We proposed a reductive elimination/oxidative addition (re/oa) mechanism for reduction of N-2 to 2NH(3) by nitrogenase, based on identification of a freeze-trapped of the alpha-70(Val -> Ile) MoFe protein as the Janus intermediate that stores four reducing equivalents on FeMo-co as two [Fe-H-Fe] bridging hydrides (denoted E-4(4H)). The mechanism postulates that obligatory re of the hydrides as H-2 drives reduction of N-2 to a state (denoted E-4(2N2H)) with a moiety at the diazene (HN=NH) reduction level bound to the catalytic FeMo-co. EPR/ENDOR/photophysical measurements on wild type (WT) MoFe protein now establish this mechanism. They show that a state freeze-trapped during N-2 reduction by WT MoFe is the same Janus intermediate, thereby establishing the alpha-70(Val -> Ile) intermediate as a reliable guide to mechanism. Monitoring the Janus state in WT MoFe during N-2 reduction under mixed-isotope condition, H2O buffer/D-2, and the converse, establishes that the bridging hydrides/deuterides do not exchange with solvent during enzymatic turnover, thereby solving longstanding puzzles. Relaxation of E-4(2N2H) to the WT resting-state is shown to occur via oa of H-2 and release of N-2 to form Janus, followed by sequential release of two H-2, demonstrating the kinetic reversibility of the re/oa equilibrium. Relative populations of E-4(2N2H)/E-4(4H) freeze-trapped during WT turnover furthermore show that the reversible re/oa equilibrium between [E-4(4H) + N-2] and [E-4(2N2H) + H2] is similar to thermoneutral (Delta(re)G(0) similar to -2 kcal/mol), whereas, by itself, hydrogenation of N-2(g) is highly endergonic. These findings demonstrate that (i) re/oa accounts for the historical Key Constraints on mechanism, (ii) that Janus is central to N-2 reduction by WT enzyme, which (iii) indeed occurs via the re/oa mechanism. Thus, emerges a picture of the central mechanistic steps by which nitrogenase carries out one of the most challenging chemical transformations in biology.