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

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^–, Ni₄L₆^2–, 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).