Stable cyclopentynes--made by metals!?

Structural limitations in small cyclic hydrocarbons are a longstanding challenge to chemistry, leading to molecules with unusual geometric parameters and reactivity. For example, incorporation of C==C==C==C or C-C[triple chemical bond]C-C arrangements, which are normally linear in open-chain compounds, into small carbocyclic rings results in enormous ring strain as a result of the angle deformation. Nevertheless, such uncommon structures have been realized by complexation of zirconocene fragments with butadiynes and butatrienes, giving five-membered metallacyclocumulenes and metallacyclopentynes. The metal atom is essential for these molecules but complicates the simplified description because of internal interactions of the metals with the multiple bonds.