Elusive Coordination of the Ag⁺ Ion in Aqueous Solution: Evidence for a Linear Structure

Abstract

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.

On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl2·6H2O and Urea

“…These values are in good agreement with those obtained from the EXAFS analysis ( Table 1 ), providing the systematic error of XANES analysis on bond lengths. 65 The analysis of the XANES spectrum of the NiCl 2 ·6H 2 O:urea:water 1:3.5:26 mixture has been also carried out, and a good agreement between the theoretical and experimental data has been obtained for the [Ni(H 2 O) 6 ] 2+ cluster ( Figure S6 and Table S7 ). The complementary information provided by the combined analysis of the XANES and EXAFS regions allowed us to find a robust model for the local structure around the Ni 2+ ion in the studied systems.…”

“…This happens because the EXAFS region is highly affected by the atomic thermal and structural disorder, resulting in a high correlation between the Debye–Waller factor and the coordination number, as both parameters affect the amplitude of the χ( k ) oscillation. 65 , 67 , 68 …”

Fate of a Deep Eutectic Solvent upon Cosolvent Addition: Choline Chloride–Sesamol 1:3 Mixtures with Methanol

“…The g ( r )’s have been multiplied by the numerical density of the observed atoms (ρ) to properly compare systems with different compositions. 15 , 54 − 57 The coordination numbers N have been computed integrating each curve up to a cutoff distance chosen at the position of the first minimum and the obtained values have been reported as a function of M .…”

Cation‐π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+(benzylamine)

“…This cation shares similar properties with alkali metal ions. For example, its closed shell d 10 configuration confers a prevailing electrostatic nature in ligand binding, while the tendency to form two‐coordinate structures in a linear geometry has been ascribed to hybridization between the and 5s orbitals resulting in hybrid orbitals [27,28] . At the same time silver is also redox active, a characteristic which is responsible for its antibacterial activity.…”