IR, UV−vis, and EPR spectroelectrochemistry at variable temperatures and in different solvents were applied to investigate in situ the formation of electroactive molecular chains with a nonbridged Os−Os backbone, in particular, the polymer [Os0(bpy)(CO)2]n (bpy ) 2,2′-bipyridine), from a mononuclear Os(II) carbonyl precursor, [OsII(bpy)(CO)2- Cl2]. The one-electron-reduced form, [OsII(bpy¥-)(CO)2Cl2]-, has been characterized spectroscopically at low temperatures. This radical anion is the key intermediate in the electrochemical propagation process responsible for the metal−metal bond formation. Unambiguous spectroscopic evidence has been gained also for the formation of [{Os0(bpy¥-)(CO)2}-]n, the electron-rich electrocatalyst of CO2 reduction. The polymer species are fairly well soluble in butyronitrile, which is important for their potential utilization in nanoscience, for example, as conducting molecular wires. We have also shown that complete solubility is accomplished for the monocarbonyl-acetonitrile derivative of the polymer, [Os0(bpy)(CO)(MeCN)2Cl]n.