Formation and stability of ternary complexes of copper(II) with ethylenediamine-N-acetic acid and amino acids in aqueous solution

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Abstract
The stability constants have been determined potentiometrically for the ternary copper(II)–edma–amino acid systems, where edma refers to ethylenediamine-N-acetate, and amino acid to glycine (Gly), L-alanine (Ala), L-norvaline (Nor), L-phenylalanine (Phe), or L-tryptophan (Trp). The tendency of mixed complex formation (Δ log K) decreased in the order Trp > Phe > Nor ≃ Ala ≃ Gly. The stability enhancement in the complex formation equilibria was found to be additive and proportional to the aromatic ring size. This trend can be explained on the basis of the proximity of an aromatic ring to the central copper(II) ion increasing the electronegativity of the metal by both copper(II)–aromatic ring interactions and decreased hydration. The co-ordination types of these complexes are discussed.