carbenes, derived by formal insertion of a divalent carbon atom into an even-membered annulene, e.g. cycloheptatrienylidene:
A00369.png
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1318 (https://doi.org/10.1351/pac199567081307)