The diradical
CH2=C(C·H2)2, 2-methylenepropane-1,3-diyl, for which no Kekulé structure can be written, and its hydrocarbyl derivatives.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1373 (https://doi.org/10.1351/pac199567081307)