Structure, Bonding, and Stability of Mercury Complexes with Thiolate and Thioether Ligands from High-Resolution XANES Spectroscopy and First-Principles Calculations

Abstract

We present results obtained from high energy-resolution L3-edge XANES spectroscopy and first-principles calculations for the structure, bonding, and stability of mercury(II) complexes with thiolate and thioether ligands in crystalline compounds, aqueous solution, and macromolecular natural organic matter (NOM). Core-to-valence XANES features that vary in intensity differentiate with unprecedented sensitivity the number and identity of Hg ligands and the geometry of the ligand environment. Post-Hartree-Fock XANES calculations, coupled with natural population analysis, performed on MP2-optimized Hg[(SR)2···(RSR)n] complexes show that the shape, position, and number of electronic transitions observed at high energy-resolution are directly correlated to the Hg and S (l,m)-projected empty densities of states and occupations of the hybridized Hg 6s and 5d valence orbitals. Linear two-coordination, the most common coordination geometry in mercury chemistry, yields a sharp 2p to 6s + 5d electronic transition. This transition varies in intensity for Hg bonded to thiol groups in macromolecular NOM. The intensity variation is explained by contributions from next-nearest, low-charge, thioether-type RSR ligands at 3.0-3.3 Å from Hg. Thus, Hg in NOM has two strong bonds to thiol S and k additional weak Hg···S contacts, or 2 + k coordination. The calculated stabilization energy is -5 kcal/mol per RSR ligand. Detection of distant ligands beyond the first coordination shell requires precise measurement of, and comparison to, spectra of reference compounds as well as accurate calculation of spectra for representative molecular models. The combined experimental and theoretical approaches described here for Hg can be applied to other closed-shell atoms, such as Ag(I) and Au(I). To facilitate further calculation of XANES spectra, experimental data, a new crystallographic structure of a key mercury thioether complex, Cartesian coordinates of the computed models, and examples of input files are provided as Supporting Information .

Mercury(II) Binding to Metallothionein in Mytilus edulis revealed by High Energy‐Resolution XANES Spectroscopy

“…The closest oxygen atom bends the S‐Hg‐S angle to 170.8°, a value significantly higher than the 163–165° angular values for the Hg(Cys) 2 and Hg(PC2) references (Figures and a). The O ligands are almost perpendicular to the S‐Hg‐S bond direction, as reported previously for the NH 2 and RSR sulfide/thioether secondary ligands . Bond analysis in terms of natural population analysis (NPA) shows that a carbonyl oxygen is a little more nucleophilic (partial atomic charge −0.6 to −0.7 e) than an amide nitrogen from the ‐CO−NH‐ peptide bond (−0.6 e), and these two atoms are much less than an amine nitrogen (−0.9 e) (Figures S2 and S5, see Supporting Information) .…”

“…According to the Natoli rule, the energy shift of the post‐edge maximum results from an increase of the interatomic distance (i.e., coordination number) between the photoabsorbing ion (Hg) and the neighboring atoms (S). Similar observations regarding the sensitivity of XANES to the nearest ligand distance were reported for the coordination geometry of cadmium in MTs and for Hg coordinated to two [ d (Hg−S)≈2.33 Å] and four [ d (Hg−S)≈2.52 Å] sulfur atoms (Figures and S2 in the Supporting Information) . Thus, region D confirms that MT has less dithiolate Hg complex than ME, but still is not conclusive on the coordination of the second Hg species, which can be three‐ or four‐fold.…”

“…Thermodynamic calculation adds further support to the α domain occupation for the Hg x S y species identified by HR‐XANES. The Gibbs free energy (Δ G ) of formation of multinuclear n [Hg(SR) 2 ] complexes from the condensation of bis‐Hg‐thiolate complexes Hg(SR) 2 increases with the nuclearity ( n ) of the cluster . Thus, a tetranuclear Hg‐α cluster is more stable than a trinuclear Hg‐β cluster.…”

Nucleation of mercury sulfide by dealkylation

“…The R group was represented in the computational work by a methyl group (CH 3 ). This simplification has been validated previously on stability calculations of Hg(II) complexes with thiolate and thioether ligands2224, and is also justified by the independence of the dissociation energy of the R−SH bond with respect to the nature of the R radical25. The direct transfer of an alkyl group between the two Hg-thiolate complexes may be regarded as a nucleophilic substitution with two sulfur atoms as nucleophilic centers.…”

Self‐Healable and Tough Thermoplastic Materials from Metal–Thioether Block Polymers

“…The molar ratio of silver ions to thioether units (Ag:T) also strongly influences the mechanical performance (Figure C). Silver and thioether complexes are molecules with a typical coordination ratio of 1:2 (Ag:T) . Interestingly, when the ratio was adjusted to 1:1, very low pulling stress level (<0.06 MPa) and modulus were observed.…”