Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier, but the current production emits more CO2 than any other chemical producing reaction in the world. The demand for decarbonizing the ammonia industry by using renewable energy has renewed research interests into catalyst development for effective N2 reduction under mild conditions, a grand scientific challenge. Conventional heterogeneous catalysts based on metallic Fe or Ru mediate dinitrogen dissociation and hydrogenation through a relatively energy-costing pathway. The ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 reported in this work, on the other hand, represent an entirely new class of compound catalysts, which are composed of the electron- and H-rich [RuH6] anionic centers for non-dissociative dinitrogen reduction, where hydridic H transports electron and proton between the centers, and the Li(Ba) cations for stabilizing NxHy (x: 0 to 2, y: 0 to 3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates a novel reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multi-step process, leading to ammonia production from N2+H2 with superior kinetics under mild conditions.