Synthesis, Structure, and Theoretical Calculations on NpO₂Br₄².

Area of Science:

• Inorganic Chemistry
• Actinide Chemistry
• Solid-State Chemistry

Background:

• Actinide-halogen complexes (AnO2X4(2-)) are crucial for understanding actinide-ligand bonding.
• Previous studies have explored various actinide complexes, but specific neptunium(VI) halide structures remain less characterized.

Purpose of the Study:

• To synthesize and characterize neptunium(VI) dioxo halide complexes, specifically NpO2X4(2-) where X = Cl, Br, and I.
• To investigate the bonding nature and electronic structure of these complexes using experimental and computational methods.
• To expand the crystal database for the AnO2X4(2-) family and understand trends with varying halide ligands.

Main Methods:

• Crystal synthesis of NpO2Cl4(2-) and NpO2Br4(2-).
• Single-crystal X-ray diffraction for structural determination.
• Spectroscopic characterization using infrared, Raman, and UV-Vis-NIR absorption.
• Density Functional Theory (DFT) calculations for vibrational frequencies, absorption spectra, and molecular orbital analysis.

Main Results:

• Successful synthesis and structural characterization of NpO2Cl4(2-) and NpO2Br4(2-); the latter is reported for the first time.
• NpO2I4(2-) could not be synthesized due to rapid reduction of Np(VI) to Np(V) by iodide ions.
• Experimental and DFT results indicate strengthened Np-O bonds and weakened Np-X bonds across the series.
• Population analysis reveals decreasing Np-O covalency and increasing Np-X covalency from chloride to iodide complexes.

Conclusions:

• The study provides new structural data for neptunium(VI) dioxo halide complexes.
• Bonding analysis highlights a trend of increasing covalency in Np-X bonds and decreasing covalency in Np-O bonds with softer halide ligands.
• The findings offer valuable insights into the influence of ligand hardness/softness on bonding in actinide complexes.