Water Defect Stabilizes the Bi³⁺ Lone-Pair Electronic State Leading to an Unusual Aqueous Hydration Structure

Area of Science:

• Inorganic Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Understanding the hydration structure of metal ions is crucial for predicting their behavior in aqueous solutions.
• Bismuth (III) ion (Bi³⁺) exhibits unique electronic properties due to its lone pair, which can significantly influence its hydration shell.

Purpose of the Study:

• To elucidate the aqueous hydration structure of the Bi³⁺ ion.
• To investigate the role of the lone-pair electronic state in controlling the ion's hydration and aqueous chemistry.

Main Methods:

• Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was employed to probe the local atomic environment around the Bi³⁺ ion.
• Density Functional Theory (DFT) simulations were utilized to model ion-water clusters and condensed-phase solutions, providing insights into electronic structure and bonding.

Main Results:

• EXAFS spectra revealed anomalous features indicative of a highly asymmetric first-solvent water shell around the Bi³⁺ ion.
• DFT calculations demonstrated that the stabilization of a lone-pair electronic state (involving 6s and 6p orbitals) dramatically influences the hydration structure.
• A distinct multimodal distribution of water molecules in the first shell was observed, with separations of approximately 0.2 Å, stabilized by collective water responses.

Conclusions:

• The aqueous structure and chemistry of Bi³⁺ are primarily governed by the water stabilization of its lone-pair electronic state.
• This lone-pair stereochemistry dictates the binding of water and ligands, significantly impacting the ion's behavior in aqueous solutions.