these compounds show a stabilization of no more than 5% of benzene due to delocalization. Anionic homoaromatics Unlike neutral homoaromatic compounds, anionic homoaromatics are widely accepted to exhibit "true" homoaromaticity. These anionic compounds are often prepared from their neutral parent compounds through lithium metal reduction. 1,2-diboretanide derivatives show strong homoaromatic character through their three-atom (boron, boron, carbon), two-electron bond, which contains shorter C-B bonds than in the neutral classical analogue. These 1,2-diboretanides can be expanded to larger ring sizes with different substituents and all contain some degree of homoaromaticity. Anionic homoaromaticity can also be seen in dianionic bis-diazene compounds, which contain a four-atom (four nitrogens), six-electron center. Experiment results have shown the shortening of the transannular nitrogen-nitrogen distance, therefore demonstrating that dianionic bis-diazene is a type of anionic bishomoaromatic compound. Peculiar feature of these systems is that the cyclic electron delocalization is taking place in the σ-plane defined by the four nitrogens. These bis-diazene-dianions are therefore the first examples for 4-center-6-electron σ-bishomoaromaticity. The corresponding 2 electron σ-bishomoaromatic systems were realized in the form of pagodane dications (see above). Antihomoaromaticity There are also reports of antihomoaromatic compounds. Just as aromatic compounds exhibit exceptional stability, antiaromatic compounds, which deviate from Huckel's rule and contain a closed loop of 4n π electrons, are relatively unstable. The bridged bicyclo[3.2.1]octa-3,6-dien-2-yl cation contains only 4 π electrons, and is therefore "bishomoantiaromatic." A series of theoretical calculations confirm that it is indeed less stable than the corresponding allyl cation. Similarly, a substituted bicyclo[3.2.1]octa-3,6-dien-2-yl cation (the 2-(4'-Fluorophenyl) bicyclo[3.2.1]oct-3,6-dien-2-yl cation) was also shown to be an antiaromate when compared to its corresponding allyl cation, corroborated by theoretical calculations as well as by NMR analysis. External links Homoaromaticity Gold Book Link References Physical organic chemistry