Mercury(II) cysteine complexes in alkaline aqueous solution

Mercury(II) complexes with L-cysteine (H(2)Cys) in alkaline aqueous solutions have been structurally characterized by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. The distribution of [Hg(Cys)(n)] (n = 2, 3, and 4) species in similar to 0.09 mol dm(-3) mercury(II) solutions with H(2)Cys/Hg-II ratios varying from 2.2 to 10.1 has been evaluated by fitting linear combinations of simulated EXAFS functions for the separate complexes to the experimental EXAFS data, aided by Hg-199 NMR and Raman results. For the [Hg(Cys)(2)](2-) and [Hg(Cys)(3)](4-) complexes and the novel four-coordinated Hg(Cys)(4) species that dominates in solutions with excess of cysteine (H(2)Cys/Hg-II > 5), the mean Hg-S bond distances were found to be 2.35(2), 2.44(2), and 2.52(2) angstrom, respectively. The minor amount of the linear [Hg(Cys)(2)](2-) complex that can still be discerned in solutions with ratios up to H(2)Cys/Hg-II = 5 was derived from the distinct S-Hg-S symmetric stretching Raman band at 334 cm(-1). From Hg-199 NMR spectra, the chemical shift of the Hg(Cys)(4) species was estimated to -340 ppm with an amount exceeding 85% in the highest excess of cysteine, consistent with the EXAFS data.