DFT Calculation of molecular orbitals of an uranyl (V) complex. Green spheres represent nitrogen atoms, red ones oxygen whereas the blue and yellow lobes represent the molecular orbitals of the complex. A) Ionic bond: uranium U has no exchange with its ligands pyridine, and we observe an atomic f orbital centered on uranium. B) π-backbonding: molecular orbitals of π type from pyridine are overlapped with atomic f orbitals from uranium.
The uranyl (V) cation UO2+ plays an important role in the chemistry of uranium. It occurs particularly in biological processes or as a disturbing element in the reprocessing of nuclear waste. However, these compounds are generally extremely difficult to study because unstable. Based on recent experimental results, a SCIB team could model such compounds. The results allow considering new route to obtain stable uranyl (V) compounds.
The uranyl cation is made up of a central uranium ion bonded to two oxygen ions in opposite directions, thus leading to a rod-like shape. Vacant coordination positions are available in the plane perpendicular to uranyl, thus various ligands may be associated to the central uranium atom, to yield uranyl complexes which charge the changes in the uranium oxidation state: (VI) or (V). In a natural medium, uranium exists mainly in two forms: the soluble uranyl (VI) and uranium (IV) species which is mostly insoluble. Yet, some uranyl (V) compounds may appear in reduction processes from (VI) to (IV) uranium species that occur in mineral or biological media. Through the precipitation of U(IV) compounds they thus participate in the accumulation of uranium species within certain ecological areas.
However, uranyl (V) compounds are very poorly known due to their intrinsic instability, which is manifested through a so-called dismutation reaction: 2UV→UVI + UIV. During this reaction, two uranyl(V) react together to yield one uranyl(VI) and one uranium(IV). Recently, chemists from the SCIB succeeded in blocking the reaction and stabilizing the uranyl (V) complexes, paving the way for further systematic studies of these compounds.
Why such stability in these particular uranyl (V) compounds? It is usually expected that the uranyl (V) species may interact with surrounding ligands through ionic bonds, thus with electrostatic interactions. One possible assumption used to explain this unusual stability is that the complexes are so hindered that there is no space left for exchanging their electron with another neighboring species. In order to answer this question, theoreticians from the SCIB have carried out DFT (Density Functional Theory) calculations with an unexpectedly original result: the uranyl (V) cation is able to covalently interact with ligands, and moreover to display π-backbonding (see insert). There, surprising results are due to the particular electronic structure of the ligands used here: one phenolate, an electron-donating group, that contributes to enhance the electron density on the uranyl, and a pyridine moiety which, in turn, will receive some electron part from the uranyl. This dialog between donor and acceptor ligands via the uranyl (V) cation stabilizes the latter long enough to stop any attempt of dismutation. Developing new uranyl (V) complex and specifying the disproportionation reactions are the two axes implemented to better understand the mechanisms of dispersion of uranium in the environment, improve the deep disposal of nuclear waste and environmental remediation strategies.
As with humans, metals M and ligands L may build several types of contacts. The first type, that could be described as “platonic”, is the ionic bond: the species are connected through electrostatic interactions through space (case of sodium chloride). The second type is more “physical” and so-called covalent, where the metal and the ligands share electrons thanks to the overlap of some of their orbitals following an axial symmetry (similarly to a π bond or single bond in organic chemistry). These interactions may also appear more “passionate” when, in addition to the σ-bond, the partners exchange their electrons through orbitals that are localized on each side of the metal-ligand axis (π-type bond or double bond). For these π bonds, the electrons may prefer to be localized on the metal (in case of a π-donor ligand) or on the ligand (in case of a π-acceptor ligand). In the latter case, apparently the metal has been able to give back some electron density to the ligand, which leads to the term back-bonding or back-donation. When a metal has several different ligands, (we can stop the comparison with human beings at the stage), a synergy may appear between a donor ligand L2 and an acceptor ligand L1, the metal center being thus an intermediate.
Further reading: V. Vetere et al., Comptes Rendus Chimie 13 (2010) 876
Maj : 19/02/2014 (950)