[1] Some authors relax the definition to include species with chains of only two double bonds between consecutive atoms,[2] also known as heteroallenes.
Most of the longer ones are very unstable and reactive, and thus have a transient existence, or can only survive when dilute or in an inert matrix.
Some definitions of heterocumulenes include compounds that contain concatenated double bonds with more than one element, but may have other parts to them.
Some energised heterocumulenes can cyclise by bending into a circle and bonding the two ends of the chain.
In the kite (or rhombic) form, a triangle of carbon has two of its atoms bonded to the heteroatom.
[3] Other known five-atom heterocumulenes include CCBCC, CCCCB, CCOCC, CCCCSi, CNCCO, HCCCO, HCCCS, and NCCCN.
Heterocumulenes are supposed to be straight, but some combinations of elements result in bent or cyclic molecules.