Cyclic voltammetry and ultraviolet-visible/infrared (UV-vis/IR) spectroelectrochemistry were used to study the cathodic electrochemical behavior of the osmium complexes mer-[Os(III)(CO) (bpy)Cl3] (bpy = 2,2'-bipyridine) and trans(Cl)-[Os(II)(CO) (PrCN)(bpy)Cl2] at variable temperature in different solvents (tetrahydrofuran (THF), butyronitrile (PrCN), acetonitrile (MeCN)) and electrolytes (Bu4NPF6, Bu4NCl). The precursors can be reduced to mer-[Os(II)(CO) (bpy(•-))Cl3](2-) and trans(Cl)-[Os(II)(CO)(PrCN) (bpy(•-))Cl2](-), respectively, which react rapidly at room temperature, losing the chloride ligands and forming Os(0) species. mer-[Os(III)(CO) (bpy)Cl3] is reduced in THF to give ultimately an Os-Os-bonded polymer, probably [Os(0)(CO) (THF)(bpy)]n, whereas in PrCN the well-soluble, probably mononuclear [Os(0)(CO) (PrCN)(bpy)], species is formed. The same products were observed for the 2 electron reduction of trans(Cl)-[Os(II)(CO)(PrCN) (bpy)Cl2] in both solvents. In MeCN, similar to THF, the [Os(0)(CO) (MeCN)(bpy)]n polymer is produced. It is noteworthy that the bpy ligand in mononuclear [Os(0)(CO) (PrCN)(bpy)] is reduced to the corresponding radical anion at a significantly less negative potential than it is in polymeric [Os(0)(CO) (THF)(bpy)]n: ΔE1/2 = 0.67 V. Major differences also exist in the IR spectra of the Os(0) species: the polymer shows a broad ν(CO) band at much smaller wavenumbers compared to the soluble Os(0) monomer that exhibits a characteristic ν(Pr-CN) band below 2200 cm(-1) in addition to the intense and narrow ν(CO) absorption band. For the first time, in this work the M(0)-bpy (M = Ru, Os) mono- and dicarbonyl species soluble in PrCN have been formulated as a mononuclear complex. Density functional theory (DFT) and time-dependent-DFT calculations confirm the Os(0) oxidation state and suggest that [Os(0)(CO) (PrCN)(bpy)] is a square planar moiety. The reversible bpy-based reduction of [Os(0)(CO) (PrCN)(bpy)] triggers catalytic reduction of CO2 to CO and HCOO(-).