Tricarbonyl rhenium(I) complexes, such as Re(bpy)(CO)(3)Cl where bpy = 2,2'-bipyridyl, have demonstrated superior activity in catalyzing CO2 reduction in the presence of sacrificial electron donors. We have utilized density functional theory (DFT) to investigate a potential pathway for formate production via a rhenium hydride insertion mechanism in the presence of triethylamine (TEA). On the basis of prior studies, we re-examined the role of TEA and studied a catalytic cycle for CO2 reduction in which TEA functions as both the hydrogen atom and the electron donor for reducing CO2 into formate. The catalytic cycle is found to be exothermic with inclusion of solvation and may be viewed as a two-electron reduction of CO2 because the net result is a transfer of hydride from TEA to CO2. In addition, we have identified structures of key intermediates in the CO2-reduction process and found that the insertion step has a very modest barrier in acetonitrile. These findings provide a molecular-level understanding to formate production via CO2 reduction mediated by transition-metal complexes. A theoretical investigation is underway to elucidate the formation of carbon monoxide, another common product in Re-catalyzed CO2 reduction.