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 π −p π delocalization. However, a strong π-donor stabilization of the carbene p π 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 π −p π delocalization. The method of electron density mapping as used by Arduengo et al. ( J. Am. Chem. Soc. 1994, 116, 6812) as evidence for negligible π-delocalization in 1 is not very useful, because it predicts negligible π-delocalization even in pyridine and pyrrole. Energetic and magnetic criteria suggest that the cyclic electron delocalization in 1 has some aromatic character.