Nitrogenases are the enzymes by which certain microorganisms convert atmospheric dinitrogen (N-2) to ammonia, thereby providing essential nitrogen atoms for higher organisms. The most common nitrogenases reduce atmospheric N-2 at the FeMo cofactor, a sulfur-rich iron-molybdenum cluster (FeMoco)(1-5). The central iron sites that are coordinated to sulfur and carbon atoms in FeMoco have been proposed to be the substrate binding sites, on the basis of kinetic and spectroscopic studies(5-7). In the resting state, the central iron sites each have bonds to three sulfur atoms and one carbon atom. Addition of electrons to the resting state causes the FeMoco to react with N-2, but the geometry and bonding environment of N-2-bound species remain unknown(5). Here we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe-S bond and binds N-2. The product is the first synthetic Fe-N-2 complex in which iron has bonds to sulfur and carbon atoms, providing a model for N-2 coordination in the FeMoco. Our results demonstrate that breaking an Fe-S bond is a chemically reasonable route to N-2 binding in the FeMoco, and show structural and spectroscopic details for weakened N-2 on a sulfur-rich iron site.