Aromaticity and antiaromaticity in the cyclic 6π and 4π molecules of carbon and silicon E <sub>6</sub> H <sub>6</sub> and E <sub>4</sub> H <sub>4</sub> (E = C, Si)
Lili Zhao, Ma Qin, Israel Fernández, Gernot Frenking
Abstract
silicon homologue 2c has a weaker preference for substitution reaction than benzene, but also tetrasilacyclobutadiene 4a prefers substitution over the addition reaction. The comparison of the calculated (pseudo) π conjugation of the cyclic compounds and acyclic reference systems suggests aromatic stabilization/destabilization for the carbon systems. The values for the silicon compounds are inconclusive and the separation of σ and π interactions is difficult due to the strong deviation of some silicon systems from planarity. The NICS values are not a reliable indicator for aromatic stabilization due to π conjugation. Chemical bonding models that have been developed and derived for compounds in the first octal series of the periodic table are only suitable to a limited extent for molecules with heavier main group atoms. This comes from the radii of the s/p valence orbitals of the atoms, which are very similar for the first octal row atoms leading to effective sp hybridization. The chemical bonds of the heavier atoms have a much higher p character because the radius is bigger than the valence s orbitals.