Metallocenes

Bis-cyclopentadienyl derivatives of transition metals whose bonding involves overlap of ns, (n − 1)d, and np orbitals of the metal with molecular orbitals of appropriate symmetry of each cyclopentadienyl ring. The resulting complexes often possess two parallel rings (sandwich structure), but in some cases, for example those involving the titanium subgroup of metals, the rings are canted (see illustration). Metals in the periodic table commonly known to form metallocene complexes are titanium, zirconium, hafnium, vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, osmium, cobalt, rhodium, and nickel. See also Coordination complexes.

ferrocene. (b) Canted cyclopentadienyl ring structure of titanocene dichloride. The distribution of the ligands about the Ti atom is tetrahedral.">
Metallocene structures. (a) Staggered sandwich structure of ferrocene. (b) Canted cyclopentadienyl ring structure of titanocene dichloride. The distribution of the ligands about the Ti atom is tetrahedral.

The reactions of metallocenes can be divided into two classes: the first is typified by the iron triad, and comprises essentially the reactions of aromatic molecules; the second consists of the reactions of the other metallocenes where the 18-electron rare-gas configuration is not found. Reactions in these latter systems often lead to a product where the 18-electron rule is obeyed.

Ferrocene is a very electron-rich system, and undergoes electrophilic substitution with great rapidity. For example, acylation proceeds about 10⁶ times faster than that of benzene under similar conditions. Ferrocene also undergoes several other typical aromatic substitution reactions besides acylation, including sulfonation, dimethylaminomethylation (Mannich reaction), metalation, and the like. Bis substitution tends to factor a product where each ring is monosubstituted, although several cases are known where two substituents are introduced into one ring. Ferrocene is oxidized and deactivated under conditions for nitration and halogenation.

Uses of metallocenes include reaction of chromocene with alumina to make a polymerization catalyst for ethylene. Ferrocene and some alkyl-substituted ferrocenes have been used as moderators in high-temperature combustions such as occur in solid rocket fuels. A cyclopentadienyl complex, CH₃C₅H₄Mn(CO)₃, briefly replaced tetraethyllead as an octane booster and antiknock agent in liquid fuels. See also Organometallic compound.