REVERSIBLE INTERCONVERSION BETWEEN A NITRIDO COMPLEX OF OS(VI) AND AN AMMINO COMPLEX OF OSMIUM(II)

The reaction between [N(n-Bu)4][Os(N)(X)4] (X = Cl, Br) and 2,2′:6′,2″-terpyridine (tpy) in acetone under reflux gave the salts [Os(N)(typ)(X)2]X. The X-ray crystal structure of [Os(N)(tpy)(Cl)2]Cl showed that the chloride ligands occupy mutually trans axial positions relative to the nitrido ligand. The effects of multiple bonding in the Os-N bond appear in the short Os-N bond length of 1.663 (5) Å, in the bending of the ligating atoms in the cis positions away from the Os-N axis, and in the elongation of the trans N atom of the bound tpy by 0.08 Å. Electrochemical or chemical reduction of the nitrido complex in acidic aqueous solution gave the corresponding ammino complex of Os(II), [Os(NH3)(tpy)(Cl)2], which, following oxidation by air, was isolated as the corresponding PF6 salt of Os(III). The Os(II)-ammine complex could be reoxidized to the nitrido complex either chemically or electrochemically. Reduction potentials were measured or estimated at pH = 3 for the intermediate Os(VI/V), Os(V/IV), Os(IV/III), and Os(III/II) couples. From those measurements, it was shown that the Os(V) intermediate, [Osv(N)(tpy)(Cl)2], is both a powerful oxidant and a strong reductant, highly unstable with respect to disproportionation into Os(VI) and Os(IV). Oxidation state IV as [OsIV(NH3)(tpy)(Cl)2]2+ is unstable with respect to disproportionation into Os(VI) and Os(III). © 1990, American Chemical Society. All rights reserved.