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any of the dicyclopentadienyl derivatives of transition metals and metal halides. Metallocenes have the general formula (C5H5)2M and a “sandwich” structure, in which the metal atom, in a +2 oxidation state, is sandwiched between the C5H5 rings and bonded to all ten carbon atoms by covalent bonds.
Metallocenes are known for almost all the transition metals. In the most stable metallocenes—ferrocene, ruthocene, and osmocene—the metal atom has the electronic configuration of an inert gas. These metallocenes display aromatic properties. The physical and chemical properties of ferrocene have been more widely studied than those of other metallocenes. The metallocenes of such metals as titanium, niobium, and tantalum are unstable and are isolated only as derivatives with the general formula (C5H5)2MXn, where n = 1, 2, or 3 and “X” stands for H or one of the halogens, such as Cl or Br. In contrast to other metallocenes, manganocene has an ionic structure, (C5H5)2– Mn2+; such metallocenes as chromocene, (C5H5)2Cr, and vanadocene, (C5H5)2V, are partially ionic.
Metallocenes are intensely colored, crystalline compounds. They are readily soluble in organic solvents. In acidic aqueous solutions they are easily oxidized to cations with the formula (C5H5)2M+. The properties of these cations are similar to those of large, singly charged metal cations, such as Cs+. The metallocenes are precipitated as, for example, silicotungstate, chloroplatinate, or tetraphenylborate. The cobaltocene cation, (C5H5)2Co+, is not destroyed by concentrated nitric or sulfuric acid, even upon heating.
Metallocenes are produced by the action of salts or acetylacetonates of transition metals on sodium cyclopentadienide, (C5H5)–Na+, or on the cyclopentadienyl magnesium halide, C5H5MgBr. They may also be produced by the action of cyclopentadiene on a transition metal salt (in the presence of a base) or on a carbonyl.
Such metallocenes as (C5H5)2TiCl2 are used as cocatalysts in homogeneous catalytic reactions. Other metallocenes are used as drugs; for example, a ferrocene derivative, ferrocerone, C5H5FeC5H4COC6H4COONa-ortho·4H2O, is used for the treatment of diseases caused by an iron deficiency.
E. G. PEREVALOVA