The synthesis and electrochemical properties of a Ru(II) complex having a redox active ligand, 1,10-phenanthroline-5,6-dione, [Ru(trpy)(PD-N,N')Cl](PF6) ([1](PF6)), and mixed-metal complexes [(PPh3)(2)Pd(O,O'-PD-N,N')Ru(trpy)-Cl](PF6) ([2](PF6)) and [(PPh3)(2)Pt(O,O'-PD-N,N')Ru(trpy)Cl](PF6) ([3](PF6)) (trpy = 2,2':6',2"-terpyridine, PD = 1,10-phenanthroline-5,6-dione) are presented. The complex of [1](+), which was prepared by the reaction of Ru(trpy)(DMSO)Cl-2 with PD in hot ethanol, underwent two reversible reductions in the PD-based redox reactions. The quinoid moiety of [1](+) was endowed with coordination ability to metals by one- and two-electron reduction of the complex. The mixed-metal complexes of [2](+) and [3](+) were synthesized by the reactions of [1](+) with M(PPh3)(4) (M = Pd, Pt), and both complexes have the [1](-) and M(II) frameworks.