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X-ray absorption near-edge structure (XANES) spectroscopy has been used, in conjunction with extended X-ray absorption fine structure (EXAFS), to determine the coordination structure of the Ag + ion in a dimethyl sulfoxide (DMSO) solution. From the EXAFS data analysis, the Ag-O first shell distance in DMSO was found to be 2.31(3) Å, with 4.1(5) oxygen atoms surrounding the Ag + ion, in fair agreement with previous results. This technique did not allow us to determine the geometry of the 4-fold coordination complex and a quantitative analysis of the XANES region was carried out to shed light on this issue. The XANES data analysis confirmed the presence of a four-coordinated complex, unambiguously showing that a regular tetrahedral [Ag(DMSO) 4 ] + complex is formed when silver triflate is dissolved in DMSO solution.

Magnesium chloride is a prototypical deliquescent material whose surface properties, although central for Ziegler-Natta cataysis, have so far remained elusive to experimental characterization. In this work, we use surface-selective X-ray absorption spectroscopy (XAS) at ambient pressure in combination with multivariate curve resolution, molecular dynamics, and XAS theoretical methods to track in real time and accurately describe the interaction between water vapor and the MgCl 2 surface. By exposing MgCl 2 to water vapor at temperatures between 595 and 391 K, we show that water is preferentially adsorbed on five-coordinated Mg 2+ sites in an octahedral configuration, confirming previous theoretical predictions, and find that MgCl 2 is capable of retaining a significant amount of adsorbed water even under prolonged heating to 595 K. As a consequence, our work provides first experimental insights into the unique surface affinity of MgCl 2 for atmospheric water. The developed technique is proven highly sensitive to the modifications induced by adsorbates on a given low-Z metal based surface and may be useful in the toolbox required to disentangle the mechanisms of interfacial chemical processes.

An approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to carry out a comparative study about the solvation properties of dilute La(NO 3 ) 3 solutions in water and methanol, with the aim of elucidating the still elusive coordination of the La 3+ ion in the latter medium. The comparison between these two systems enlightened a different behavior of the nitrate counterions in the two environments: while in water the La(NO 3 ) 3 salt is fully dissociated and the La 3+ ion is coordinated by water molecules only, the nitrate anions are able to enter the metal first solvation shell to form inner-sphere complexes in methanol solution. The speciation of the formed complexes showed that the 10-fold coordination is preferential in methanol solution, where the nitrate anions coordinate the La 3+ cations in a monodentate fashion and the methanol molecules complete the solvation shell to form an overall bicapped square antiprism geometry. This is at variance with the aqueous solution where a more balanced situation is observed between the 9- and 10-fold coordination. An experimental confirmation of the MD results was obtained by La K-edge XAS measurements carried out on 0.1 M La(NO 3 ) 3 solutions in the two solvents, showing the distinct presence of the nitrate counterions in the La 3+ ion first solvation sphere of the methanol solution. The analysis of the extended X-ray absorption fine structure (EXAFS) part of the absorption spectrum collected on the methanol solution was carried out starting from the MD results and confirmed the structural arrangement observed by the simulations.

X-ray absorption spectroscopy (XAS) has been employed to carry out structural characterization of the local environment around mercury after the dissolution of the HgCl 2 molecule. A combined EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) data analysis has been performed on the Hg L 3 -edge absorption spectra recorded on 0.1 M HgCl 2 solutions in water, methanol (MeOH), acetone and acetonitrile. The Hg-Cl distance determined by EXAFS (2.29(2)-2.31(2) Å) is always comparable to that found in the HgCl 2 crystal (2.31(2) Å), demonstrating that the HgCl 2 molecule dissolves in these solvents without dissociating. A small sensitivity of EXAFS to the solvent molecules interacting with HgCl 2 has been detected and indicates a high degree of configurational disorder associated with this contribution. XANES data analysis, which is less affected by the disorder, was therefore carried out for the first time on these systems to shed light into the still elusive structural arrangement of the solvent molecules around HgCl 2 . The obtained results show that, in aqueous and MeOH solutions, the XANES data are compatible with three solvent molecules arranged around the HgCl 2 unit to form a trigonal bipyramidal structure. The determination of the three-body Cl-Hg-Cl distribution shows a certain degree of uncertainty around the average 180° bond angle value, suggesting that the HgCl 2 molecule probably vibrates in the solution around a linear configuration.

The metal-based deep eutectic solvent (MDES) formed by NiCl 2 ·6H 2 O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios ( W ) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV-vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni 2+ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni 2+ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni-Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl 2 ·6H 2 O 1:3.5 MDES, the Ni 2+ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) have been combined to study the coordination of the Co 2+ and Ni 2+ ions in ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide ([Tf 2 N] - ) anion and having different organic cations, namely, 1-butyl-3-methylimidazolium ([C 4 mim] + ), 1,8-bis(3-methylimidazolium-1-yl)octane ([C 8 (mim) 2 ] 2+ ), N , N , N -trimethyl- N -(2-hydroxyethyl)ammonium ([choline] + ), and butyltrimethylammonium ([BTMA] + ). Co and Ni K-edge XAS data have been collected on 0.1 mol L -1 Co(Tf 2 N) 2 and Ni(Tf 2 N) 2 solutions and on the metallic salts. MD simulations have been carried out to obtain structural information on the metal ion coordination. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the atomistic description provided by MD, and the studied ILs have been found to be able to dissolve both the Co(Tf 2 N) 2 and Ni(Tf 2 N) 2 salts giving rise to a different structural arrangement around the metal ions as compared to the solid state. The combined EXAFS and MD results showed that the Co 2+ and Ni 2+ ions are surrounded by a first solvation shell formed by six [Tf 2 N] - anions, each coordinating in a monodentate fashion by means of the oxygen atoms. The nature of the IL organic cation has little or no influence on the overall spatial arrangement of the [Tf 2 N] - anions, so that stable octahedral complexes of the type [M(Tf 2 N) 6 ] 4- (M = Co, Ni) have been observed in all the investigated ILs.

X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag + ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag-O bond distance of 2.34(2) Å, different from the first generally accepted tetrahedral model with a longer mean Ag-O bond distance. Ab initio molecular dynamics simulations with the Car-Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag + ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag + ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag + in water. Altogether these data show how the description of the hydration of the Ag + ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag-O bond distance in the linear [Ag(H 2 O) 2 ] + ion to be ca. 0.2 Å longer than in isolated ions in solid state.

Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) were employed to study the solvation of Zn2+ ion in dry [Cnmim][Tf2N] (n = 2, 4; 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) room temperature ionic liquids (RTILs). For [C4mim][Tf2N] also a water-saturated system was considered. The thermodynamic parameters of single ion solvation and transfer from water to the RTIL, as well as the structural information, were calculated by means of MD simulations and our study evidences that both structural and thermodynamic features can be reproduced by MD in good agreement with the experimental data. In the case of [C4mim][Tf2N], enthalpies and entropies of solvation and transfer from water to the RTIL were also obtained. The Zn2+ ion results more favorably solvated in water than in the RTIL and this seems to be caused by the more negative entropy of solvation in the latter liquid. This can be explained by the higher ordering imposed by the metal ion when solvated in [C4mim][Tf2N] with respect to water. When the transfer of Zn2+ in a water/[C4mim][Tf2N] biphasic system is considered, the results show a positive value for the free energy of transfer and the metal ion always prefers to remain solvated in water than in the water-saturated RTIL phase. This theoretical result was confirmed by the analysis of XANES (X-ray absorption near edge structure) and the EXAFS (extended X-ray absorption fine structure) spectra of Zn(Tf2N)2 solutions in [C4mim][Tf2N] in the presence of water.