Equilibria of some α-amino-acids containing sulphur atoms in the chain with metal(II) ions. Part 1. DL-4,4′-dithiobis(2-aminobutyric acid) with manganese(II), cobalt(II), and nickel(II) in aqueous solution

Abstract

The formation constants of the complexes formed between H⁺, Mn²⁺, Co²⁺, and Ni²⁺ and DL-4,4′-dithiobis(2-aminobutyric acid)(dtba) have been measured potentiometrically at 25 °C and I= 0.1 mol dm^–3 KCl. Titration curves of the ligand in the presence of Mn²⁺, Co²⁺, and Ni²⁺ ions did not reveal appreciable complexing in acidic media. At intermediate pH values (ca. 6–8) the equilibria, after neutralization of two COOH groups, have been shown to include a large range of protonated complexes with the ligand behaving as tri- or quadri-dentate chelate depending on the pH. The determination of the formation constants of complexes formed by dtba with divalent metals has shown that there are the following species in solution: [Mn(HL)]⁺, [MnL₂]^2–; [Co(HL)]⁺, [CoL], [CoL₂]^2–; [Ni(HL)]⁺, [NiL], [NiL₂]^2–, where the ligand L^2–=[^–O₂CCH(NH₂)CH₂CH₂S–]₂. These complexes are very likely to be N,O-chelates of a glycine type. The proton of the hydrogen complexes is probably bound to the amine group which is not a part of the chelate ring. The cumulative and stepwise formation constants follow the Irving–Williams series. Probable structures for the various complexes are discussed by comparing them with the crystal structure of the solid ligand and by considering the values of log (K_n/K_n+1).