A variety of (diphosphine)platinum(II) carbonate complexes, (LL)Pt(CO3), are readily prepared from the corresponding (diphosphine)platinum dichlorides by treatment with silver carbonate in dichoromethane solution provided that water is present. This reaction also permits facile preparation of analogous C-13-labeled complexes. The carbonate ligands in these complexes have been characterized by IR and C-13 NMR spectroscopy. Alternative preparative routes involve conversion of the precursor dichlorides to the corresponding dialkoxides or diphenoxides, followed by treatment with water and carbon dioxide. Various reaction intermediates have been spectroscopically observed in the latter syntheses. Two crystalline modifications of (Ph(2)PCH(2)CH(2)CH(2)PPh(2))Pt(CO3), one with and one without a dichloromethane of solvation, have been studied by single-cgrstal X-ray diffraction. Crystal data for PtP2O3C28H26 : P2(1)/c, Z = 4, T = 200 K, a = 10.362(8) Angstrom, b = 14.743(6) Angstrom, c = 19.183(10) Angstrom, beta = 122.69(6)degrees. Crystal data for PtP2O3C28H26. CH2Cl2 : P2(1)/c, Z = 4, T approximate to 298 K, a = 11.744(2) Angstrom, b = 15.526(3) Angstrom, c = 15.866(3) Angstrom, beta = 101.58(1)degrees.