Pentalene

A fused pair of five-membered carbon rings with formula ${\mathrm{C}}_8{\mathrm{H}}_6$. Its neutral conjugated framework has eight pi electrons. Coordination and reduction alter its electron distribution and allow several metal-binding hapticities.

Two metal fragments attached to the fused ring system are syn when they lie on the same face and anti when they lie on opposite faces. A direct metal–metal bond between adjacent fragments generally requires the syn arrangement.

A ${\mathrm{C}}_8{\mathrm{H}}_8$ hydrocarbon with two more hydrogen atoms than pentalene; double-bond isomers are possible. One and two deprotonations give chemically distinct hydropentalenide and pentalenide charge states.

The monoanionic ${\mathrm{C}}_8{\mathrm{H}}_7^{-}$ coordination ligand obtained by removing one proton from dihydropentalene. An aromatic five-membered portion can bind a metal in an eta-five mode while the fused ring retains an uncoordinated alkene.

The dianion ${\mathrm{C}}_8{\mathrm{H}}_6^{2-}$, with ten pi electrons over the fused framework. The two overlapping five-membered rings must not be counted as two independent cyclopentadienyl donors.