Disulfide-bridged binuclear ruthenium complexes, [{RuCl(P(OMe)(3))(2)}(2)(mu-Cl)(2)(mu-S-2)] (1), [{RuCl(P(OMe)(3))(2)}(mu-Cl)(2)(mu-S-2){Ru(CH3CN)(P(OMe)(3))(2)}](+) ([2](+)), [{Ru(CH3CN)(3)(P(OMe)(3))(2)}(2)(mu-S-2)](3+) ([3](3+)), [{Ru(CH3CN)(P(OMe)(3))(2)}(2)(mu-Cl)(2)(mu-C-2)](2+) ([4](2+)), and [{Ru(CH3CN)(3)(P(OMe)(3))(2)}(2)(mu-S-2)](4+) ([5](4+)), have been synthesized, and their crystal structures have been solved. Compounds 1, [2](+), and [4](2+) have a triply bridged Ru-III (mu-Cl)(2)(mu-S-2)Ru-III core:, in which the S-2(2-) ligand bridges the two Ru atoms in a cis configuration. Compounds [3](3+) and [5](4+) have a singly bridged trans-RuSSRu core, whereby [3](3+) corresponds to a one-electron reduced form of [5](4+). Compound [3](3+) is the first example of a well-characterized mixed-valent compound with a trans-MSSM core, where M is any metal. All the compounds have intense absorption bands at around 700 nm, which can be explained for [3](3+) and [5](4+) as a pi-pi* transition of the distinct trans-RuSSRu core. Resonance Raman spectroscopy of 1, [2]+, and [3](3+) and comparison with several literature values for cis-RuSSRu compounds show that only [3](3+) exhibits a strong nu(S-S) Raman band, when excited by lambda(e) = 647.1 nm, whereas all the others show strong to medium nu(Ru-S) and very weak nu(S-S) bands. The ESR spectrum of [3](3+) shows a rhombic signal with g(1) = 2.12, g(2) = 2.05, and g(3) = 1.995. This anisotropy is unusually small, compared to most mononuclear and binuclear Ru(III) compounds with various ligands. Analysis of the g values by use of the matrix of spin-orbit coupling Hamiltonian has revealed a very small spin-orbit coupling constant of 100 cm(-1), which is a result of the extensive covalency of the metal-disulfide bond. The X-ray photoelectron spectrum of [3](3+) did not give any of the expected double peaks of the Ru(II) and Ru(III) components; the observed peaks are Ru-5/2(3d) 281.0 eV, 3P(3/2) 462.4 eV, S(S-2(2-)) 2P(3/2) 162.7 eV. Compound [3](3+) does not give any intervalence-transition band in the longer-wavelength visible to near-IR region, other than the UV-vis band similarly observed in the one-electron oxidized compound [5](3+). These characteristics are reasonably understood, if [3](3+) is regarded as a mixed-valent complex with valence-averaged ground state (class III of the Robin and Day classification).