carbenes in which the carbenic carbon atom has a double bond to another carbon atom
R2C=C:, e.g.
H2C=C: ethenylidene.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1374 (https://doi.org/10.1351/pac199567081307)