Pentazole aromatic ring

The molecular structure of 4-dimethylaminopentazole was determined by X-ray diffraction and proved to display practically coplanar rings with bond lengths agreeing with the aromatic character of the pentazole ring with a re-electron sextet (83CC910). 

As dramatic as the tale of the pentazoles is the story of the unsuccessful attempt to detect a six-membered nitrogen ring. The first calculations were performed in the early seventies. In 1980 Vogler et al. [8] found indications that Ne is formed on UV irradiation from cis-[Pt(N3)2(PPh3)2] embedded in a matrix, but it is only stable at low temperature. At that time hexaazabenzene (9) was considered the only feasible structure for Ne. This report unleashed an avalanche of quantum chemical calculations, in particular because 9 is isoelec-tronic with benzene and thus should likewise display aromatic stabilization. The calculations over the decade produced every conceivable answer hexaazabenzene has Den symmetry. .. has Dsh symmetry. .. does not exist, because 9 is the transition structure for nitro-... 

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