C5H5

C5H5 is a chemical formula that can refer to several related species in organic and organometallic chemistry. As a neutral hydrocarbon with five carbons and five hydrogens, C5H5 cannot exist as a stable, isolable molecule. A hydrogen-deficiency calculation yields a noninteger degrees of unsaturation, indicating that any neutral form would require a radical or a charge. In practice, C5H5 most often appears in charged or radical forms based on the cyclopentadienyl ring.

Cyclopentadienyl anion (C5H5−) is a planar, five-membered ring with alternating double bonds that carries a negative charge. It is aromatic, containing six π electrons, and serves as a highly important ligand in organometallic chemistry, commonly denoted Cp−. The Cp− ligand stabilizes many metal complexes, including metallocenes such as those with iron or nickel centers.

Cyclopentadienyl radical (C5H5•) is a neutral radical with five π electrons. It is less stable than the anion and is typically short-lived, studied mainly in gas-phase spectroscopy and as an intermediate in reactions involving cyclopentadiene derivatives or metal centers.

Cyclopentadienyl cation (C5H5+) contains four π electrons and is antiaromatic. It is highly reactive and not isolable under ordinary conditions, but can be generated and studied under special laboratory conditions or in matrix isolation experiments.

In common usage within chemistry, the term Cp refers to the cyclopentadienyl ligand as a unit, highlighting the central role of the C5H5 ring in a wide range of coordination compounds and catalytic systems.