An EXAFS Spectroscopic, Large‐Angle X‐Ray Scattering, and Crystallographic Study of Hexahydrated, Dimethyl Sulfoxide and Pyridine 1‐Oxide Hexasolvated Mercury(II) Ions

Abstract

The structure of the solvated mercury(II) ion in water and dimethyl sulfoxide has been studied by means of large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The distribution of the Hg-O distances is unusually wide and asymmetric in both solvents. In aqueous solution, hexahydrated [Hg(OH(2))(6)](2+) ions in a distorted octahedral configuration, with the centroid of the Hg-O distance at 2.38(1) A, are surrounded by a diffuse second hydration sphere with HgO(II) distances of 4.20(2) A. In dimethyl sulfoxide, the six Hg-O and HgS distances of the hexasolvated [Hg{OS(CH(3))(2)}(6)](2+) complex are centered around 2.38(1) and 3.45(2) A, respectively. The crystal structure of hexakis(pyridine 1-oxide)mercury(II) perchlorate has been redetermined. The space group R(-)3 implies six equal Hg-O distances of 2.3416(7) A for the [Hg(ONC(5)H(5))(6)](2+) complex at 100 K. However, EXAFS studies of this compound, and of the solids hexaaquamercury(II) perchlorate and hexakis(dimethyl sulfoxide)mercury(II) trifluoromethanesulfonate, also with six equidistant Hg-O bonds according to crystallographic results, reveal in all cases strongly asymmetric Hg-O distance distributions. Vibronic coupling of valence states in a so-called pseudo-Jahn-Teller effect probably induces the distorted configurations.

The Molecular Structure of Aqueous Hg(II)-EDTA As Determined by X-ray Absorption Spectroscopy

“…Overlaying our XANES spectra of Hg­(II)-EDTA in pure Milli-Q at 298 K onto the spectra of Hg 2+ (aq) shows that both spectra share the same edge position as well as the same shoulder feature at approximately 12 285 eV (Figure C). One study that analyzed multiple known hexacoordinated Hg­(II) compounds noted a XANES feature they all had in common is the shoulder we also observe at ∼12 285 eV . A fit of the EXAFS of Hg 2+ (aq) reveals a Hg­(II) coordination number of 5.6 ± 0.3 (see Supporting Information), and other studies have also determined that aqueous Hg 2+ is hexacoordinated. , We note that according to speciation calculations, our Hg 2+ (aq) sample contains ∼2% HgNO 3 + , which could explain why our fit results provide a coordination number slightly lower than 6.…”

“…25 Therefore, we chose to set the initial bond distance of the axial carboxyl oxygens and the equatorial carboxyl oxygen and nitrogens to corresponding values. As we calculated in the Supporting Information and others have reported, 20 the water molecules in Hg 2+ (aq) are on average ∼2.3 Å from Hg. Thus, we assigned a similar initial bond distance for the one suspected coordinating water molecule in Hg(II)-EDTA (aq) .…”

“…One study that analyzed multiple known hexacoordinated Hg(II) compounds noted a XANES feature they all had in common is the shoulder we also observe at ∼12 285 eV. 20 A fit of the EXAFS of Hg 2+ (aq) reveals a Hg(II) coordination number of 5.6 ± 0.3 (see Supporting Information), and other studies have also determined that aqueous Hg 2+ is hexacoordinated. 20,21 We note that according to speciation calculations, our Hg 2+ (aq) sample The Journal of Physical Chemistry A Article contains ∼2% HgNO 3 + , which could explain why our fit results provide a coordination number slightly lower than 6.…”

“…20 A fit of the EXAFS of Hg 2+ (aq) reveals a Hg(II) coordination number of 5.6 ± 0.3 (see Supporting Information), and other studies have also determined that aqueous Hg 2+ is hexacoordinated. 20,21 We note that according to speciation calculations, our Hg 2+ (aq) sample The Journal of Physical Chemistry A Article contains ∼2% HgNO 3 + , which could explain why our fit results provide a coordination number slightly lower than 6. The shared edge position and shoulder feature at ∼12,285 eV for Hg 2+ (aq) and Hg(II)-EDTA (aq) strongly support that Hg(II)-EDTA (aq) is hexacoordinated.…”

Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques

“…The widespread diffusion of the EXAFS technique since the 90s limited the use of WAXS methods. Nevertheless, few recent examples can be found [489][490][491][492][493], often combined with EXAFS analysis. In the SAXS region, the X-ray scattering signal is sensitive to shape and dimensions of macromolecules, typically between 1 and 100 nm in size [482] and to characteristic distances up to 150 nm [480] periodically repeating in partially ordered systems.…”