Graphical abstract Highlights • A new synthesis of redox-active bis-chelate Ru(II) complex Ru(ONO) 2. • The RuIII/RuII conversion through the formation of RuIII (ONO) 3 intermediate. • Ru(ONO) 2 enters into ligand substitution only via its radical-anion form. • The structure of Ru(ONO)Py 3 is X-ray established. Abstract The redox-active bis-chelate hexacoordinate ruthenium complex 2 Ru(ONO) 2 comprising two redox-active 2,4,6,8-tetrakis(tert. -butyl)-9-hydroxyphenoxazin-1-one ligands 1 was synthesized by coupling ruthenium trichloride with 1 or its thallium salt. Reduction of 2 with zinc amalgam produces its radical-anion 2a , which readily reacts with pyridine in aerobic conditions resulting in substitution of one of the tridentate hydroxyphenoxazinone ligand by three pyridine molecules and the formation of a salt of tris(pyridine)(tetra-(tert. -butyl)oxyphenoxazin-1-onate) Ru(II) cation 5. The structure of this salt was established by X-ray crystallography and its redox activity investigated using cyclic voltammetry. The mechanism of the RuIII/RuII conversion observed in this reaction was suggested based on the data of ESR, UV-Vis spectra and DFT B3LYP/6-311++G(d,p)/SDD calculations. [ABSTRACT FROM AUTHOR]