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MetalloceneIn chemistry, and in particular, in organometallic chemistry, a metallocene is a compound consisting of an aromatic organic ligand bound to a metal. A key feature of metallocenes is planar aromatic organic ligands in which multiple of the carbon atoms in the ring form metal-carbon bonds that are equivalent. The prototypical metallocene is ferrocene, consisting of two cyclopentadienyl rings bound on opposite sides of a central iron atom, forming an organometallic sandwich compound. In the case of ferrocene, for instance, the length and strength and bond angles about the metal-carbon bonds are identical. The nature of the bonding of the organic ligand to the metal is referred to as its "hapticity" and is indicated by the Greek letter eta. The equivalent bonding of all 5 carbon atoms of each cyclopentadienyl ring in ferrocene is denoted as eta-5. The systematic name of ferrocene is, therefore, bis(eta-5-cyclopentadienyl)iron. A notable feature of all metallocenes is their high thermal stability (up to and over 500C). All neutral metallocenes are soluble in common organic solvents and can be purified by vacuum sublimation. Metallocenes of all 3d-row elements are known, but with the exception of ferrocene all of these are sensitive to air, paramagnetic and do not possess a closed-shell electron structure. X-ray diffraction studies have shown that all metallocenes have a sandwich structure, with the cyclopentadienyl rings either eclipsed or staggered. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only 8 kJ/mol. Ferrocene and osmocene take on an eclipsed conformation at low temperatures, while pentamethylcyclopentadienyl ligands are always in a staggered conformation to minimize steric hindrance between the methyl groups. A similar structure to metallocenes, including just one facially-bound planar organic ligand instead of two, are the piano stool compounds.
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