Based upon the known reactions of nitrogenase and the principles of coordination catalysis,a model of nitrogenase active-center is proposed.An octa-atomic cluster,Fe₂S₂·Mo₂O₂,of pseudo-onbane-type structure is supposed to form a coupled twin of trinuclear(2Mo-1Fe) active-centers,which also catalyze the reduction of H⁺ to H₂.With this model,mechanisms of all the known nitrogenase-catalyzed reactions are explained,together with the non-inhibition of the hydrogen-evolution reaction by CO,and the mixed character of inhibition of other nitrogenase-catalyzed reactions.