Large Stabilization Effects by Intramolecular Beryllium Bonds in Ortho-Benzene Derivatives

Resonance-assisted intramolecular triel bonds

The possibility that the intramolecular Tr⋯S triel bond is strengthened by resonance is examined by quantum chemical calculations within the planar five-membered ring of TrH₂-CRCR-CRS (Tr = Al, Ga, In; R = NO₂, CH₃). This internal bond is found to be rather short (2.4-2.7 Å) with a large bond energy between 12 and 21 kcal mol^-1. The pattern of bond length alternation and atomic charges within the ring is consistent with resonance involving the conjugated double bonds. This resonance enhances the triel bond strength by some 25%. The electron-withdrawing NO₂ group weakens the bond, but it is strengthened by the electron-donating CH₃ substituent. NICS analysis suggests the presence of a certain degree of aromaticity within the ring.