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 PaO 2 + in aqueous phase. Instead, it manifests as a monooxo PaO 3+ cation or a Pa 5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH) 2 (X)(H 2 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)(H 2 O)→Pa(OH) 4 (X), we find that the PaO(OH) 2 (X)(H 2 O) complex is stabilized with Cl - , Br - , I - , NCS - , NO 3 - , and SO 4 2- ligands, while it is not favored with OH - , F - , and C 2 O 4 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 NO 3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.