Cyclopentadienyl metal carbonyls and related compounds

A number of ff-cyclopentadienyl metal complexes are known which contain clusters of three or more metal atoms, held together by metal- 

TT-Cyclopentadienyl metal carbonyls are usually rath stable thermally, and in the solid state most resist oxidation by oxygen at room temperature. They can often be reduced by sodium amalgam in tetrahydrofuran to yield rr-cyclopentadienyl carbonyl anions. This reduction may proceed either by loss of carbon monoxide, e.g. 

Acidification of solutions containing salts of the anions leads to the liberation of the metal hydrides which contain covalent metal-hydrogen bonds, e.g. 

Tricarbonyl(methylcyclopeiitadienyl)]nanganese, MeC5H4Mn(CO)3, is the only cyclopentadienyl to have found industrial application on a significant scale. It is used as an antiknock, mixed with tetra-methyl and -ethyl lead and their redistribution products, and in some ways its antiknock properties are complementary to those of the lead compounds. Its antiknock property is due to the ability of manganese oxides, produced in the combustion, to end radical chains by electron-transfejr processes (as PbO does). The methylcyclopentadienyl compound, which is liquid at room temperature, is preferred to ir-CsH5Mn(CO)3 (mp 77°) because liquids are more convenient to handle industrially than solids. It is also used as a combustion improver since it helps to reduce smoke. 

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