The pyrazine-bridged diosmium(II) complex {(mu-C4H4N2)[OS(CN)(5)](2)}(6-) has been synthesized as the hexapotassium salt and was converted to the hexakis(tetrabutylammonium) compound by ion exchange. Its stepwise oxidation to the (Os2OsII)-Os-III and (Os2OsIII)-Os-III states was monitored spectroelectrochemically in acetonitrile/0.1 M Bu4NPF6 in the UV-vis-NIR and IR regions and by EPR. The Os-2(II), species exhibits long-wavelength solvatochromic MLCT bands at 14 800 and 18 480 cm(-1). The pentaanionic mixed-valent ion has a comproportionation constant K-c of 10(5.8) and is distinguished by several electronic absorptions in the IR region. The bands at 1972, 2480, 4000, 5000, 7170, and 11 900 cm(-1) are similar but appreciably lower in energy than those reported for the analogous complex {(mu-C4H4N2)[Os(NH3)(5)](2)}(5+). In addition to CN vibrational stretching features shifted on electron transfer there is an additional sharp band only for the 5- ion at 1582 cm(-1) which is attributed to a pyrazine ring vibration. The intensity of this band suggests an unsymmetrical situation and thus valence localization on the vibrational time scale. The EPR signals at g(perpendicular to) = 2.0563 and g(parallel to) = 1.761 point toward excited states lying close to the doublet ground state and to significant metal/cyanide interaction. Compared to the Creutz-Taube ion {(mu-C4H4N2)[Ru(NH3)(5)](2)}(5+) and its osmium homologue, the 5-ion described here exhibits a lower degree of metal-metal coupling.