carbenes
R2C: formed by mono or dialkyl substitution of methylene,
H2C:, e.g.
CH3CH2CH: propylidene.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1314 (https://doi.org/10.1351/pac199567081307)