Several new ruthenium(II) complexes containing 8-(dimethylphosphino)quinoline (Me(2)Pqn) were synthesized, and their structures and electrochemical/spectroscopic properties have been investigated. In addition to the mono(Me2Pqn) complex [Ru(bpy or phen)(2)(Me(2)Pqn)](PF6)(2) (1 or 1'; bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline), the geometrical isomers trans(P)- and C-1-[Ru(bpy)(Me(2)Pqn)(2)](PF6)(2) (tP-2 and C-1-2) and mer- and fac-[Ru(Me(2)Pqn)(3)](PF6)(2) (m-3 and f-3) were also selectively synthesized and isolated. It was found that complexes tP-2 and m-3 were converted quantitatively to the corresponding C1-2 and f-3 isomers, respectively, by irradiation of light corresponding to the MLCT transition energy. The strong trans influence of the Me2P- donor group of Me(2)Pqn was confirmed by the X-ray structural analyses for 1, tP-2, m-3, and f-3. Cyclic voltammetry of a series of complexes, [Ru(bPY)(3)](PF6)(2), 1, C-1-2, and f-3, exhibited a reversible one-electron oxidation wave and two or three one-electron reduction waves. The oxidation potentials of the complexes gave a large positive shift with increasing number of coordinated Me(2)Pqn molecules, indicating a larger pi-acceptability of the Me2P- group compared with bpy or qn. Complex f-3 in EtOH/MeOH (4:1) glass at 77 K exhibited an intense long-lived (T = 920 mus) emission arising from the quinoline-based (3)(pi-pi(*)) excited state. In contrast, the mixed-ligand complexes 1, 1', and C-1-2 showed a characteristic dual emission, giving a double-exponential emission decay, and the dual emission originates from both the bpy-based (MLCT)-M-3 and the quinoline-based emitting states.