Mercury(II) penicillamine complex formation in alkaline aqueous solution.

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