Dicationic dicarbonyl [Ru(bpy)(2)(CO)(2)](2+) (bpy = 2,2'- bipyridyl) exists as equilibrium mixtures with [Ru(bpy)(2)(CO)(COOH)](+) and [Ru(bpy)(2)(CO)(CO2)](0) depending on the pH in H2O. Those three complexes work as the precursors to CO, HCOOH production, and CO2 carrier, respectively, in electro- and photochemical CO2 reduction in aqueous solutions. However, [Ru(bpy)(2)(CO)(2)](2+) loses the catalytic activity toward CO2 reduction under aprotic conditions because [Ru(bpy)(2)(CO)(2)](2+) is not regenerated from [Ru(bpy)(2)(CO)(CO2)](0) in the absence of proton sources. Analogous monocarbonylruthenium complexes such as [Ru(tpy)(bpy)(CO)](2+) and [Ru(bpy)(2)(qu)(CO)](2+) catalyze CO2 reduction in the absence and presence of proton sources. Both complexes are reproduced through oxide transfer from the corresponding Ru-CO2 to CO2 in CO2 reduction and produce the same amount of CO and CO32- in the absence of proton donors. The reduction of CO2 catalyzed by polypyridylrhenium complexes in the presence of proton sources takes place via essentially the similar mechanism as that in the case of ruthenium complexes. On the other hand, CO evolution in CO2 reduction under aprotic conditions is ascribed to the dissociation of CO from a dimeric Re-C(O)OC(O)O-Re scaffold. Visible-light irradiation to a catalytic system composed of [Ru(bpy)(2)(CO)(2)](2+)/[Ru(bpy)(3)](2+)/Me2NH2+Me2NH as the catalyst, photosensitizer, proton donor, and nucleophile in addition to the electron donor, respectively, in CO2-saturated CH3CN selectively produces N,N-dimethylformamide without concomitant CO and HCOOH formation. Structurally robust mu 3-S of reduced metal-sulfur clusters provides a suitable site for reductive activation of CO2 with retention of the framework. Indeed, CO2 activated on mu 3-S of [Fe6Mo2S8(SEt)(3)](5-) is fixed at the carbonyl carbon of thioesters trapped on a neighboring iron of the cluster, and a-keto acids are produced catalytically. Furthermore, two-electron reduction of [(CpMen)(3)M3S3](2+) (n = 1, M = Co; n = 5, M = Rh, Ir) creates the catalytic ability to produce oxalate through the coupling of two CO2 molecules possibly activated on mu 3-S and a metal ion.