JRP CALL information
Supported By

European Commission

Short description of the work

The hexavalent state is the prevalent oxidation state for uranium in aqueous solution under oxic conditions, where it occurs as a linear, dioxo uranyl cation, UO2(2+). Under strongly oxidizing conditions also neptunium and plutonium occur as linear NpO2(2+) and PuO2(2+) cations. The multiple bonds in these cations are strong and normally unreactive, making the oxo groups to weak Lewis bases. However, the reactivity and the Lewis basicity of the oxo groups depend strongly on the ligands in the equatorial plane and their binding to the metal. To deepen the knowledge about the actinyl-ligand bonds and effect of the ligands onto the oxo bonds, various experimental and theoretical methods are possible. Direct determination of bond energies in actinyl complexes is difficult. Therefore, other methods should be utilized, such as different spectroscopic methods, among them vibrational spectroscopy accompanied with force field calculations is regarded as the most prominent one. Theoretical quantum chemical methods at different levels of theory may also be used to gain information about the structure and the relative stability of a complex, although it should be emphasized that the results may depend strongly on the molecular cluster size, boundary conditions, and solvent effects, the latter being particularly crucial for aqueous systems due to the problems with correct description of hydrogen bond network. As a result, data obtained by theoretical methods typically require comparison with the experimental information.

In the current project we have undertaken the comprehensive spectroscopic studies of the wide range of actinide nitrate hydrates, trinitrato, pentafluoro, triacetato and tricarbonatocomplexes has been undertaken. The complete assignment of vibrational modes above 200 cm-1 has been achieved and force field calculations have been performed. The advantages of the treatment of complete molecule instead of separate fragments have been shown. The data obtained were supported by DFT quantum chemistry calculations at different levels of theory. The next conclusions have been drawn:
– The clear violation of Badger’s rule has been observed in all the series studied both
experimentally and theoretically by means of quantum chemical calculations. The degree of violation decrease upon increase in ligand strength (AnO2(NO3)3(-) → AnO2(H2O)2(NO3)2 →AnO2(CH3COO)3(-)).
– The increase in ligand strength leads to decrease in axial actinyl bonding that results from the increase in occupancy of actinide atom 5f-orbitals and decrease in donor-acceptor bonding between axial oxoligands and central atom.
– Effect of counter-ion on metal-ligand bonding is more pronounced in pentafluorocomplexes where alkaline cation lies in equatorial UF5 plane. In other complexes under study alkaline ion served as the linker between UO2 linear groups and influence mostly only on axial bonding.
– The dominating effect of steric repulsion on metal-ligand equatorial bonding is shown for the
actinide nitratocomplexes: actinide-nitrate bonding obviously weakens from mono- to tetranitratocomplex. As to the effect of actinide atom on metal-ligand bonding, less atomic radii results in increase of this interaction.

Main visitor contact data
Name: Dr Mikhail Skripkin
Organisation: Saint-Petersburg State University

JRP Identification
JRP nr: TALI-C05-12
JRP title: Effect of strong and weak ligands on bonding in dioxoactinyl(VI) complexes.
JRP scope: Scope 2: Actinide in the geological environment

Visited Associated Pooled Facility
Visited APF during the stay: KIT-INE - Laboratories
Name of the APF Contact Person: Dr. Thorsten Schafer

Other APF and organisation involved in the JRP
Other organisations involved:
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 8/1/2015
End date of the stay: 8/29/2015
Quantity of access: 40
Access Unit: Days

Other APF visitors of the JRP during the stay
Visitor 2: Mr Nikita Bogachev