Isolation of a compound (stable above room temperature) containing the
pentazole (ring of 5 N atoms) anion:
Science Vol. 355 Issue 6323 p. 374 Jan. 27
science.sciencemag.org/content/355/6323/374
doi: 10.1126/science.aah3840
(A 5-nitrogen cation was previously known; it has a V shape
(isoelectronic with cyanogen azide). It's unlikely the salt of these
two can be prepared.)