Methylidenecarbene (systematically named λ2-ethene and dihydrido-1κ2H-dicarbon(C—C)) is an organic compound with the chemical formula C=CH2 (also written [CCH2] or C2H2).

Ethenylidene (without the -1- locant) or vinylidene, is used, systematically, to refer to the substituent group (=C=CH2).

With a half-life on the order of hundreds of femtoseconds, free methylidenecarbene will spontaneously decay via tautomerisation to acetylene,[2] with an energy barrier of between 4 and 21 kJ mol−1.

[1] Besides tautomerisation, methylidenecarbene can also autopolymerise to form various oligomers, the simplest of which, is the cumulene butatriene.

[1] Although, free methylidenearbene is quickly destroyed by tautomerisation into acetylene, its adducts are quite stable.