Heterocumulene

A heterocumulene is a molecule containing a chain of at least three double bonds between consecutive atoms, in which one or more atoms in the doubly bonded chain is a heteroatom. Such a molecule is analogous to a cumulene in which the chain of doubly bonded atoms contains only carbon, except that at least one carbon is replaced by a heteroatom. Some authors extend the definition to include molecules with chains of only two double bonds between consecutive atoms, also known as heteroallenes.

Similar molecules termed cumulenic, have a chain of alternating triple and single bonds. C≡C-C≡C-.... Heterocumulenes are commonly found in space or flames. 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. Molecular clouds in space are very dilute and allow heterocumulenes to exist long enough to be detected. Some simple heterocumulenes are common chemicals or ions. These include carbon dioxide, carbon disulfide, carbon diselenide, cyanate, thiocyanate. Some definitions of heterocumulenes include compounds that contain concatenated double bonds with more than one element, but may have other parts to them. This class includes ketene, sulfur diimide, and dicyclohexylcarbodiimide. Some heterocumulenes can act as ligands with various metals.

Some energised herterocumulenes can cyclise by bending into a circle and bonding the two ends of the chain. Molecules that can do this are CCCB, CCCAl, CCCSi, CCCN, or CCCP.

Other four atom heterocumulenes include CCBO, tricarbon monoxide (CCCO) and CCCS Four atom heterocumulenes when cyclised can have two forms. In the kite (or rhombic) form, a triangle of carbon has two of its atoms bonded to the heteroatom. In the fan form the hetero atom links to three carbon atoms arranged in a fan shape. CCCSi has linear, rhombic or fan isomers. The rhombic form is known in space near the carbon star IRC+10216.

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