Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of Protactinium (Z = 91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, Protactinium in the pentavalent state diverges by not forming the typical dioxo pro- tactinyl moiety PaO2^{+} in aqueous phase. Instead, it manifests as a monooxo PaO^{3+} cation. Employing first-principle calculations with implicit and explicit solvation, we investigate two sto- ichiometrically equivalent neutral complexes: PaO(OH)2(X)(H2O) and Pa(OH)4(X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs’ free energy for the reaction PaO(OH)2(X)(H2O) −−→ Pa(OH)4(X), we find that the PaO(OH)2(X)(H2O) complex is stabilized with Cl– , Br– , I– , NCS– , NO3 – , and SO42– ligands, while it is not favored with OH–, F–, and C2O42– ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl–, Br–, I–, NCS–, and NO3–. These findings elucidate Protactinium’s unique chemical 1 attributes and provide insights into the conditions supporting the stability of relevant complexes.