The Electron-Density Distribution of UCl4 and Its Topology from X-ray Diffraction

The chemistry of 5f electrons in actinide complexes and materials is still poorly understood and represents a serious challenge and opportunity for experiment and theory. The study of the electron density distribution of the ground state of such systems through X-ray diffraction represents a unique opportunity to quantitatively investigate different chemical bonding interactions at once, but was considered "almost impossible" on heavy-atom systems, until very recently. Here, we present a combined experimental and theoretical investigation of the electron density distribution in UCl_ 4 crystals and comparison with the previously reported spin density distribution from polarized neutron diffraction. All approaches provide a consistent picture in terms of electron and spin density distribution, and chemical bond characterization. More importantly, the synergy between experiments and quantum-mechanical calculations allows to highlight the remarkable sensitivity of X-ray diffraction to 5f electrons in materials.