A platinum(II) complex bearing 1,10-phenanthroline-5,6-dione (phenO(2)), [PtCl(phenO(2)-N,N')(PPh3)](+) ([2](+)) was newly prepared and characterized. An X-ray crystallographic study of [2](PF6) revealed that the phenO(2) ligand coordinates to platinum with the N,N'-chelate form. The basicity of the quinoid part of phenO(2) in [2](+) and of the diimine one in [Pt(phenO(2)-O,O')(PPh3)(2)] with an O,O'-chelate ring (3) was evaluated in the absence and the presence of Li+ by means of UV-vis spectroscopy and cyclic voltammetry. The quinoid group in [2](+) practically had no ability to bind Li+, but induced on anodic potential shift of the phenO(2) localized redox potential by 850 mV due to the strong interaction between the reduced form of [2](+) and Li+. Undeniably, a weak interaction of Li+ with the diimine moiety of free phenO(2) (K = 3.6 M-1) is greatly enhanced in the interaction between Li+ and the diimine site in 3 because of the electron donation from the catecholato group of the complex.