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

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 AgI and AuI. 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.