Wiktionary
cyclopentadienone
n. (context organic compound English) Am unsaturated alicyclic ketone derived from cyclopentadiene; any derivative of this compound
Wikipedia
Cyclopentadienone
Cyclopentadienone is an organic compound with molecular formula CHO. 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.