page_title: Sandwich compound
section_title: Multidecker sandwiches
hierarchical_section_title: Sandwich compound / Classes / Multidecker sandwiches
caption_attribution_description: English: Molecular structure of a Co2C2B3 triple-decker complex
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: The first isolated multidecker sandwich was the tris(cyclopentadienyl)dinickel triple-decker complex [Ni₂Cp₃]BF₄, a highly air- and water-sensitive compound reported in 1972, with X-ray crystallographic confirmation in 1974.
  In 1973 the electrically neutral air-stable triple-decker cobaltacarborane sandwiches 1,7,2,3- and 1,7,2,4-CpCo(RHC₂B₃H₃)Cp (where R = H, Me) were isolated and characterized by multinuclear NMR and X-ray studies (the structure of the 1,7,2,3 isomer is shown).
   
  Since then many three-, four-, five-, and six-decker sandwich complexes have been described. The largest structurally characterized multidecker sandwich monomer is the hexadecker shown at lower right.
   
  An extensive family of multidecker sandwiches incorporating  planar (R₂R′C₃B₂R″₂)³⁻ (diborolyl) ligands has also been prepared.
  Numerous multidecker sandwich compounds featuring hydrocarbon bridging rings have also been prepared, especially triple deckers.  A versatile method involves the attachment of Cp*Ru⁺ to preformed sandwich complexes.
caption: 1,7,2,3-CpCo(MeC2B3H4)CoCp, the first structurally confirmed multidecker sandwich.[12]
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