Stability-charge and stability-structure relationships in the binding of dicarboxylic ligands by open-chain polyammonium cations

The stability of complexes formed by 5 dicarboxylic ligands with 11 open-chain polyammonium cations (40 systems) was studied potentiometrically, in aqueous solution, at 25 degrees C. In all the systems ALH, species (A = amine, L = dicarboxylic ligand, r =1 ... n; diamines n = 3, triamines n = 4, tetraamines n = 4 or 5) were found. Formation constants for the various complexes studied in this work, together with data for the analogous systems previously studied (20 systems); were examined as a function of charges involved in the formation reaction and of the structure of both the dicarboxylic ligand and the polyammonium cation. Structure generally has little effect on stability, while charges play a fundamental role. The most general relationship, valid for all the complexes (the formation constants of 249 complexes were examined) is log K = 0.925(+/-0.010)zeta(2/3) (zeta = \z(anion)z(cation)\). The likely role of the formation of these mixed species in the speciation of natural waters and biological fluids is also discussed.