Sandwich compound

[1] 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.

[2][page needed] The best known members are the metallocenes of the formula M(C5H5)2 where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, Os, Sm, Ti, V, Mo, W, Zn.

[8] Half sandwich complexes have only one facially-bound planar organic ligand instead of two gives rise to a still larger family of half-sandwich compounds.

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,[9] with X-ray crystallographic confirmation in 1974.

[10] 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[11] (the structure of the 1,7,2,3 isomer is shown).

[15] Numerous multidecker sandwich compounds featuring hydrocarbon bridging rings have also been prepared, especially triple deckers.

[17] Monomeric double-decker and multidecker sandwiches have been used as building blocks for extended systems, some of which exhibit electron delocalization between metal centers.