Sandwich compound

A sandwich compound in organometallic chemistry is any chemical compound containing a metal atom sandwiched between two arene units.

The term sandwich compound was introduced in organometallic nomenclature in during the mid-1950s when J.D. Dunitz, L.E. Orgel and R.A. Rich confirmed the structure of ferrocene by X-ray crystallography. The correct structure had been proposed several years previously by Robert Burns Woodward. Of particular interest, 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, accelerating the growth of organometallic chemistry.

Classes of sandwich compounds
The best known members are the metallocenes of the formula M(C5H5)2 where M = Cr, Fe, Co, Ni. These species are also called Bis(cyclopentadienyl)metal complexes.
 * Mixed cyclopentadienyl complexes: M(C5H5)(CnHn). An example is  Ti(C5H5)(C7H7)
 * Bis(benzene) complexes: M(C6H6)2, the best known example being the chromium derivative.
 * Bis(cyclooctatetraenyl) complexes: U(C8H8)2, the Th derivative is also known.

Sandwich complexes are even known containing purely inorganic ligands, such as Fe(C5Me5)(P5) and [(P5)2Ti]2-.

Monometallic half sandwich compounds
Replacement of one ring in any of the above classes of compounds 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 "piano stool" compounds, at least when there are three diatomic ligands in addition to the hydrocarbon "seat" of the piano stool.

A piano stool compound is a metallocene compound including just one facially-bound planar organic ligand instead of two. The name derives from the similarity of the structure to such a "stool" with the seat being a facial planar organic compound, e.g. benzene or cyclopentadiene, and the legs being ligands such as CO. Below is an example of such a rhodium piano stool compound.



Dimetallic half sandwich compounds
Compounds such as the cyclopentadienyliron dicarbonyl dimer and cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they are dimetallic. A structurally related species is [Ru(C6H6)Cl2]2.

Multimetallic sandwich compounds
The compound depicted below has four palladium atoms joined in a chain sandwiched between two perylene units. The counterions are bulky tetraarylborates.

Multidecker sandwiches
The first multidecker sandwich complex was the dicationic triscyclopentadienyl dinickel complex [Ni2Cp3](BF4)2. This motif has been replicated many times, especially by attachment of Cp*Ru+ to preformed sandwich complexes.