Complexation and bonding studies on [Ru(NO)(H₂O)₅]³⁺ with nitrate ions by using density functional theory calculation

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Nuclear Waste Management

Background:

• Ruthenium-nitrosyl complexes are key species in high-level radioactive liquid waste (HLLW) solutions.
• Understanding their complexation behavior in nitric acid is crucial for waste treatment and disposal.
• Previous experimental data provides a basis for theoretical validation.

Purpose of the Study:

• To predict the stability and complexation reactions of ruthenium-nitrosyl complexes in nitric acid solutions using DFT.
• To elucidate the reaction mechanisms and energetic favorability of ligand substitutions.
• To provide a computational model for ruthenium speciation in HLLW.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to determine equilibrium structures and energies.
• Gibbs energy comparisons were used to analyze geometrical isomer stability.
• Electron density analyses were performed to understand coordination bond strengths.
• Stepwise complexation reactions and association energies were estimated.

Main Results:

• DFT calculations accurately reproduced experimental Ru-ligand bond lengths and IR frequencies.
• Complexation with nitrate (NO3-) occurs at the equatorial plane, influenced by the Ru-NO axis.
• The model successfully reproduced the fraction of Ru-NO species in concentrated nitric acid.
• Ruthenium complexes without axial nitrate are more stable due to ligand trans influence.

Conclusions:

• DFT is a reliable tool for modeling ruthenium-nitrosyl complexation in nitric acid.
• The study provides insights into the factors governing ruthenium stability in HLLW.
• This work facilitates precise modeling of platinum-group metal complexation in acidic nuclear waste solutions.