A Transition in the Ni²⁺Complex Structure from Six- to Four-Coordinate upon Formation of Ion Pair Species in Supercritical Water: An X-ray Absorption Fine Structure, Near-Infrared, and Molecular Dynamics Study

Abstract

The coordination structure about Ni2+ in water at temperatures up to 525 °C was measured by the X-ray absorption fine structure (XAFS) technique. Solutions containing 0.2 m NiBr2 and 0.2 m NiBr2/0.8 m NaBr were explored at pressures up to 720 bar. For certain systems, both Ni and Br XAFS data were acquired and a global model was used to fit the two independent sets of XAFS data. These two independent measurements gave excellent agreement on the coordination structure of the Ni2+/Br- contact-ion pairing. The result is a complete picture of the coordination structure of the contact-ion pairing including the coordination numbers, distances for the water−ion and ion−ion associations, and also a high-quality measurement of the binding strength and amount of disorder (Debye−Waller factor and the anharmonicity) of the Ni2+/Br- association. The XAFS measurements strongly indicate a transitional change in the coordination of Ni2+ from the octahedral Ni2+(H2O)6 species at room temperature to the 4-coordinate structures at supercritical conditions (e.g., T > 375 °C). Specifically, the equilibrium structure at 425 °C is Ni2+(Br-)3.3(H2O)1.0 for the aqueous solution of 0.2 m NiBr2 with 0.8 m NaBr. At 325 °C, the octahedral species still exists but it is in equilibrium with new species of lower coordination. Above 425 °C, at moderate pressures up to 700 bar, the stable structures are a family of 4-coordinated species (NiBr(H2O)3·Br, NiBr2(H2O)2, NiBr3(H2O)·Na), where the degree of Br- adduction and replacement of H2O in the inner shell depends on the overall Br- concentration. The most likely symmetry of these species is a distorted tetrahedron. Measurements of the Ni preedge 1s → 3d and to 1s → 4d transitions using X-ray absorption spectroscopy confirm that a symmetry change occurs at high temperature, and the results are consistent with the XAFS and molecular dynamics (MD) conclusion of a distorted tetrahedral structure. This observation is further confirmed by near-infrared (NIR) spectra of the same system. The MD simulations under identical conditions were used to verify the experimental findings. Although we found qualitative agreement between the experimental and simulated first-shell coordination structure, it is clear that refinements of the intermolecular potentials are required to quantitatively capture the true high-temperature structure.