Cumulene

Unlike most alkanes and alkenes, cumulenes tend to be rigid, comparable to polyynes.

Cumulene carbenes H2Cn for n from 3 to 6 have been observed in interstellar molecular clouds[2][3] and in laboratory experiments[4] by using microwave and infrared spectroscopy.

(The more stable cumulenes H2CnH2 are difficult to detect optically because they lack an electric dipole moment.)

[6] The most common synthetic method for butatriene synthesis is based on reductive coupling of a geminal dihalovinylidene.

[8] The rigidity of cumulenes arises from the fact that the internal carbon atoms carry double bonds.