Probing Cyclic π‐Electron Delocalization in an Imidazol‐2‐ylidene and a Corresponding Imidazolium Salt

The extent of cyclic π‐electron delocalization in the N‐heterocyclic ring of an imidazol‐2‐ylidene (i.e., 1,3,4,5‐tetramethylimidazol‐2‐ylidene) and its corresponding imidazolium salt (i.e., 1,3,4,5‐tetramethylimidazolium chloride) has been investigated theoretically by using Bader's quantum theory of atoms in molecules (QTAIM) descriptors, delocalization indices, electron localizability indicators (ELI‐Ds), and the source function tool. In addition, the experimental electron density distribution for the imidazolium salt has been obtained and analyzed from 100 K X‐ray diffraction data. A significant drop is found in the ellipticity of the electron density along the Ccarbene−N bond path in the imidazol‐2‐ylidene. This is shown to be a natural consequence of the σ lone pair of the Ccarbeneatom, which overwhelms the π‐electron density, rather than a sign of a significantly diminished degree of π‐electron delocalization in the imidazol‐2‐ylidene compared to its imidazolium salt. In fact, the source functions, the ELI‐Ds, and the delocalization indices all probe a quite similar extent of cyclic π‐electron delocalization in the N‐heterocyclic rings of the two compounds.