page_title: Sandwich compound
section_title: Monometallic half-sandwich compounds
hierarchical_section_title: Sandwich compound / Classes / Half-sandwich compounds / Monometallic half-sandwich compounds
context_page_description: In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CₙHₙ, substituted derivatives and heterocyclic derivatives. 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. The correct structure 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, the molecule features an iron atom sandwiched between two parallel cyclopentadienyl rings. This result further demonstrated the power of X-ray crystallography and accelerated the growth of organometallic chemistry.
context_section_description: Metallocenes including just one facially-bound planar organic ligand instead of two gives rise to a still larger family of half-sandwich compounds.  The most famous example is probably methylcyclopentadienyl manganese tricarbonyl.  Such species are occasionally referred to as piano-stool compounds, at least when there are three diatomic ligands in addition to the hydrocarbon "seat" of the piano stool.  The name derives from the similarity of the structure to such a "stool" with the seat being a facial planar organic compound, such as benzene or cyclopentadiene, and the legs being ligands such as CO or allyl.
caption: Ball-and-stick model of methylcyclopentadienyl manganese tricarbonyl, a "piano stool" compound
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