Uncovering Heavy Actinide Covalency: Implications for Minor Actinide Partitioning

Area of Science:

• Nuclear chemistry
• Computational chemistry
• Materials science

Background:

• Heavy actinides exhibit trivalent oxidation states, resembling rare earth elements.
• Separating actinides from lanthanides is crucial for nuclear fuel reprocessing but challenging due to chemical similarities.
• Conventional understanding attributes actinide-ligand interactions to electrostatics, termed actinide contraction.

Purpose of the Study:

• To investigate the nature of actinide-ligand bonding across the heavy actinide series (Am to Cf).
• To challenge the prevailing electrostatic model by exploring the role of covalency.
• To provide insights into the electronic structure and bonding characteristics of heavy actinide complexes.

Main Methods:

• Density functional theory (DFT) calculations incorporating relativistic effects (ZORA, spin-orbit coupling).
• Optimization and evaluation of heavy actinide complexes with various ligands (N-, O-, S-donors, varying denticity).
• Energy decomposition analysis (EDA) to quantify metal-ligand interaction energies.

Main Results:

• Demonstrated a steady increase in covalency across the actinide series from Americium (Am) to Californium (Cf).
• Observed increased covalency irrespective of ligand donor atom type (hard/soft) or denticity.
• Identified enhanced bonding and orbital contributions along the Am-Cf series, linked to increased covalency.

Conclusions:

• The study reveals a significant and increasing covalency in heavy actinide-ligand bonds, contradicting the traditional electrostatic model.
• This enhanced covalency is a key factor in understanding heavy actinide chemistry and separation processes.
• Findings complement recent theories on orbital degeneracy-induced covalency in actinides.