Ferrocene

Ferrocene is the prototypical metallocene, a type of organometallic chemical compound, consisting of two cyclopentadienyl rings bound on opposite sides of a central iron atom and forming an organometallic sandwich compound.

It was first made unintentionally from the reaction of cyclopentadiene and iron powder, originally designed to couple the diene. Instead a lightly orange powder was obtained. The structure was confirmed by NMR.