Fully conjugated nonaromatic rings

Cyclopentadienone is an organic compound with molecular formula C5H4O. The parent cyclopentadienone is rarely encountered, because it rapidly dimerizes. Many substituted derivatives are known, notably tetraphenylcyclopentadienone. Such compounds are used as ligands in organometallic chemistry. thumb|left|The Knölker complex, derived from a substituted cyclopentadienone, is a catalyst for [[transfer hydrogenation.]]