When it comes to the elements, few are as rare as protactinium, the actinide metal that sits between thorium and uranium on the periodic table. Because of the element’s scarcity and radioactivity, chemists pretty much gave up studying protactinium in the 1960s after developing the chemical reactions and separations necessary to handle it while processing nuclear reactor fuel. But given the recent interest in developing next-generation nuclear fuels, Stéphanie M. De Sio and Richard E. Wilson of Argonne National Laboratory have been taking a new look at protactinium’s structure and bonding properties. The researchers prepared seven Pa5+ compounds, including (NH4)2PaF7, Na3PaF8, and [(CH3)4N]PaF6, and determined their crystal structures and Raman spectra (Inorg. Chem. 2014, DOI: 10.1021/ic402877a). “Our hypothesis is that protactinium is at the electronic intersection of transition-metal d and actinide f chemistries,” Wilson says. Protactinium is the first actinide to have a 5f valence electron, and it has empty 6d orbitals, he explains. Theory suggests that the 6d and 5f orbitals are nearly equal in energy and that protactinium may act like the group 5 transition metals niobium and tantalum. By contrast, thorium’s 5f orbitals are higher in energy (less stable) than its 6d orbitals, a situation that is reversed for uranium. As a consequence, thorium dioxide has a bent structure and uranium dioxide is linear. A next step for the Argonne team will be to try to prepare a protactinium oxide to shed more light on actinide trends.