The story
From metallo- + c(yclopentadi)ene. First used in ferrocene.
Etymology from Wiktionary, CC BY-SA 4.0.
metallocene
Today it means: Any of a class of organic compounds containing two cyclopentadienyl anions and a transition metal whose bonding involves overlap of ns and np orbitals of the metal with molecular orbitals of appropriate symmetry of each cyclopentadienyl ring; the rings are parallel to each other, or sometimes canted. Full definition →