The electrically neutral species H₂C: and its derivatives, in which the carbon is covalently bonded to two univalent groups of any kind or a divalent group and bears two nonbonding electrons, which may be spin-paired (singlet state) or spin-non-paired (triplet state). In systematic name formation, carbene is the name of the parent hydride:CH₂ hence, the name dichlorocarbene for :CCl₂. However, names for acyclic and cyclic hydrocarbons containing one or more divalent carbon atoms are derived from the name of the corresponding all--hydrocarbon using the suffix -ylidene. E.g. prop-2-en-1-ylidene, H₂C=CHCH: ethenylidene, H₂C=C:; cyclohexylidene, Subclasses of carbenes include acyl carbenes, imidoyl carbenes, and vinyl carbenes. PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1092