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Living systems carry out the reduction of N2to ammonia (NH3) 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 N2but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH3on addition of H2to N2have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp′Fe)3(μ3-N)2] (in which Cp′ = η5-1,2,4-(Me3C)3C5H2), which is prepared by reduction of [Cp′Fe(μ-I)]2under an N2atmosphere and comprises three iron centres bridged by two μ3-nitrido ligands. In solution, this complex reacts with H2at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH3. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp′Fe)3(μ3-NH)2], by addition of H2(10 bar) through an unusual solid–gas, single-crystal-to-single-crystal transformation. In solution, [(Cp′Fe)3(μ3-NH)2] further reacts with H2or H+to form NH3.