Living systems carry out the reduction of N to ammonia (NH) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber-Bosch process hydrogenates N but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH on addition of H to N have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp'Fe)(μ-N)] (in which Cp' = η-1,2,4-(MeC)CH), which is prepared by reduction of [Cp'Fe(μ-I)] under an N atmosphere and comprises three iron centres bridged by two μ-nitrido ligands. In solution, this complex reacts with H at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp'Fe)(μ-NH)], by addition of H (10 bar) through an unusual solid-gas, single-crystal-to-single-crystal transformation. In solution, [(Cp'Fe)(μ-NH)] further reacts with H or H to form NH.