The reaction of the [Ru(bpy)(NO2)(4)](2-) (bpy = 2,2'-bipyridine) ion in aqueous solutions produces two different nitrosyl complexes, depending on the pH of the solution. At acidic pH, complex cis, cis-Ru (bpy) (NO2)(2)(ONO)(NO) was isolated. At neutral or basic pH, [Ru(bpy)(NO2)(4)](2-) reacts to give cis,trans-Ru(bpy)(NO2)(2)(NO)(OH). Both new complexes were fully characterized by elemental analysis and UV-vis, IR, H-1 NMR, and N-15 NMR spectroscopy. A single-crystal X-ray structure of cis,trans-Ru(bpy)(NO2)(2)(NO)(OH) was also obtained. cis,cis-Ru(bpy)(NO2)(2)(ONO)(NO) isomerizes in acetone or water solution to give a mixture of the trans, cis-Ru (bpy) (NO2)(2)(ONO)(NO) and cis,cis-Ru(bpy)(ONO)(2)(NO2)(NO) linkage isomers as determined by H-1 and N-15 NMR spectroscopy. A single-crystal X-ray structure of a solid solution of cis,cis-Ru(bpy)(ONO)(2)(NO2)(NO)/trans,cis-Ru(bpy)(NO2)(2)(ONO)(NO) was also obtained. This pair of isomers is the first crystallographically characterized compound with nitro, nitrito, and nitrosyl ligands. The kinetic studies of the Ru-NO2 --> Ru-NO conversion reactions of [Ru(bpy)(NO2)(4)](2-) in buffered solutions from pH 3 to pH 9 complement previous studies of the reverse reaction. The reactions are first order in [Ru(bpy)(NO2)(4)](2-). At high pH, the reaction is independent of the concentration of H+ while, at low pH, the reaction is first order in the concentration of H+. The rate determining step of the high pH reaction involves breakage of the Ru-NO2 bond while, at low pH, the mechanism involves a rapid reversible protonation of a NO2 ligand followed by the rate determining loss of hydroxide to produce a nitrosyl ligand.