Stability of the Protactinium(V) Mono-Oxo Cation Probed by First-Principle Calculations

Abstract

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)(H₂ O) 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)(H₂ O)→Pa(OH)₄ (X), we find that the PaO(OH)₂ (X)(H₂ O) complex is stabilized with Cl^- , Br^- , I^- , NCS^- , NO₃ ^- , and SO₄ ^2- ligands, while it is not favored with OH^- , F^- , and C₂ O₄ ^2- 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.

Keywords: bond analyses; coordination chemistry; protactinium; quantum chemistry; solvation; stability.