The electrochemical reduction of dinitrogen to ammonia promises a sustainable route to fertilizers and fuels. A mechanism based on direct N2 splitting into metal nitride complexes, followed by proton-coupled electron transfer (PCET) to generate NH3, will be considered. Electrochemical kinetic and spectroscopic studies of rhenium and molybdenum complexes capable of reductive N2 binding and splitting provide molecular-level mechanistic details on key steps relevant to ammonia synthesis. The mechanistic studies are complemented by molecular orbital theory analyses probing fundamental aspects of dinitrogen bonding and reactivity, guiding the design of catalysts for ammonia synthesis.