We have investigated the reduction of CO2 to CO mediated by the anionic niobium nitride complex [NNb(NtBuMe)3]−, a model complex of [NNb(NtBuAr)3]−, using density functional theory. We mainly calculated the energetics for the interesting reaction sequence, reported recently by Cummins and his co-workers, involving reactions of the anionic niobium nitride complex [NNb(NtBuAr)3]− with CO2 to first give a carbamate complex. The carbamate complex then reacts with MeC(O)Cl to give an isocyanate–acetate, which can be reduced by SmI2 to give the isocyanate complex (OCN)Nb(NtBuAr)3. Further reduction of the isocyanate complex by Na/Hg extrudes CO and regenerates [NNb(NtBuAr)3]−. In addition, we compare the reaction pathways for the reduction reaction of the isocyanate complexes (OCN)M(NtBuMe)3 (M = Nb, V), model complexes of (OCN)M(NtBuAr)3, with sodium and explain why these two reduction reactions give remarkably different products.