The interaction of cysteine methyl ester with metal ions. Part I. The nature of the complex species

Journal of the Chemical Society A: Inorganic, Physical, Theoretical Pub Date: DOI: 10.1039/J19690000118

Abstract

The proton ionisation constants of cysteine methyl ester have been determined at 25°, 37°, and 50° and I= 0·1M(KCl). Values of ΔH° and ΔS°298 for the various ionisation equilibria have been calculated. Stability constants have been determined potentiometrically at 25° and I= 0·1M for complexes present in aqueous solutions of cysteine methyl ester and the bivalent ions of nickel, zinc, cadmium, mercury, and lead. In the case of nickel(II) the complexes NiL+, NiL2, NiL3–, Ni4L62–, NiHL2+, and NiHL2+ occur. Confirmatory evidence for the existence of polynuclear species in this system has been obtained spectrophotometrically. Polynuclear complexes do not occur with zinc(II) and lead(II), but protonated species (ZnHL2+, ZnHL2+) and hydrolysed species [ZnL(OH)] occur with zinc in addition to simple mononuclear complexes. Protonated complexes also occur with lead(II). A polynuclear complex Cd2L3+ occurs with cadmium, in addition to the simple mono- and bis-species. In the case of mercury(II) it is suggested that the complex HgL+ is completely formed at the commencement of pH-tritrations at a 1 : 2 metal-to-ligand ratio (i.e., log β1 is very large). Using this assumption the titration data can be satisfactorily described by the set of complexes HgL2, Hg2L3+, and Hg3L42+. A number of solid complexes have been isolated and their infrared spectra studied. Of particular interest is the isolation of a number of trinuclear complexes of the type [M{Ni[SCH2CH(NH2)COCH3]2}2]MCl4 where M = Zn(II), Cd(II), and Hg(II).

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