A Proposed Complete‐Cycle Mechanism for Conversion of N ₂ to NH ₃ by Mo‐Nitrogenase

How does the enzyme nitrogenase convert N 2 to NH 3 under ambient conditions? This perspective expounds the Dance chemical mechanism comprising 40 explicit intermediates and 31 reaction steps, from start to finish, to effect the enzyme stoichiometry N 2 + 8H + + 8e − → 2NH 3 + H 2 . Energy profiles and kinetic barriers are derived from density functional simulations with a 483+ atom quantum model. The key to the mechanism is discovery of the unique N 2 capture step, in which N 2 tumbles into a preformed gallery of H atoms, and is activated there by concerted formation of two HN and three FeN bonds, forming bound HNNH. This sets up subsequent hydrogenations that break N–N and form NH 3 in concert. Retention of bridging S2B adjacent to bound intermediates is a crucial stereochemical component. An inconsistency with kinetic data is still to be resolved. The well‐known H 2 /N 2 exchange occurs because H 2 is formed where its dissociation creates the space required for N 2 capture. Possible quantum tunneling by H atoms is considered. I discuss architecture and function in the reaction space and surrounding protein. An enabling attribute of this mechanism is the favourable geometry and propitious stereochemistry involved in each of the chemical steps.