Term: mesoionic compounds
https://doi.org/10.1351/goldbook.M03842

Definition:
Dipolar five- (possibly six-) membered heterocyclic compounds in which both the negative and the positive charge are delocalized, for which a totally covalent structure cannot be written, and which cannot be represented satisfactorily by any one polar structure. The formal positive charge is associated with the ring atoms, and the formal negative charge is associated with ring atoms or an exocyclic nitrogen or chalcogen atom. Mesoionic compounds are a subclass of betaines.

Related Terms:
1) munchnones (http://doi.org/10.1351/goldbook.M04068).
2) sydnones (http://doi.org/10.1351/goldbook.S06207).
3) heterocyclic compounds (http://doi.org/10.1351/goldbook.H02798).
4) betaines (http://doi.org/10.1351/goldbook.B00637).

Source: PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1349 (https://doi.org/10.1351/pac199567081307)

Citation: 'mesoionic compounds' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.M03842

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