High-valent dichloride and dimethylniobium complexes 1 and 2 bearing tert-butylimido and N,NY-bis(2,4,6-trimethylpheny1)-beta-diketiminate (BDIAr) ligands were prepared. The dimethyl complex reacted with dihydrogen to release methane and generate the hydride bridged diniobium(IV) complex 3 in high yield. One-electron oxidation of 3 with silver salts resulted in the release of dihydrogen and conversion to a mixed-valent Nb-III-Nb-IV complex, 4, that displayed a frozen-solution X-band electron paramagnetic resonance signal consistent with a slight dissymmetry between the two Nb centers. Spectroscopic and computational analysis supported the presence of Nb-Nb sigma-bonding interactions in both 3 and 4. Finally, one-electron reduction of 4 resulted in conversion to the highly dissymmetric Nb-V-Nb-V dimer 5 that formed from the reductive C-N bond cleavage of one of the BDIAr supporting ligands.