Hexazine

Hexazine (also known as hexaazabenzene) is a hypothetical allotrope of nitrogen composed of 6 nitrogen atoms arranged in a ring-like structure analogous to that of benzene. It would be the final member of the azabenzene (azine) series, having all of the methylidyne groups of the benzene molecule replaced with nitrogen atoms. The two last members of this series, hexazine and pentazine, have not been observed, although many other members of the azine series have (such as pyridine, pyrimidine, pyrazine, and 1,3,5-triazine).

Stability
The hexazine molecule bears a structural similarity to the very stable benzene molecule. Like benzene, it has been calculated that hexazine is likely an aromatic molecule. Despite this, it has yet to be synthesized. In fact, it has been predicted computationally that the hexazine molecule is highly unstable. It has been suggested that this predicted instability is caused by the lone pairs on the nitrogen atoms, which may repel each other electrostatically and/or cause electron-donation to sigma antibonding orbitals.