Cumulenes

thumb|Butatriene, the simplest cumulene

Cyclo[6]carbon is an allotrope of carbon with molecular formula . This theoretical molecule is a ring of six carbon atoms, connected by alternating double bonds. It is, therefore, a member of the [[cyclocarbon|cyclo[n]carbon]] family.

Cyclopropatriene is a hypothetical compound () which is an allotrope of carbon. It was once proposed as a candidate for a spectroscopically observed tricarbon species. It is a cyclic cumulene.