Accurate single-crystal X-ray diffraction data afford a total electron density distribution for crystalline materials by employing an aspherical atomic model with comparable accuracy to that of theoretical calculations. Chemical bonds and intermolecular interactions in the crystalline state are characterized based on the electron density distribution of valence electrons, as well as structural parameters. Herein, the bonding nature of weak chemical bonds in labile compounds, such as hypervalent bonds and delocalized π-bonds, is explored on the basis of electronic structures derived from experimental electron density distribution analyses. In addition, the visualization of a radicalic orbital distribution on an sp2-hydridized carbon atom is demonstrated.

An electron density analysis of the 1-zirconacyclopent-3-yne complex has been carried out based on X-ray diffraction data, showing that the coordination mode of the but-2-yne-1,4-diyl ligand to the metal is a resonance hybrid between η2-σ, σ-coordination, as the major contributor, and η4-π, π-coordination.