A study of the formation constants of ternary and quaternary complexes of some bivalent transition metals

The formation of hetero-ligand 1:1:1, M(II)-Opda-Sal/Gly ternary and 1:1:1:1, M(II)-Opda-Sal-Gly quaternary complexes, where M(II) = Ni, Cu, Zn and Cd; Opda = o-phenylenediamine, Sal = salicylic acid, Gly = glycine, was studied pH-metrically in aqueous medium. The formation constants for the resulting ternary and quaternary complexes were evaluated at a constant ionic strength, μ = 0.20 mol dm -3 and temperature, 30±0.1 °C. The order of the formation constants in terms of the metal ion for both type of complexes was found to be Cu(II) > Ni(II) > Zn(II) > Cd(II). This order was explained based on the increasing number of fused rings, the coordination number of the metal ions, the Irving - William order and the stability of various species. The ex- pected species formed in solution were pruned with the Fortran IV program SPEPLOT and the stability of the ternary and quaternary complexes is explained.