Organometallics: counting to 18 electrons

An organometallic compound contains at least one direct metal–carbon bond (diethylzinc, $1849$; ferrocene, $1951$). The hapticity $\eta^n$ gives the number of carbons bound to the metal at once: $\eta^2$ alkene, $\eta^4$ diene, $\eta^5$ cyclopentadienyl (Cp), $\eta^6$ arene.