Equilibria of some α-amino-acids containing sulphur atoms in the chain with metal(II) ions. Part 2. SS′-methylenebis(L-cysteine) with cobalt(II), nickel(II), and zinc(II) in aqueous solution

Abstract

Studies of the complexes between bivalent metal ions Co²⁺, Ni²⁺, and Zn²⁺, and SS′-methylenebis(L-cysteine)(H₂L), and H⁺ have been undertaken. The quantitative equilibrium investigations of H₂L with the above mentioned metal ions are described in detail. The proton ionization proton, metal, and ligand ion association constants have been calculated from potentiometric data at 25 °C and I= 0.1 mol dm^–3(KCl). The ligand forms with metals the complexes [Co(HL)]⁺, [CoL], [CoL₂]^2–; [Ni(HL)]⁺, [NiL], [NiL₂]^2–; [ZnL], [ZnL₂]^2–, where the ligand L^2–=^–O₂C(H₂N)CHCH₂SCH₂SCH₂CH(NH₂)CO₂^–. The equilibrium constants follow the Irving–Williams series. All the complexed species very likely contain penta-atomic chelate rings of the glycine type. The probable structures of the chelated compounds formed in aqueous solution are deduced by structural analysis of the solid ligands, and their stability constants are compared with those of analogous chelated compounds. The H⁺ of the protonated complexes is probably bound to the amine group not involved in the chelate ring. The importance of the protonated complexes in almost all systems studied has been demonstrated and the relative structures are discussed.