This paper describes the 4e(-)/3H(+) interconversion between NH3 and N3-, which is reversible in the coordination spheres of Os complexes containing either tpm (tpm = tris(1-pyrazolyl)methane) or Tp (Tp = hydrotris(1-pyrazolyl)-borate anion) ligands. Electrochemical or chemical reduction of the nitrido complexes [Os-VI(tpm)(Cl)(2)(N)(+) (1) and Os-VI(Tp)(Cl)(2)(N) (2) in acidic asueous solution gives the corresponding Os-II-ammine complexes, which, after air oxidation and workup, are isolated and structurally characterized as [Os-III(tpm)(Cl)(2)(NH3)](PF6) (3) and Os-III(Tp)(Cl)(2)(NH3) (4). The Os-III-ammine complexes are reoxidized electrochemically to the nitrido complexes by stepwise mechanisms involving the loss of both electrons and protons and sequential Os(III --> IV) and Os(IV --> VI) oxidations.