The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ and the pyrimidine-nucleoside 5'-diphosphates CDP3-, UDP3-, and dTDP(3-) (= NDP3-) were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3; 25 degrees C). For comparison, the same values were measured for the corresponding complexes with the simple diphosphate monoesters (R-DP3-) phenyl diphosphate, methyl diphosphate, and n-butyl diphosphate. The acidity constants for H-3(CDP)(+/-), H-2(UDP)(-), H-2(dTDP)(-), and H-2(R-DP)(-) were measured also via potentiometric pH titration and various comparisons with related constants are made. By plotting log KM(R-DP)(M) versus pK(H(R-DP))(H) for the complexes of all six diphosphates mentioned and by a careful evaluation of the deviation of the various data pairs from the straight-line correlations, the expectation is confirmed that in the M(UDP)(-) and M(dTDP)(-) complexes the metal ion is only diphosphate-coordinated. The straight-line equations, which result from the mentioned correlations, together with the pK(a) value of a given monoprotonated diphosphate monoester allow now to predict the stability of the corresponding M(R-DP)(-) complexes. In this way, the experimentally determined stability constants for the M(CDP)(-) complexes are evaluated and it is concluded that the pyridine-like N3 of the cytosine residue does not participate in complex formation; i.e., the stability of the M(CDP)(-) complexes is also solely determined by the coordination tendency of the diphosphate residue. In all the monoprotonated M(H;NDP) and M(H;R-DP) complexes both, H+ and M2+, are bound at the diphosphate group. Only the Cu(H;CDP) complex exists in aqueous solution in the form of three different isomers: about 15% of the species have Cu2+ and H+ the diphosphate residue, in about 13% Cu2+ is bound at N3 and H+ at the terminal beta-phosphate group, and the dominating isomer with about 72% carries the proton at N3 and the metal ion at the diphosphate residue. Several general features of phosphate-metal ion coordination are discussed, and estimations for the stabilities of the Fe2+ complexes formed with mono-, di-, and triphosphate monoesters are provided.