Reversible osmium(VI) nitrido to osmium(II) ammine interconversion in complexes containing polypyrazolyl ligands.

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 aqueous 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.