<p>The well-defined catalytic conversion of dinitrogen (N₂) to ammonia (NH₃) by molecular complexes is of fundamental interest and important for providing an atomic-level understanding of reactivity that can be related to industrial and biological nitrogen-fixation processes. Molecular catalytic N₂ to NH₃ conversion currently involves the reduction and protonation of terminal or bridging end-on bound metal–N₂ complexes. However, catalytic N₂ to NH₃ conversion by side-on bound metal–N₂ molecular complexes is relevant to both the industrial and biological nitrogen-fixation processes. Here, using a uranium triamidoamine complex, we report catalytic N₂ to NH₃ conversion involving side-on bound N₂ binding. Stoichiometric reactions reveal stepwise reduction of N₂ from free N₂ to bridging side-on bound forms and subsequently to bridging nitrides, uniquely accessing four different states of side-on bound N₂ for the same molecular system. This reveals the roles of N₂, N₂²⁻, N₂³⁻, N₂⁴⁻ and N³⁻ in the catalytic conversion of N₂ to NH₃ when involving side-on bridging N₂.</p><p></p>