Biological N_2 fixation to NH_3 may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e–/H+ transfer steps of iron-ligated N_2 in well-defined synthetic systems is hence of much interest but remains a significant challenge. While iron complexes have been recently discovered that catalyze the formation of NH_3 from N_2, mechanistic details remain uncertain. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate Fe═NNH_2+ species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N_2– complex. The Fe═NNH_2+ complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = 1/2 Fe═NNH_2. At warmer temperatures, Fe═NNH_2 rapidly converts to an iron hydrazine complex, Fe-NH_2NH_2+, via the additional transfer of proton and electron equivalents in solution. Fe-NH_2NH_2+ can liberate NH_3, and the sequence of reactions described here hence demonstrates that an iron site can shuttle from a distal intermediate (Fe═NNH_2+) to an alternating intermediate (Fe-NH_2NH_2+) en route to NH_3 liberation from N_2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N_2 reduction may be operative in biological N_2 fixation
