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)Cl-3] (bpy = 2,2'-bipyridine) and trans(Cl)- [Os-II(CO) (PrCN)(bpy)Cl-2] 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(center dot-))Cl-3](2-) and trans(Cl)-[Os-II(CO)(PrCN) (bpy(center dot-))Cl-2](-), respectively, which react rapidly at room temperature, losing the chloride ligands and forming Os(0) species. mer-[Os-III(CO) (bpy)Cl-3] is reduced in THF to give ultimately an Os-Os-bonded polymer, probably [Os-0(CO) (THF)(bpy)], whereas in PrCN the well-soluble, probably mononuclear [Os (CO) (PrCN)(bpy)], species is formed. The same products were observed for the 2 electron reduction of trans(Cl)-[Os-II(CO)(PrCN) (bpy)Cl-2] in both solvents. In MeCN, similar to THF, the [Os-0(CO) (MeCN)(bpy)],, 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) : Delta E-1/2 = 0.67 V. Major differences also exist in the IR spectra of the Os(0) species: the polymer shows a broad v(CO) band at much smaller wavenumbers compared to the soluble Os(0) monomer that exhibits a characteristic v(Pr-CN) band below 2200 cm(-1) in addition to the intense and narrow v(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-.