Electronic structure of stable carbenes, silylenes, and germylenes

Quantum mechanical ab initio calculations at the MP4/6-311G(d,p)//MP2/6-31G(d) level of theory have been carried out for the parent compounds of the stable carbenes imidazol-2-ylidene (1) and the silylene (8) and germylene (9) analogues. The energies of hydrogenation of 1, 8, and 9 and the structures of the saturated derivatives 2, 10, and 11 have also been calculated. The analysis of the electronic structure shows clearly that the higher stability of 1 than that of 2 is caused by the enhanced p(pi)-p(pi) delocalization. However, a strong pi-donor stabilization of the carbene p(pi) orbital by the nitrogen lone pairs is found already in the C-C-saturated imidazolin-2-ylidene 2. This explains why the N-mesityl derivative of 2, which is sterically protected against dimerization, could recently be synthesized. Also the silylene and germylene systems 8 and 9 are stabilized by enhanced p(pi)-p(pi) delocalization. The method of electron density mapping as used by Arduengo et al. (J. Am. Chern. Sec. 1994, 116, 6812) as evidence for negligible pi-delocalization in 1 is not very useful, because it predicts negligible pi-delocalization even in pyridine and pyrrole. Energetic and magnetic criteria suggest that the cyclic electron delocalization in 1 has some aromatic character.