ON THE QUESTION OF STABILITY, CONJUGATION, AND AROMATICITY IN IMIDAZOL-2-YLIDENES AND THEIR SILICON ANALOGS

Thermodynamic, structural, and magnetic criteria, the properties of th e charge distributions, and low-energy ionization processes are theore tically analyzed to learn about the role of pi-electron delocalization in recently synthesized stable singlet carbenes (Arduengo et al. J. A m, 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 relat ed model systems. The different approaches show consistently that cycl ic electron delocalization does indeed occur in the C=C unsaturated im idazol-2-ylidene systems, in particular with respect to the correspond ing C-C saturated imidazolin-2-ylidenes. However, the conclusion regar ding the degree of conjugation and aromaticity depends on the criteria used, being quite small according to the ''atoms-in-molecules'' charg e analysis but more significant according to the energetic and the mag netic properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to benzene or the imi dazolium cation. pi-Electron resonance is found to be less extensive i n the silylenes compared to their carbene analogs.