Ferrocene

Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule consists of two cyclopentadienyl rings bound on opposite sides of a central iron atom. It is a orange solid with a camphor-like odor, that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule consists of two cyclopentadienyl rings bound on opposite sides of a central iron atom. It is a orange solid with a camphor-like odor, that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. The rapid growth of organometallic chemistry is often attributed to the excitement arising from the discovery of ferrocene and its many analogues (metallocenes). Ferrocene was discovered by accident—thrice. In 1951, Peter L. Pauson and Thomas J. Kealy at Duquesne University attempted to prepare the fulvalene ((C5H4)2) by oxidative dimerization of cyclopentadiene (C5H6). To that end, they reacted the Grignard compound cyclopentadienyl magnesium bromide in diethyl ether with ferric chloride as an oxidizer. However, instead of the expected fulvalene, they obtained a light orange powder of 'remarkable stability', with the formula C10H10Fe. However, that was only the third known time that the compound had been observed. The second time was around 1950, when S. Miller, J. Tebboth, and J. Tremaine, researchers at British Oxygen, were attempting to synthesize amines from hydrocarbons and nitrogen in a modification of the Haber process. When they tried to react cyclopentadiene with nitrogen at 300 °C, at atmospheric pressure, they were disappointed to see the hydrocarbon react with some source of iron, yielding ferrocene. While they too observed its remarkable stability, they put the observation aside and did not publish it until after Pauson reported his findings. In fact, Kealy and Pauson were provided with a sample by Miller et al., who confirmed that the products were the same compound. The first known synthesis, may have been made in the late 1940s by unknown researchers at Union Carbide, who tried to pass hot cyclopentadiene vapor through an iron pipe. The vapor reacted with the pipe wall, creating a 'yellow sludge' that clogged the pipe. Years later, a sample of the sludge that had been saved was obtained and analyzed by E.Brimm, shortly after reading Kealy and Pauson's article, and was found to consist of ferrocene. Pauson and Kealy conjectured that the compound had two cyclopentadienyl groups, each with a single covalent bond from the saturated carbon atom to the iron atom. However, that structure was inconsistent with then-existing bonding models and did not explain the unexpected stability of the compound, and chemists struggled to find the correct structure. The structure was deduced and reported independently by three groups in 1952: The 'sandwich' structure of ferrocene was shockingly novel, and required new theory to explain. Application of molecular orbital theory with the assumption of a Fe2+ centre between two cyclopentadienide anions C5H−5 resulted in the successful Dewar-Chatt-Duncanson model, allowing correct prediction of the geometry of the molecule and explaining its remarkable stability became clear. Ferrocene was not the first organometallic compound known. Zeise's salt K · H2O was reported in 1831, Mond's discovery of Ni(CO)4 occurred in 1888, and organolithium compounds were developed in the 1930s. However, it can be argued that it was ferrocene's discovery that began organometallic chemistry as a separate area of chemistry. It also led to an explosion of interest in compounds of d-block metals with hydrocarbons.