The reaction of NO with highly unsaturated, triplet spin state (PNP)(RuCl)-Cl-II ("PNP" = ((Bu2PCH2SiMe2)-Bu-t)(2)N) in benzene at 20degreesC is reported. The reaction proceeds through three major intermediate species, ultimately forming [(PNP)Ru(NO)(2)(+)][Ru(NO)(OH)(NO2)(2)Cl-2(-)], whose structure is determined by X-ray diffraction. The implications of PNP ligand loss, NO2- production, and partial oxidation of ruthenium to Ru(III) (the anion above) are discussed, together with the observed oxygen transfer which represents NO disproportionation. The two NO ligands in the cation are chemically inequivalent (one bent, NO-, and one linear, NO+), features which are studied by density functional theory (DFT) geometry optimization. Two isomers of (PNP)Ru(NO)(2)Cl, as well as (PNP)Ru(NO)CI are evaluated as possible reaction intermediates by DFT geometry optimization. (C) 2004 Elsevier B.V, All rights reserved.