Mercury(ii) penicillamine complex formation in alkaline aqueous solution

Abstract

The complex formation between mercury(II) and penicillamine (H₂Pen = 3,3′-dimethyl cysteine) in alkaline aqueous solutions (pH ∼ 2) has been investigated with extended X-ray absorption fine structure (EXAFS) and ¹⁹⁹Hg NMR spectroscopy. By varying the penicillamine concentration (C_H2Pen = 0.2–1.25 M) in ∼0.1 M Hg(II) solutions, two coexisting major species [Hg(Pen)₂]²⁻ and [Hg(Pen)₃]⁴⁻ were characterized with mean Hg–S bond distances 2.34(2) and 2.44(2) Å, respectively. The [Hg(Pen)₂]²⁻ complex with two deprotonated penicillamine ligands forms an almost linear S–Hg–S entity with two weak chelating Hg–N interactions at the mean Hg–N distance 2.52(2) Å. The same type of coordination is also found for the corresponding [Hg(Cys)₂]²⁻ complex in alkaline aqueous solution with the mean bond distances Hg–S 2.34(2) Å and Hg–N 2.56(2) Å. The relative amounts of the [Hg(Pen)₂]²⁻ and [Hg(Pen)₃]⁴⁻ complexes were estimated by fitting linear combinations of the EXAFS oscillations to the experimental spectra. Also their ¹⁹⁹Hg NMR chemical shifts were used to evaluate the complex formation, showing that the [Hg(Pen)₃]⁴⁻ complex dominates already at moderate excess of the free ligand ([Pen²⁻] > ∼ 0.1 M).