The Stability of Some Metal Complexes in Mixed Solvents

Acidity constants and stability constants of certain metal complexes (nickel(II), zinc(II), and manganese(II)) with ethylenediamine and glycine were determined at 25 °C and in 0.1 M ionic strength in mixed aqueous solutions of methanol (MeOH), dioxane (DOX), acetonitrile (MeCN), and dimethylformamide (DMF). In general, the stability constants of the metal complexes and the first protonation constant of glycine increase with increasing composition of the co-solvent in the order[Formula: see text]The protonation constants of ethylenediamine pass through a minimum at some particular solvent composition, while the second protonation constant of glycine exhibits behavior characteristic of the particular solvent employed.These results are discussed in terms of medium and solvent effects and properties of the solvent that might affect these, but find more satisfactory explanation in terms of solvent displacement during the association reactions. The observations can be accounted for in terms of a complete equilibrium constant incorporating the concentration (activity) of each solvent substance.