Characterizing Ni(II) hydration in aqueous solution using DFT and EXAFS.

Area of Science:

• Inorganic Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Understanding metal ion hydration is crucial for various chemical and biological processes.
• Nickel(II) plays a significant role in catalysis and biological systems, necessitating detailed study of its hydration structure.

Purpose of the Study:

• To elucidate the hydration structure of nickel(II) ions in aqueous solutions.
• To compare computational predictions with experimental spectroscopic data for Ni(II) hydration.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP functional were employed to model [Ni(H2O)n](2+) clusters.
• Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was performed on aqueous NiSO4 and Ni(NO3)2 solutions, and Ni(NO3)2·6H2O crystal.

Main Results:

• DFT calculations predicted a preferred coordination number of six for the first hydration shell of Ni(II), with outer shells showing variability.
• EXAFS analysis confirmed a first coordination shell of six water molecules with a Ni-O bond distance of 2.04 Å.
• No evidence of sulfur or nitrogen coordination in the first shell, nor substitution of water by sulfate or nitrate ions in the outer shell was found.

Conclusions:

• The study successfully characterized the Ni(II) hydration structure in aqueous solutions.
• Computational DFT results showed excellent agreement with experimental EXAFS findings.
• The findings confirm the stability and structure of the Ni(II) first hydration sphere.