Cumulene

A cumulene is an chemical compound with two or more cumulative (consecutive) double bonds, for example 1,2-propadiene. A longer member is 1,2,3-butatriene, H2C=C=C=CH2. Unlike alkanes and most alkenes, cumulenes tend to be rigid, which makes them appealing for molecular nanotechnology. Polyynes are another kind of rigid carbon chains. Cumulenes are found in regions of space where hydrogen is rare. See astrochemistry. The parent compound, 1,2-propadiene, simply is sometimes called cumulene, although the name allene is more common.



Inorganic cumulenes include carbon dioxide, carbon disulfide, and carbon suboxide. An important class of organic cumulenes are ketenes, which are intermediate in character between CO2 and allene.

Structure
See allene

The rigidity of cumulenes arises from the fact that the internal carbon atoms carry two double bonds. Their sp hybridisation results in two bonds separated by 90°. Interestingly, cumulenes with (1) an even number of consecutive double bonds and (2) dissimilar substituents on either end can be chiral even though they lack a classical stereocenter. For example, 2,3-pentadiene and 1,3,4-hexatriene are chiral. On the other hand odd numbers of double bonds in a cumulated system with proper substituents show cis/trans-isomerism.

Reactions
The reactions of cumulene are those of the isolated double bond. Although this molecule possesses two π-bonds in very close vicinity, in essence these act as isolated double bonds. This is due to the fact that the two π-bonds on the central carbon atom are formed by the non-hybridised p-orbitals. As these orbitals are perpendicular towards each other and occupy each other's nodal planes the two bonds are in essence isolated.

Transition metal cumulenes
The first reported complex containing a vinylidene ligand was (Ph2C2Fe2(CO)8, derived from the reaction of diphenylketene and Fe(CO)5 Structurally, this molecule resembles Fe2(CO)9, wherein one &mu;-CO ligand is replaced by 1,1-diphenylvinylidene, Ph2C2.  The first monometallic vinylidene complex was (C5H5)Mo(P(C6H5)3)(CO)2[C=C(CN)2]Cl.