Sandwich compound

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In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula C_nH_n, substituted derivatives (for example C_n(CH₃)_n) and heterocyclic derivatives (for example BC_nH_n+1). Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the metallocenes.

The term sandwich compound was introduced in organometallic nomenclature in 1956 in a report by J. D. Dunitz, L. E. Orgel and R. A. Rich, who confirmed the structure of ferrocene by X-ray crystallography.^[1] The correct structure, in which the molecule features an iron atom sandwiched between two parallel cyclopentadienyl rings, had been proposed several years previously by Robert Burns Woodward and, separately, by Ernst Otto Fischer. The structure helped explain puzzles about ferrocene's conformers. This result further demonstrated the power of X-ray crystallography and accelerated the growth of organometallic chemistry.^{[2][page needed]}