nitrogenases are found in some microorganisms , and these enzymes convert atmospheric n2 to ammonia , thereby providing essential nitrogen atoms for higher organisms . 
 some nitrogenases reduce atmospheric n2 at the femoco , a sulfur - rich iron - molybdenum cluster15 . 
 the iron centers that are coordinated to sulfur and carbon atoms in femoco have been proposed as the substrate binding sites , based on kinetic and spectroscopic studies5,6 . 
 studies on the enzyme indicate that iron atom fe6 and possibly also adjacent belt iron sites are involved.58 in the resting state , the central fe sites ( including fe6 ) have identical environments consisting of three sulfides and a carbide . 
 addition of electrons to the resting state causes the femoco to react with n2 , but the geometry and bonding environment of n2-bound species remain unknown5 . in this manuscript 
 , we describe a synthetic complex with a sulfur - rich coordination sphere that , upon reduction , breaks an fe - s bond and binds n2 . 
 the product is the first synthetic fe 
 n2 complex in which iron has bonds to sulfur and carbon atoms , providing a model for n2 coordination in the femoco . 
 our results demonstrate that breaking an fe - s bond is a chemically reasonable route to n2 binding in the femoco , and show structural and spectroscopic details for weakened n2 on a sulfur - rich iron site .