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Organometallic compound: Fe(II) sandwiched between two cyclopentadienyl rings
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| Verifiedfields = changed
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| verifiedrevid = 457631192
| ImageFileL1 = Ferrocene. svg
| ImageFileR1 = Ferrocene from xtal 3D balls. png
| ImageFileL2 = Ferrocene 3d model 2. png
| ImageFileR2 = Photo of Ferrocene (powdered). JPG
| PIN = Ferrocene
| OtherNames = Dicyclopentadienyl iron, Bis(η5 cyclopentadienyl)iron, Iron(II) cyclopentadienide
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| Section4 = Chembox Hazards
|NFPA F = 2
|NFPA H = 3
|NFPA R = 1
|NFPA ref =
|PEL = TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)
|IDLH = N. D.

Mössbauer spectroscopy indicates that the iron center in ferrocene should be assigned the +2 oxidation state. Each cyclopentadienyl (Cp) ring should then be allocated a single negative charge. Thus ferrocene could be described as iron(II) bis(cyclopentadienide), |Fe(2+)[C5H5( )]2. Each ring has six π electrons, which makes them aromatic according to Hückel's rule. These π electrons are then shared with the metal via covalent bonding. Since Fe2+ has six d electrons, the complex attains an 18 electron configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as |Fe(η^{5}\ C5H5)2, where η denotes hapticity, the number of atoms through which each ring binds. The carbon–carbon bond distances around each five membered ring are all 1.40 Å, and all Fe–C bond distances are 2.04 Å. From room temperature down to 164 K, X ray crystallography yields the monoclinic space group; the cyclopentadienide rings are a staggered conformation, resulting in a centrosymmetric molecule, with symmetry group D5d. The Cp rings rotate with a low barrier about the Cp(centroid)–Fe–Cp(centroid) axis, as observed by measurements on substituted derivatives of ferrocene using 1H and 13C nuclear magnetic resonance spectroscopy. For example, methylferrocene (CH3C5H4FeC5H5) exhibits a singlet for the C5H5 ring. In solution, and at room temperature, eclipsed D5h ferrocene was determined to dominate over the staggered D5d conformer, as suggested by both Fourier transform infrared spectroscopy and DFT calculations.

Ferrocene undergoes many reactions characteristic of aromatic compounds, enabling the preparation of substituted derivatives. A common undergraduate experiment is the Friedel–Crafts reaction of ferrocene with acetic anhydride (or acetyl chloride) in the presence of phosphoric acid as a catalyst. Under conditions for a Mannich reaction, ferrocene gives N,N dimethylaminomethylferrocene. Ferrocene itself can be used as the backbone of a ligand, e. g. 1,1' bis(diphenylphosphino)ferrocene (dppf). Ferrocene can itself be oxidized to the ferrocenium cation (Fc+); the ferrocene/ferrocenium couple is often used as a reference in electrochemistry. just as acylation of benzene yields acetophenone under similar conditions. Protonation of ferrocene allows isolation of [Cp2FeH]PF6. In the presence of aluminium chloride, Me2NPCl2 and ferrocene react to give ferrocenyl dichlorophosphine, whereas treatment with phenyldichlorophosphine under similar conditions forms P,P diferrocenyl P phenyl phosphine. Ferrocene reacts with P4S10 forms a diferrocenyl dithiadiphosphetane disulfide.

Ferrocene reacts with butyllithium to give 1,1′ dilithioferrocene, which is a versatile nucleophile. In combination with butyllithiium, tert butyllithium produces monolithioferrocene.

Ferrocene undergoes a one electron oxidation at around 0.4 V versus a saturated calomel electrode (SCE), becoming ferrocenium. This reversible oxidation has been used as standard in electrochemistry as Fc+/Fc = 0.64 V versus the standard hydrogen electrode, however other values have been reported. Ferrocenium tetrafluoroborate is a common reagent. The remarkably reversible oxidation reduction behaviour has been extensively used to control electron transfer processes in electrochemical and photochemical systems. thumb|left|132px|The one electron oxidized derivative of biferrocene has attracted much research attention. Substituents on the cyclopentadienyl ligands alters the redox potential in the expected way: electron withdrawing groups such as a carboxylic acid shift the potential in the anodic direction (i. e. made more positive), whereas electron releasing groups such as methyl groups shift the potential in the cathodic direction (more negative). Thus, decamethylferrocene is much more easily oxidised than ferrocene and can even be oxidised to the corresponding dication. Ferrocene is often used as an internal standard for calibrating redox potentials in non aqueous electrochemistry.

Disubstituted ferrocenes can exist as either 1,2 , 1,3 or 1,1′ isomers, none of which are interconvertible. Ferrocenes that are asymmetrically disubstituted on one ring are chiral – for example [CpFe(EtC5H3Me)]. This planar chirality arises despite no single atom being a stereogenic centre. The substituted ferrocene shown at right (a 4 (dimethylamino)pyridine derivative) has been shown to be effective when used for the kinetic resolution of racemic secondary alcohols. Several approaches have been developed to asymmetrically 1,1′ functionalise the ferrocene.

Ferrocene and its numerous derivatives have no large scale applications, but have many niche uses that exploit the unusual structure (ligand scaffolds, pharmaceutical candidates), robustness (anti knock formulations, precursors to materials), and redox (reagents and redox standards).

Chiral ferrocenyl phosphines are employed as ligands for transition metal catalyzed reactions. Some of them have found industrial applications in the synthesis of pharmaceuticals and agrochemicals. For example, the diphosphine 1,1′ bis(diphenylphosphino)ferrocene (dppf) is a valued ligand for palladium coupling reactions and Josiphos ligand is useful for hydrogenation catalysis. They are named after the technician who made the first one, Josi Puleo. [[File:Josiphos. png|thumb|Josiphos ligand. Petrol additive solutions containing ferrocene can be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol. The iron containing deposits formed from ferrocene can form a conductive coating on spark plug surfaces. Ferrocene polyglycol copolymers, prepared by effecting a polycondensation reaction between a ferrocene derivative and a substituted dihydroxy alcohol, has promise as a component of rocket propellants. These copolymers provide rocket propellants with heat stability, serving as a propellant binder and controlling propellant burn rate. Ferrocene has been found to be effective at reducing smoke and sulfur trioxide produced when burning coal. The addition by any practical means, impregnating the coal or adding ferrocene to the combustion chamber, can significantly reduce the amount of these undesirable byproducts, even with a small amount of the metal cyclopentadienyl compound.

Ferrocene derivatives have been investigated as drugs, with one compound ferrocerone approved for use in the USSR in the 1970s, though it is no longer marketed today. Only one drug has entered clinical trials in recent years, Ferroquine (7 chloro N (2 ((dimethylamino)methyl)ferrocenyl)quinolin 4 amine), an antimalarial, which has reached Phase IIb trials. Ferrocene containing polymer based drug delivery systems have been investigated. The anticancer activity of ferrocene derivatives was first investigated in the late 1970s, when derivatives bearing amine or amide groups were tested against lymphocytic leukemia. Some ferrocenium salts exhibit anticancer activity, but no compound has seen evaluation in the clinic. Ferrocene derivatives have strong inhibitory activity against human lung cancer cell line A549, colorectal cancer cell line HCT116, and breast cancer cell line MCF 7. An experimental drug was reported which is a ferrocenyl version of tamoxifen. Ferrocifens are exploited for cancer applications by a French biotech, Feroscan, founded by Pr. Gerard Jaouen.

Ferrocene and related derivatives are used as powerful burn rate catalysts in ammonium perchlorate composite propellant.

Ferrocene analogues can be prepared with variants of cyclopentadienyl. For example, bisindenyliron and bisfluorenyliron. This compound on boiling under reflux in benzene is converted to ferrocene. Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand, features a cyclopentadienyl anion derivatized with five ferrocene substituents. In hexaferrocenylbenzene, C6[(η5 C5H4)Fe(η5 C5H5)]6, all six positions on a benzene molecule have ferrocenyl substituents (R). X ray diffraction analysis of this compound confirms that the cyclopentadienyl ligands are not co planar with the benzene core but have alternating dihedral angles of +30° and −80°. Due to steric crowding the ferrocenyls are slightly bent with angles of 177° and have elongated C Fe bonds. The quaternary cyclopentadienyl carbon atoms are also pyramidalized. Also, the benzene core has a chair conformation with dihedral angles of 14° and displays bond length alternation between 142.7 pm and 141.1 pm, both indications of steric crowding of the substituents. The synthesis of hexaferrocenylbenzene has been reported using Negishi coupling of hexaiodidobenzene and diferrocenylzinc, using tris(dibenzylideneacetone)dipalladium(0) as catalyst, in tetrahydrofuran: The vinylferrocene can be made by a Wittig reaction of the aldehyde, a phosphonium salt, and sodium hydroxide. The vinyl ferrocene can be converted into a polymer (polyvinylferrocene, PVFc), a ferrocenyl version of polystyrene (the phenyl groups are replaced with ferrocenyl groups). Another polyferrocene which can be formed is poly(2 (methacryloyloxy)ethyl ferrocenecarboxylate), PFcMA. In addition to using organic polymer backbones, these pendant ferrocene units have been attached to inorganic backbones such as polysiloxanes, polyphosphazenes, and polyphosphinoboranes, (–PH(R)–BH2–)n, and the resulting materials exhibit unusual physical and electronic properties relating to the ferrocene / ferrocinium redox couple. Both PVFc and PFcMA have been tethered onto silica wafers and the wettability measured when the polymer chains are uncharged and when the ferrocene moieties are oxidised to produce positively charged groups. The contact angle with water on the PFcMA coated wafers was 70° smaller following oxidation, while in the case of PVFc the decrease was 30°, and the switching of wettability is reversible. In the PFcMA case, the effect of lengthening the chains and hence introducing more ferrocene groups is significantly larger reductions in the contact angle upon oxidation.