Nitron: a stable N-heterocyclic carbene that has been commercially available for more than a century.

The analytical reagent Nitron is a mesoionic compound whose structure consists of a cationic triazolium unit with an anionic PhN substituent. We provide clear evidence for the existence of a singlet diaminocarbene tautomer whose structure is composed of a 1,2,4-triazol-5-ylidene unit bearing an NHPh substituent.