Bond Ellipticity as a Measure of Electron Delocalization in Structure and Reactivity

The ellipticity of the electron density at the bond critical points is a parameter computed in the framework of the Atoms in Molecules (AIM) analysis. This parameter provides a quantitative measurement of the anisotropy of the electron density at the BCP. The ellipticity has been originally associated to the π character of bonds, and therefore, it has been also employed as a measurement of delocalization and, ultimately, aromaticity. The number of applications of this parameter, however, has increased significantly in recent years and studies on the description of unusual bonds in charge transfer interactions, steric contacts, organometallic complexes, etc include the ellipticity as a useful chemical index. In this work a revision of the uses of ellipticity to characterize both molecular bonds and reactivity is provided.