Chemistry and speciation of protactinium – a first principles study

National audienceIt is of fundamental interest to understand and predict the chemistry of rare radioelements. In this work, we focus on protactinium (Z = 91), an element that is located in between thorium and uranium in the periodic table. It might be considered as an unpredictable actinide, because it be-haves differently than any other actinide and in particular its immediate neighbours. For instance, in solution, the +V oxidation state dominates, and related complexes display a mono-oxo bond (thorium would not while uranium would display trans-di-oxo bonds). For a first study, we start with two series of protactinium(V) complexes of similar nature, that have been already experimentally investigated, and that successively form in sulfuric acid and oxalic acidic media [1-3]. As experimental information, we may rely on the structure of one of the complexes, determined by EXAFS, and on apparent formation constants. Based on this in-formation, research hypotheses and a preliminary methodological study, we aim at revealing the structure and coordination of all the complexes and at defining a computational strategy to derive relative complexation constants (corresponding to ligand-exchange reactions). For instance, if the formed 1:1, 1:2 and 1:3 (Pa:L) complexes all mutually have similar natures, we may simply de-rive the relative complexation constants from the global formation constants of both the relevant complexes. Note that owing to the few available computational studies of protactinium complex-es and to the near degeneracy of the protactinium 5f and 6d shells, this study may prove to be quite challenging.From our computational models, geometry optimizations performed both in the gas phase and in solution lead to a first view on the actual coordination sphere of protactinium(V). It involves an oxygen atom from the Pa=O mono-oxo bond and also oxygen atoms from the bidentate sulfate and oxalate ligands, and in some cases from additional water molecules. The exchange formation constants are deduced and found to be in fair agreement with experiment for the reactions involving two or three bidentate ligands; i.e. when the ligands nearly fill by themselves the coordination sphere.[1]Le Naour C. et al. (2019) Radiochim. Acta, 107, 979-991.[2]Le Naour C. et al. (2005) Inorg. Chem. 44, 9542.[3]Mendes M. et al. (2010) Inorg. Chem. 49, 9962-9971.