Minimum, antiaromatic reactive

Non-planarity is the result of the dominance of the destabilizing interactions of the sulfur lone pair and tt- occupied MOs of the pentadienyl anion over the stabilizing interaction of that lone pair and the LUMO of the anion fragment. In fact thiabenzene is antiaromatic in a planar configuration. Pyramidalization reduces the antiaromaticity induced by the sulfur. Although no X-ray data are available on the parent system, kinetic data have been obtained supporting a minimum barrier to inversion at the pyramidal sulfur of a 2-thianaphthalene of 99.1 kJ mol-1 (75JA2718). The formulation of the system as a cyclic ylide is supported by the chemical reactivity of the compounds as related in the reactivity section below. 

Chattaraj, P. K., Roy, D. R., Elango, M., and Subramanian, V. 2005. Stabihty and reactivity of aU-metal aromatic and antiaromatic systems in hght of the principles of maximum hardness and minimum polarizability. J. Phys. Chem. A 109 9590-9597. 

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