The electrochemical reduction of HOC₂H₄N₃ as a nitrogenase substrate with a (n-BU₄N)₃ [Mo₂Fe₆S₈(SPh)9]-modified glassy carbon electrode ([Mo-Fe]/GCE) in water at -1.25 V vs. SCE affords not only HOC₂H₄NH₂ and N₂ but also NH₃ as an eight electron reduction product with the turnover number about 12000 in 2 h. Similar reductions of n-C₅H₁₁N₃ can be achieved chemically in an aqueous Triton X-100 micellar solution containing Na₂S₂O₄ (reducing agent), methylviologen (electron mediator), and [MoFe₃S₄(SC₆H₄-p-n-C₈H₁₇)₃(O₂C₆-Cl₄) ((CH₃)₂CO)]^2- (catalyst) to give NH₃ as well as n-C₅H₁₁NH₂ and N₂. The [Mo-Fe]/GCE can successfully be used also for the catalytic reductions of No_n‾ (n =2, 3). On the other hand, the controlled potential electrolysis of a CO₂-saturated CH₃CN solution containing [Fe₄S₄(SPh)₄]^2-, NO₂‾, PhCOCH₃, and molecular sieves 3A at -1.25 V vs. SCE produces PhCOCH₂COO‾ (β-keto acid) and N₂ with the mole ratio about 7:1. Analogous CO₂ fixation coupled with NO₂‾ reduction occurs also in the presence of CH₃CO-CH₃, PhC≡CH, and cyclohexanone as proton sources. Moreover, the CO₂ fixation at the carbonyl carbon of RC(O)SEt (R = CH₃, Et, Ph) can be catalyzed by [Mo₂Fe₆S₈(SEt)₉]^3- under the electrolysis conditions at – 1.55 V vs. SCE to afford α -keto acids.