe < Ru < Os.