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.