Pt(II) complexes of 4-substituted o-phenylenediamine (opda-R; R = CH3, OCH3, H, Cl, COOH, NO2 and SO3H), PtX2 (opda-R), were synthesized and tested for antitumor activity against [mouse] P-388 leukemia. The IR spectra of those complexes were analyzed to assign the Pt-N stretching vibrations on the basis of the halogeno trans effect and Hammett''s .sigma. values. The assigned Pt-N stretching vibrations were related to the antitumor activities of the opda-R Pt(II) complexes.