Ab initio calculations on the complex Cl2Pt(malonate)(-) have been performed to estimate the magnitude and the source of the splitting between the symmetric and antisymmetric C=O stretches. It is shown that a large part of the splitting can result from pure electronic coupling mediated via the pi -structure of the ligand. Calculations have also been performed on the free ligand malonate(-) and the neutral malonaldehyde that show decreasing electronic coupling, which is consistent with the decreasing resonance stabilization. A simple effective C=O interaction potential has been produced for Cl2Pt(malonate)- that is used to estimate the electronic splitting of the 1600 cm(-1) IR band to 137 cm(-1). Anharmonic contributions seem to reduce this value somewhat, but the magnitude and sign of the splitting is similar to the experimental value of 183 cm(-1) found for the similar Cl2Pt(acac)(-) complex.