Sandwich compound

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 , substituted derivatives (for example ) and heterocyclic derivatives (for example ). 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, 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. The best known members are the metallocenes of the formula where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, Os, Sm, Ti, V, Mo, W, Zn. These species are also called bis(cyclopentadienyl)metal complexes. Other arenes can serve as ligands as well. Mixed cyclopentadienyl complexes: . Some examples are where the fullerene ligand is acting as a cyclopentadienyl analogue. Bis(benzene) complexes: , the best known example being bis(benzene)chromium. Bis(cyclooctatetraenyl) complexes: , such as and (both actinocenes). Metal–carborane complexes (metallacarboranes), a very large and diverse family in which main-group or transition metal ions are coordinated to carborane ligands to form polyhedral cages ranging in size from 6 to 15 vertices. Examples include bis(dicarbollide) complexes, such as and , and small-carborane sandwiches such as and where M is a transition metal and R and R′ are methyl or ethyl. Closely related are the metal complexes containing (diborolyl) ligands.