The observed chemistry and properties of the early actinide elements are governed by their changing electron configurations. the filling of the 5f electron orbitals, and the relative energetics of these orbitals with respect to the 6d orbitals. This phenomenon gives rise to the well-known redox chemistry and multitude of oxidation states among the early actinides in contrast to the lanthanide series where the stabilized 4f orbitals impart the universally observed trivalent oxidation states. Importantly, the 5f and 6d orbital energies intersect at protactinium, imparting a unique behavior that may be typical of both actinide f- and transition metal d- chemistries. In order to explore this hypothesis we have undertaken a study of protactinium investigating its chemical and structural properties in a series of ligand systems that typify actinide and transition metal behavior, structure, and symmetry. Experimental efforts have been aimed at the chemistry of Pa(V) in aqueous solution containing ligands classically thought of as strong donor ligands, and associated with both the actinides as well as the transition metals Nb and Ta: fluoride and peroxide. Synthesis of a series of fluoride and peroxide complexes of protactinium, niobium and tantalum, and characterization of these complexes using X-ray diffraction is complemented by vibrational spectroscopy, X-ray absorption spectroscopy, and computational studies. Experimentally, stark contrasts between the behavior of protactinium, niobium, tantalum, and the early actinide series are observed. Computational studies have been conducted to shed light on the underlying electronic properties that cause the experimentally observed effects. Studies of these systems provide a framework for the quantitative description of the chemistry of Pa, the element at the intersection of transition metal d- and actinide f- behaviors. Understanding this chemistry is fundamental not only to our understanding of metal-ligand interactions in the actinide series, but in our description of these interactions across the entire periodic table. This work was performed at Argonne National Laboratory, operated by UChicagoArgonne LLC, and supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Early Career Research Program, under contract DE-AC02-06CH11357.

Protactinium: Chemistry at the intersection of the 5f and 6d elements

249th National Meeting (2015)

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