In organometallic chemistry, a sandwich compound is a chemical compound featuring a metalbound by hapticcovalent bonds to two areneligands. 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.
Classes
The best known members are the metallocenes of the formula M2 where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, Sm, Ti, V, Mo, W, Zn. These species are also called bismetal complexes. Other arenes can serve as ligands as well.
Mixed cyclopentadienyl complexes: M. Some examples are Ti and Fe where the fullerene ligand is acting as a cyclopentadienyl analogue.
Bis complexes: M2, the best known example being bischromium.
Bis complexes: M2, such as U2 and Th2.
Metal–carborane complexes, 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 complexes, such as z− and 2−, and small-carborane sandwiches such as M and M where M is a transition metal and R and R′ are methyl or ethyl.
File:KIWJOP.png|thumb|Structure of 2+, showing only one Me4N+. Closely related are the metal complexes containing H3C3B2R2 ligands. In addition to these, other sandwich complexes containing purely inorganic ligands are known, such as Fe and 2−.
Half-sandwich compounds
Monometallic half-sandwich compounds
s 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.
The first isolated multidecker sandwich was the trisdinickel triple-decker complex 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-CpCoCp were isolated and characterized by multinuclear NMR and X-ray studies.
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 3− 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.
Linked sandwiches
Monomeric double-decker and multidecker sandwiches have been used as building blocks for extended systems, some of which exhibit electron delocalization between metal centers. An example of a cyclic poly complex is the octahedral "carbon-wired" system shown below, which contains a planar C16B8 macrocycle.
Inverse sandwiches
In these anti-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include 2 and 2COT.
Double- and multimetallic sandwich compounds
Another family of sandwich compound involves more than one metal sandwiched between two carbocyclic rings. Examples of the double sandwich include V22, Ni22 and Cr22. Depicted at right is an example of a multimetallic sandwich compound, which has four palladium atoms joined in a chain sandwiched between two perylene units. The counterions are bulky tetraarylborates.
