All the possible three O,O’-, O,S-, and S,S’-chelation modes were observed for diamineplatinum(II) complexes of sulfur-containing dicarboxylate ligands S-(CH2)(n)-S-C=C(COO-)(2)(n=2,3,4) depending on their dithioether ring size. Such variable coordination modes are explicable in terms of their ring size effects affording different degrees of contribution to the resonance structures of the planar C2C=CS2 moiety, which gives rise to dramatic change in the basicity of the ring sulfur atoms competing with the carboxylate oxygen atoms for coordination to patinum(II).