Sandwich compounds

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 (for example Cn(CH3)n) and heterocyclic derivatives (for example BCnHn+1). 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 during the mid-1950s in a report by J.D. Dunitz, L.E. Orgel and R.A. Rich, who confirmed the structure of ferrocene by X-ray crystallography.[1] The correct structure had been proposed several years previously by Robert Burns Woodward. 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.[2]

Classes of sandwich compounds

The best known members are the metallocenes of the formula M(C5H5)2 where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, 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: M(C5H5)(CnHn). some examples are Ti(C5H5)(C7H7) and (C60)Fe(C5H5Ph5) where the fullerene ligand is acting as a cyclopentadienyl analaogue.
  • Bis(benzene) complexes: M(C6H6)2, the best known example being Bis(benzene)chromium.
  • Bis(cyclooctatetraenyl) complexes: M(C8H8)2, such as U(C8H8)2 and Th(C8H8)2.
  • Bis(cyclobutadienyl) complexes: M(C4H4)2, such as Fe(C4H4)2.

Sandwich complexes are even known containing purely inorganic ligands, such as Fe(C5Me5)(P5) and [(P5)2Ti]2-.[3]

Half sandwich compounds

Monometallic half sandwich compounds

Main article: Monometallic half sandwich compound

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, e.g. benzene or cyclopentadiene, and the legs being ligands such as CO or allyl.[4][5]

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.[4] A structurally related species is [Ru(C6H6)Cl2]2.

Multidecker sandwiches

The first multidecker sandwich complex was the dicationic triscyclopentadienyl dinickel complex [Ni2Cp3](BF4)2. Since that discovery, many related multidecker sandwich compounds have been discovered, especially triple deckers.[6] A versatile method involves the attachment of Cp*Ru+ to preformed sandwich complexes.[7]

Inverse sandwiches

In these anti-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include {(THF)3Ca}2(1,3,5-triphenylbenzene)[8] and {(Ar) Sn}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 V2(indenyl)2,[9] Ni2(COT)2[10] and Cr2(pentalene)2. Depicted below is an example of a multimetallic sandwich compound, which has four palladium atoms joined in a chain sandwiched between two perylene units.[11] The counterions are bulky tetraarylborates.


Ferrocene and methylcyclopentadienyl manganese tricarbonyl have been used as antiknock agents. Certain bent metallocenes of zirconium and hafnium are effective precatalysts for the polymerization of propylene. Many half sandwich complexes of ruthenium, such as those derived from (cymene)ruthenium dichloride dimer catalyse transfer hydrogenation, a useful reaction in organic synthesis.[12]


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