A chemical molecule known as a sandwich has a metal attached to two arene (ring) ligands by haptic, covalent connections. The formula for arenes is CnHn, and their derivatives can be either substituted or heterocyclic. It is referred regarded as being "sandwiched" because the metal is often positioned in between the two rings. The metallocenes are a distinct class of sandwich complexes. When ferrocene's structure was verified by X-ray crystallography in a study by J. D. Dunitz, L. E. Orgel, and R. A. Rich in 1956, they coined the phrase "sandwich compound" to describe it. It had been suggested some years prior by Robert Burns Woodward and, separately, by Ernst Otto Fischer that the molecule would include an iron atom wedged between two parallel cyclopentadienyl rings.