Persistent Mono-Oxo Bonding with Protactinium(V) Revealed in Highly Acidic Chloride Solutions

Protactinium is one of those long discovered elements with poorly known physico-chemical properties. In aqueous solution, protactinium(V), highly prone to hydrolysis, may or may not display oxo bonding, depending on the medium, in a way that still appears unpredictable due to a critical lack of data. Here, we probe the stability of this bond in hydrochloric acid solutions. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy, complemented by ab initio calculations, reveal that a short Pa─O_oxo bond indeed exists at moderate hydrochloric acid concentration 3 M and unexpectedly persists even in highly acidic conditions (12 M). Our analysis identifies the coexistence of PaO(OH)Cl_{1, 2} species at moderate acidity and PaOCl_{4, 5} species at high acidity, with both pairs unambiguously displaying the characteristic mono-oxo bond, with a total coordination number of seven. Notably, EXAFS spectral fitting using geometries obtained from relativistic quantum calculations reveals a Pa─O_oxo bond length of approximately 1.83 Å, in excellent agreement with computational predictions. This finding overturns expectations and provides new insight into early actinide bonding behavior in extreme chemical conditions. Understanding such coordination chemistry is critical for advancing nuclear fuel management and more broadly actinide science.