An orange-red crystalline solid, Fe(C₅H₅)₂; m.p. 173°C. It can be made by adding the ionic compound Na⁺C₅H₅⁻ (cyclopentadienyl sodium, made from sodium and cyclopentadiene) to iron(III) chloride. In ferrocene, the two rings are parallel, with the iron ion sandwiched between them (hence the name sandwich compound: see formula). The bonding is between pi orbitals on the rings and d-orbitals on the Fe²⁺ ion. The compound can undergo electrophilic substitution on the C₅H₅ rings (they have some aromatic character). It can also be oxidized to the blue ion (C₅H₅)₂Fe⁺. Ferrocene is the first of a class of similar complexes called sandwich compounds. Its systematic name is di-π‎-cyclopentadienyl iron(II).

Ferrocene