Hydrazine nitrosation of [Ru(NO)(py(si)S(4))] Br, THF ( 1) (py(si)S(4)(2-) = 2,6-bis(3-triphenylsilyl-2-sulfanylphenylthiomethyl)pyridine(2-)) in methanol/DMF led to the formation of mononuclear ammine complex [Ru(NH3)(py(si)S(4))] ( 2) and N2O, whereas the reaction performed in THF/CH2Cl2/toluene afforded thioether-bridged dinuclear ammine complex [(NH3)Ru( mu-py(si)S(4)) Ru(py(si)S(4))] ( 3). Compound 2 dimerizes in solution at room temperature to form 3 and is regenerated upon treatment of 3 with NH3. A plausible mechanism for the hydrazine nitrosation of 1 has been proposed. The reaction of 1 with NH3 or N-3(-) does not lead to a nucleophilic attack at the NO+ ligand but to a deprotonation that yields neutral nitrosyl complex [Ru(NO) {py(si)S(4)(H+)}] ( 4), which is supported by density functional theory calculations.