Hydration and ion pair formation in aqueous Y(3+)-salt solutions

Area of Science:

• Inorganic Chemistry
• Solution Chemistry
• Spectroscopy

Background:

• Yttrium(III) is a trivalent lanthanide ion with significant applications.
• Understanding the hydration and complexation of yttrium(III) in aqueous solutions is crucial for various chemical processes.
• Previous studies have explored yttrium speciation, but detailed spectroscopic analysis across a wide concentration range is needed.

Purpose of the Study:

• To investigate the hydration and complexation of yttrium(III) in aqueous solutions of perchlorate, triflate, chloride, and nitrate salts.
• To determine the stability of the yttrium octa-aqua ion and identify the formation of ion pairs and complexes at different concentrations.
• To validate experimental findings with theoretical calculations using Density Functional Theory (DFT).

Main Methods:

• Raman spectroscopy was employed to analyze aqueous solutions of yttrium salts (perchlorate, triflate, chloride, nitrate) across a broad concentration range (0.198–3.252 mol L⁻¹).
• Spectra were recorded from low wavenumbers up to 4200 cm⁻¹.
• Density Functional Theory (DFT) geometry optimizations and frequency calculations were performed on yttrium-water clusters, both in the gas phase and within a polarizable continuum model.

Main Results:

• In dilute perchlorate solutions (∼0.5 mol L⁻¹), the octa-aqua yttrium(III) ion, [Y(OH2)8]³⁺, is stable. At higher concentrations, outer-sphere and contact ion pairs form.
• [Y(OH2)8]³⁺ was also observed in triflate solutions, with weak depolarized modes confirming its presence and incompatibility with hexa-hydration.
• Contact ion pair formation was evident in yttrium chloride solutions across the measured concentration range, weakening upon dilution. Chloro-complexes, [Y(OH2)8-nCln]³⁺⁻ⁿ (n=1,2), formed with added HCl.
• Nitrato-complexes, [Y(OH2)8-n(NO3)n]³⁺⁻ⁿ (n=1,2), were detected in yttrium nitrate solutions, disappearing at concentrations below 0.1 mol L⁻¹.
• DFT calculations for the [Y(OH2)8]³⁺ cluster in a polarizable continuum showed good agreement with experimental bond distances, angles, and the Y-O stretching frequency (372 cm⁻¹).

Conclusions:

• The yttrium(III) ion predominantly exists as the octa-aqua complex, [Y(OH2)8]³⁺, in dilute aqueous solutions.
• Ion pairing and the formation of specific complexes (chloro-, nitrato-) depend on the counter-anion and concentration.
• DFT calculations provide valuable insights into the structure and vibrational properties of hydrated yttrium(III) ions, complementing experimental spectroscopic data.