The formation of 1:1 adducts of Pt(bpy)(2)(2+) (bpy = 2,2'-bipyridine) with various N bases (B) has been ascertained in water at ambient temperature by spectrophotometric titration and electrospray ionization mass spectroscopy. A pseudo-five-coordinated complex, [Pt(bpy)(2)(B)](2+) with a monodentating bpy, is proposed based on density functional theory calculation. The formation constants (K-c) increase with the nucleophilicity of B except for sterically hindered N-bases, indicating an associative ligand-substitution mechanism.