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

Protactinium (Z = 91) exhibits a distinct behaviour among the actinides. Unlike its neighbors uranium, neptunium or plutonium in the pentavalent oxidation state, it does not form the dioxo protactinyl moiety PaO2^{+}, but it can exist in the form of a monooxo PaO^{3+} cation. In this article, with the standard first-principle calculations and by including implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes PaO(OH)2(X)(H2O) and Pa(OH)4(X) where X is a monodentate ligand (X = OH-, F-, Cl-, Br-, I- and NCS-), as well as bidentate ligands (X = NO3^{-}, SO4^{2-}, and C2O4^{2-}). We evaluate the relative stability of the complexes by calculating the Gibbs' free energy of the reaction PaO(OH)2(X)(H2O) -> Pa(OH)4(X). The PaO(OH)2(X)(H2O) complex is stabilized with Cl-, Br-, I-, NCS-, NO3^{-} and SO4^{2-} ligands, while it is not energetically favored with OH-, F- and C2O4^{2-} ligands, implying the stability of the Pa(OH)4(X) complexes over PaO(OH)2(X)(H2O) ones. Moreover, the analysis utilizing the QTAIM (Quantum Theory of Atoms in Molecules) and NBO (Natural Bond Orbital) methods confirms that the Pa mono-oxo bond is a triple bond with significant contributions from the 5f and 6d shells to the bonding orbitals. The covalency of the Pa mono-oxo bond in the PaO(OH)2(X)(H2O) complexes increases with the Cl-, Br-, I-, NCS- and NO3^{-} ligands. These findings shed light on the unique chemical behavior of protactinium and offer valuable insights into the specific experimental conditions necessary for their stability.