Though Lewis acids (LAs) have been shown to have profound effects on carbon dioxide (CO_2) reduction catalysis, the underlying cause of the improved reactivity remains unclear. Herein, we report a well-defined molecular system for probing the role of LA additives in the reduction of CO_2 to carbon monoxide (CO) and water. Mo(0) CO_2 complex (2) forms adducts with a series of LAs, demonstrating CO_2 activation that correlates linearly with the strength of the LA. Protons induce C–O cleavage of these LA adducts, in contrast to the CO_2 displacement primarily observed in the absence of LA. CO_2 cleavage shows dependence on both bond activation and the residence time of the bound small molecule, demonstrating the influence of both kinetic and thermodynamic factors on promoting productive CO_2 reduction chemistry
