We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr]2; Ar = 2,6-iPr2-C6H3) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(NtBu)(NAr)(L′) (L′ = py, thf) were formed by either KC8 reduction of (BDI)Nb(NtBu)Cl2(py) in the absence of strong π-acids or by H2 reduction of the Nb(V) dimethyl complex (BDI)Nb(NtBu)Me2 in THF. These products are likely formed though an intramolecular, 2 e− reductive C–N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC8 reduction of (BDI)Nb(NtBu)Cl2(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(NtBu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (Aiso{93Nb} = 120.5×10−4 cm−1, Aiso{31P} = 31.0×10−4 cm−1, giso = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(NtBu)(py) prior to reductive cleavage of the ligand C–N bond, the thermally stable Nb(III) species (BDI)Nb(NtBu)(CX)2(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me2-C6H3) were obtained in good yields. The Nb(III) complexes (BDI)Nb(NtBu)py, (BDI)Nb(NtBu)(CO)2(py) and (BDI)Nb(NtBu)(CO)2 were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(NtBu)(py) and diphenylsulfoxide by (BDI)Nb(NtBu)(CO)2 led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(NtBu)(NMes)(py) and [(BDI)Nb(NtBu)]2(μ2-O)2, respectively. MeLi addition to (BDI)Nb(NtBu)(CO)2(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb–Cacylate bond distance (2.059(4) Å), consistent with multiple Nb–C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(NtBu)(CO)2 with two equivalents of 4,4′-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(NtBu)(CO)2 formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this s...