On the question of stability, conjugation, and ''aromaticity'' in imidazol-2-ylidenes and their silicon analogs

Thermodynamic, structural, and magnetic criteria, the properties of the charge distributions, and low-energy ionization processes are theoretically analyzed to learn about the role of pi-electron delocalization in recently synthesized stable singlet carbenes (Arduengo et al. J. Am, Chem. Sec. 1991, 113, 361) and silylenes (Denk et al. J. Am. Chem. Sec. 1994, 116, 2691) of the imidazol-2-ylidene type and also in related model systems. The different approaches show consistently that cyclic electron delocalization does indeed occur in the C=C unsaturated imidazol-2-ylidene systems, in particular with respect to the corresponding C-C saturated imidazolin-2-ylidenes. However, the conclusion regarding the degree of conjugation and aromaticity depends on the criteria used, being quite small according to the ''atoms-in-molecules'' charge analysis but more significant according to the energetic and the magnetic properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to benzene or the imidazolium cation. pi-Electron resonance is found to be less extensive in the silylenes compared to their carbene analogs.