Covalency in AnCl₃ (An = Th-No)

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Actinide Chemistry

Background:

• Understanding the bonding characteristics of actinide compounds is crucial for predicting their chemical behavior and properties.
• Previous studies suggest trends in actinide-trichloride (AnCl3) bond properties, but a comprehensive computational analysis is needed.

Purpose of the Study:

• To computationally investigate the geometric and electronic structures of AnCl3 compounds.
• To analyze the covalency of the An-Cl bond using various theoretical metrics.
• To assess the influence of different computational methods on the interpretation of bonding in AnCl3.

Main Methods:

• Scalar relativistic, hybrid density functional theory (DFT) calculations, specifically the PBE0 functional.
• Analysis using Natural Bond Orbital (NBO), Natural Resonance Theory (NRT), and Quantum Theory of Atoms-in-Molecules (QTAIM) methods.
• Comparison of results obtained with PBE0 and the generalized gradient approximation PBE functional.

Main Results:

• An-Cl bond lengths generally decrease across the 5f series, with minor deviations indicating increasing metal(II) character.
• Covalency metrics show complex trends; some suggest later An-Cl bonds are less ionic than expected due to degeneracy-driven covalency.
• Discrepancies arise between delocalization indices and bond critical point metrics, highlighting metric-dependent interpretations of covalency.

Conclusions:

• The interpretation of An-Cl bond covalency is highly sensitive to the chosen theoretical metric.
• Energy degeneracy significantly contributes to covalency in later actinides, influencing delocalization indices.
• The selection of the electronic structure method (hybrid DFT vs. GGA) impacts the calculated degree of covalency.