The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N-2) to ammonia (NH3) is an iron surface promoted with potassium cations (K+), but soluble iron complexes have neither reduced the N-N bond of N-2 to nitride (N3-) nor produced large amounts of NH3 from N-2. We report a molecular iron complex that reacts with N-2 and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe3N2 core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H-2 to give substantial yields of N-2-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N-2 cleavage and N-H bond-forming reactions of iron.