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. (Figure presented.)