A bifunctional, chelating N-heterocyclic carbene-pyridine (NHC-pyridine) containing Mn(I) complex [MnBr(NHC-pyridine)(CO)3] displays a strong selectivity for CO2 reduction over proton reduction. Interestingly, the two-electron reduction of this complex occurs at a single potential, as opposed to MnBr(bpy)(CO)3, which is reduced by two electrons in two separate one-electron reductions. Here, the Gibbs free energy barriers, reduction potentials, rate constants, and pKa values are predicted with theory to understand the one- vs two-electron reduction mechanism. The effects of weak and strong Brønsted acids [HCl, TFE (2,2,2-trifluoroethanol), PhOH, CH3OH, and H2O] are studied to gauge the preference for CO2 vs proton reduction; water is found to be an ideal proton donor that allows for strong CO2 selectivity. © 2016 American Chemical Society.