The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N₂) to ammonia (NH₃) is an iron surface promoted with potassium cations (K⁺), but soluble iron complexes have neither reduced the N-N bond of N₂ to nitride (N^3–) nor produced large amounts of NH₃ from N₂. We report a molecular iron complex that reacts with N₂ 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 H₂ to give substantial yields of N₂-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N₂ cleavage and N-H bond-forming reactions of iron.