Reaction of NO₂^– with [RuCl₂(py)₄] gave [Ru(NO₂)₂(py)₄] which on treatment with HCl gave [RuCl(py)₄(NO)]²⁺, isolated as ClO₄^– or PF₆^– salts. Use of HBr or HClO₄, instead of HCl gave [RuBr(py)₄(NO)]²⁺ or [Ru(OH)(py)₄(NO)]²⁺ respectively. The nitrosyl ligand in [RuX(py)₄(NO)]²⁺ behaved as an electrophile [ν(NO) was 1 910 cm^–1 for X = Cl, 1 901 cm^–1 for X = Br, and 1 868 cm^–1 for X = OH]. With OH^–[RuX(NO₂)(py)₄] was formed reversibly. With an excess of N₃^– and [RuCl(py)₄(NO)][ClO₄]₂ a mixture of [RuCl(N₃)(py)₄] and [RuCl(H₂O)(py)₄]⁺ was formed, N₂ and N₂O being evolved. The less soluble [RuCl(py)₄(NO)][PF₆]₂ reacted with an equimolar amount of N₃^– to give [RuCl(N₂)(py)₄]PF₆, which was unstable with respect to N₂ loss in solution, and was contaminated with a small quantity of a reduced nitrosyl complex, believed to be [Ru(H₂O)(py)₄(NO)][PF₆]₂ or [Ru(py)₄(NO)][PF₆]₂·H₂O. The formation of [RuCl(N₂)(py)₄]⁺ indicates that the reaction between [RuCl(py)₄(NO)]²⁺ and N₃^– proceeds via a cyclic [graphic omitted] intermediate, as was confirmed by labelling experiments. Electrochemical one-electron reduction of [RuCl(py)₄(NO)]²⁺ gave [Ru(H₂O)(py)₄(NO)]²⁺, isolated as the PF₆^– salt; it is not known how strongly the H₂O molecule is attached to the ruthenium, if at all. Electrochemical six-electron reduction of [RuCl(py)₄(NO)]²⁺ gave [RuCl(NH₃)(py)₄]⁺; this same product could be isolated as the PF₆^– salt from zinc amalgam reduction of [RuCl(py)₄(NO)]²⁺. Polarographic, coulometric, and cyclic voltammetry experiments showed that [RuCl(py)₄(NO)]²⁺ is reduced in two successive reversible one-electron steps followed by an irreversible four-electron reduction.