Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy

Our knowledge of actinide chemical bonds lags far behind our understanding of bonding regimes of any other series of elements. This is a major issue given the technological as well as fundamental importance of f-elements. Some key chemical differences between actinides and lanthanides, and between different actinides, can be ascribed to minor differences in covalency, i.e. the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Herein we report the first pulsed electron paramagnetic resonance (EPR) spectra of actinide compounds. We apply the hyperfine sublevel correlation (HYSCORE) technique to quantify the electron spin density at ligand nuclei (via the weak hyperfine interactions) in molecular thorium(III) and uranium(III) species and therefore the extent of covalency. Such information will be important in developing our understanding of chemical bonding, and therefore reactivity, of actinides.