The reactivity of ruthenium(II)- and ruthenium(III)-chloride-dimethyI sulfoxide precursors and of the antimetasatic drug [ImH][trans-RuCl4(dmso-S)(Im)] (NAMI-A, Im = imidazole, dmso = dimethyl sulfoxide) toward NO was investigated. Treatment of [(dmso)(2)H][trans-RuCl4(dmso-S)(2)] and mer-RuCl3(dmso)(3) with gaseous NO yielded [(dmso)(2)H][trans-RuCl4(dmso-O)(NO)] (1) and mer, cis-RuCl3(dmso-O)(2)(NO) (2), respectively. Thus, coordination of the strong pi-acceptor NO induces a S to O linkage isomerization of the dmso trans to it to avoid competition for pi-electrons. In light-protected nitromethane solutions, complex 2 equilibrates slowly with the two isomers mer-RuCl3(dmso-S)(dmso-O)(NO) (3), with NO trans to Cl, and mer-RuCl3(dmso-S)(dmso-O)(NO) (4), with NO trans to dmso-O; the equilibrium mixture consists of ca. 64% 2, 3% 3, and 33% 4. Treatment of the Ru(II) precursor trans-RuCl2(dmso-S)(4) with gaseous NO in CH2Cl2 solution yielded the nitrosyl-nitro derivative trans, cis, cis-RuCl2(dmso-O)(2)(NO)(NO2) (5). Finally, [(IM)(2)H][trans-RuCl4(Im)(NO)] (6) was prepared by treatment of [ImH][ trans-RuCl4(dmso-O)(NO)] (1Im) with an excess of imidazole in refluxing acetone. The spectroscopic features are consistent with the {Ru(NO)}(6) formulation for all complexes, that is, a diamagnetic Ru-II nucleus bound to NO+. Compounds 1, 2, 5, and 6 were characterized also by X-ray crystallography; they all show a linear nitrosyl group, with short Ru-NO bond distances consistent with a strong d(pi) --> pi* NO back-bonding. An unusual inertness of O-bonded dmso was observed in compound 1. Complexes 1, 2, 3, 5, and 6 are all redox active in DMF solutions showing irreversible reductions whose peak potentials depend on the other ligands attached to the Ru metal center. The site of reduction is the NO+ moiety. The reduced complexes are not stable and release a Cl- or NO2-ligand followed by the NO. radical. The chemical reactions following electron transfer are all fast (rate constant >100 s(-1) at 293 K). The Ru product species are not redox active within the DMF window.