A dinuclear, Cu(I)-catalyzed reductive CO2 coupling reaction was recently developed to selectively yield a metal oxalate product through electrochemical means, instead of the usual formation of carbonate and CO (Science 2010, 327, 313). To shed light on the mechanism of this important and unusual reductive coupling reaction, extensive and systematic density functional theory (DFT) calculations on several possible pathways and spin states were performed in which a realistic system up to 164 atoms was adopted. Our calculations support the observation that oxalate formation is energetically more favorable than the formation of carbonate and CO products in this cationic Cu(I) complex. Spatial confinement of the realistic catalyst (a long metal metal distance) was found to further destabilize the carbonate formation, whereas it slightly promotes oxalate forMation. Our study does not support the proposed diradical coupling mechanism. Instead, our calculations suggest a new incchanism. in, which one CO2 molecule is first reduced cooperatively by two Cu(I) metals to give a new, fully delocalized mixed-valence Cu-2(I/II)(CO2 center dot-) radical anion, intermediate (analogues to Type 4 Cu center, CuA), followed by further partial reduction of the metal-ligated CO2 molecule and (metal-mediated) nudeophilic-like attack on the carbon. atom of an incoming second CO2 molecule to afford the dinuclear Cu(II) oxalate product. Overall, our proposed reaction-mechanism involves a closed-shell reactant as well as two open-shell transition states and products. The effects of size, charge; and catalyst metal on the oxalate formation were also investigated and compared.