The birth of cyclopentadienyl transition-metal chemistry occurred in 1951 when Pauson and Kealy discovered bis(η5–cyclopentadienyl)iron (ferrocene). Sodium cyclopentadienides containing aldehyde, ketone, or ester substituents can be synthesized easily following a method developed originally by Thiele in 1900. Peters has shown that a reaction between equimolar amounts of methyl chloroformate and sodium cyclopentadienide gave two main products. Cyclopentadienide anions possessing electron-withdrawing groups generally have greater air stability than do the corresponding unsubstituted cyclopentadienide anions. Excluding ferrocene and cymantrene, which can be halogenated indirectly via electrophilic substitution, there are two methods available for the synthesis of halocyclopentadienylmetal compounds. Wulfsberg and West have synthesized thallium pentachlorocyclopentadienide as well as other M+C5Cl5– salts and have studied their properties. In 1900, Thiele obtained the first cyclopentadienylmetal compound––sodium nitrocyclopentadienide––from a reaction between cyclopentadiene and ethyl nitrate in the presence of sodium ethoxide.Various nucleophiles reacted with 6-diene to give, after hydrolysis, substituted cyclopentadienes. Schlenk and Bergmann first observed sodium isopropenylcyclopentadienide as the product of a reaction between 6,6-dimethylfulvene and triphenylmethylsodium. Polymer-supported cyclopentadienyl compounds have been synthesized mainly for possible catalytic applications. In general, polystyrene–divinylbenzene copolymers have been used as the polymer supports.