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Covalency in AnCl2 (An = Th-No)
Sophie Cooper1, Nikolas Kaltsoyannis1
1Department of Chemistry, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK. nikolas.kaltsoyannis@manchester.ac.uk.
Dalton Transactions (Cambridge, England : 2003)
|March 29, 2022
Summary
Covalency in transcurium chlorides is driven by energy matching between actinide 5f orbitals and ligand 3p, not the +2 oxidation state. This study analyzes actinide dichloride (AnCl2) bonding, revealing distinct covalent behaviors across the series.
Area of Science:
- Computational chemistry
- Inorganic chemistry
- Actinide chemistry
Background:
- A link between unexpected covalent behavior in transcurium complexes and the stability of the +2 oxidation state in later actinides has been proposed.
- Previous computational studies on actinide trichlorides (AnCl3) indicated energy degeneracy-driven covalency in later actinides.
Purpose of the Study:
- To present a comparative computational study of actinide dichlorides (AnCl2).
- To analyze the covalency of An-Cl bonds in AnCl2 using various computational metrics.
- To elucidate the factors driving covalent behavior in transcurium chlorides.
Main Methods:
- Computational methods were employed to study AnCl2 complexes.
- Analysis included Natural Bond Orbital (NBO), Natural Resonance Theory (NRT), and Quantum Theory of Atoms In Molecules (QTAIM) metrics.
- An-Cl bond lengths were systematically analyzed and compared between different actinide series groups.
Main Results:
- An-Cl bond lengths in AnCl2 divide into two sets, with significant increases between Np-Pu and Bk-Cf.
- The first group of dichlorides (Th-Np, Cm, Bk) exhibit more covalent bonds than the second group (Pu, Am, Cf-No).
- An-Cl covalency decreases across the second half of the series for AnCl2, unlike in AnCl3.
Conclusions:
- The observed change in covalent behavior in AnCl2 is attributed to decreased 6d orbital involvement in later elements.
- Covalency in AnCl2 is driven by energy matching between actinide 5f orbitals and ligand 3p, not the stability of the +2 oxidation state.
- The larger energy difference between 5f and 3p Natural Atomic Orbitals in AnCl2, compared to AnCl3, limits energy degeneracy-driven covalency.
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