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 protactinyl moiety PaO2+ in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+. Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically 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 attributes and provide insights into the conditions supporting the stability of relevant complexes. Protactinium (Z=91) deviates from the neighboring actinides by forming PaO3+ in solution instead of PaO2+. Complex stability, evaluated through Gibbs free energy, favors PaO(OH)2(X)(H2O) with Cl-, Br-, I-, NCS-, NO3- and SO42-, but not with OH-, F- and C2O42-. QTAIM and NBO analyses reveal a triple bond in Pa mono-oxo bond, with increased covalency Cl-, Br-, I-, NCS-, NO3- providing insights into protactinium's unique chemical behavior. image