Ferrocene is an organometallic compound with the formula Fe(C5H5)2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds. The rapid growth of organometallic chemistry is often attributed to the excitement arising from the discovery of ferrocene and its many analogues.
3D model (JSmol)
|Molar mass||186.04 g/mol|
|Appearance||light orange powder|
|Density||1.107 g/cm3 (0 °C), 1.490 g/cm3 (20 °C)|
|Melting point||172.5 °C (342.5 °F; 445.6 K)|
|Boiling point||249 °C (480 °F; 522 K)|
|Insoluble in water, soluble in most organic solvents|
|log P||2.04050 |
|Main hazards||Very hazardous in case of ingestion. Hazardous in case of skin contact (irritant), of eye contact (irritant), of inhalation|
|US health exposure limits (NIOSH):|
|TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)|
|TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp)|
IDLH (Immediate danger)
|cobaltocene, nickelocene, chromocene, ruthenocene, osmocene, plumbocene|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Ferrocene was first prepared unintentionally. In 1951, Peter Pauson and T. J. Kealy at Duquesne University reported the reaction of cyclopentadienyl magnesium bromide and ferric chloride with the goal of oxidatively coupling the diene to prepare fulvalene. Instead, they obtained a light orange powder of "remarkable stability". A second group at British Oxygen also unknowingly discovered ferrocene. Miller, Tebboth and Tremaine were trying to synthesize amines from hydrocarbons such as cyclopentadiene and ammonia in a modification of the Haber process. They published this result in 1952 although the actual work was done three years earlier. The stability of the new organoiron compound was accorded to the aromatic character of the negatively charged cyclopentadienyls, but they were not the ones to recognize the η5 (pentahapto) sandwich structure.
Robert Burns Woodward and Geoffrey Wilkinson deduced the structure based on its reactivity. Independently Ernst Otto Fischer also came to the conclusion of the sandwich structure, but he originally termed ferrocene as a "double-cone structure", which is the english translation of Doppelkegelstruktur. At this time, he began to synthesize other metallocenes such as nickelocene and cobaltocene.
The structure of ferrocene was confirmed by NMR spectroscopy and X-ray crystallography. Its distinctive "sandwich" structure led to an explosion of interest in compounds of d-block metals with hydrocarbons, and invigorated the development of the flourishing study of organometallic chemistry. In 1973 Fischer of the Technische Universität München and Wilkinson of Imperial College London shared a Nobel Prize for their work on metallocenes and other aspects of organometallic chemistry.
The carbon–carbon bond distances are 1.40 Å within the five-membered rings, and the Fe–C bond distances are 2.04 Å. At room temperature down to 164K, X-ray crystallography points to the Cp rings being in a staggered conformation due to imposed molecular centrosymmetric symmetry in the monoclinic space group. However, Below 110 K, ferrocene crystallizes in an orthorhombic crystal lattice in which the Cp rings are ordered and eclipsed. It has been shown through gas phase electron diffraction and computational studies that in the gas phase the Cp rings are eclipsed. The point group of the staggered conformation is D5d and the point group of the eclipsed conformation is D5h.
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 terms of bonding, the iron center in ferrocene is usually assigned to the +2 oxidation state, consistent with measurements using Mössbauer spectroscopy. Each cyclopentadienyl (Cp) ring is then allocated a single negative charge, bringing the number of π-electrons on each ring to six, and thus making them aromatic. These twelve electrons (six from each ring) are then shared with the metal via covalent bonding. When combined with the six d-electrons on Fe2+, the complex attains an 18-electron configuration.
The first reported syntheses of ferrocene were nearly simultaneous. Pauson and Kealy synthesised ferrocene using iron(III) chloride and a Grignard reagent, cyclopentadienyl magnesium bromide. Iron(III) chloride is suspended in anhydrous diethyl ether and added to the Grignard reagent, which is prepared by reacting cyclopentadiene with magnesium and bromoethane in anhydrous benzene. An iron(III) salt was chosen as they sought to couple the cyclopentadienyl moieties to form dihydrofulvalene and then fulvalene, but ferrocene was formed instead as the oxidative formation of dihydrofulvalene also produced iron(II) by reduction, which in turn reacts with the Grignard.
The other early synthesis of ferrocene was by Miller et al., who reacted metallic iron directly with gas-phase cyclopentadiene at elevated temperature. An approach using iron pentacarbonyl was also reported.
More efficient preparative methods are generally a modification of the original transmetalation sequence using either commercially available sodium cyclopentadienide or freshly cracked cyclopentadiene deprotonated with potassium hydroxide and reacted with anhydrous iron(II) chloride in ethereal solvents. A modern modification of the Grignard approach is also known:
As expected for a symmetric, uncharged species, ferrocene is soluble in normal organic solvents, such as benzene, but is insoluble in water. Ferrocene is an air-stable orange solid that readily sublimes, especially upon heating in a vacuum. It is stable to temperatures as high as 400 °C. The following table gives typical values of vapor pressure of ferrocene at different temperatures:
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.
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 butyllithium to give 1,1′-dilithioferrocene, which is a versatile nucleophile. Tert-Butyllithium produces monolithioferrocene. Dilithioferrocene reacts with S8, chlorophosphines, and chlorosilanes. The strained compounds undergo ring-opening polymerization.
Ferrocene undergoes a one-electron oxidation at around 0.5 V versus a saturated calomel electrode (SCE). This reversible oxidation has been used as standard in electrochemistry as Fc+/Fc = 0.40 V versus the standard hydrogen electrode. Ferrocenium tetrafluoroborate is a common reagent.
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.
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 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.
Ferrocene and its derivatives are antiknock agents used in the fuel for petrol engines; they are safer than tetraethyllead, previously used. 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 the spark plug surfaces. What is more, ferrocene polyglycol copolymers, prepared by effecting a polycondensation reaction between a ferrocene derivative and a substituted dihydroxy alcohol, has especially promising applications as a component of rocket propellants. In particular, these copolymers provide the rocket propellants with heat stability, serving as a propellant binder and controlling the burn rate of the propellant.
In a similar light, ferrocene also has been found to be effective at reducing the smoke and sulfur trioxide produced when burning coal. The addition by any practical means, impregnating the coal or simply adding ferrocene to the combustion chamber, can significantly cut down the amount of these undesirable byproducts, even with a small amount of the metal cyclopentadienyl compound.
Ferrocene derivatives have been investigated as drugs. Only one drug has entered clinic trials, Ferroquine (7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine), an antimalarial. 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. In particular, 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. The idea is that the tamoxifen will bind to the estrogen binding sites, resulting in cytotoxicity.
Carbon atoms can be replaced by heteroatoms as illustrated by Fe(η5-C5Me5)(η5-P5) and Fe(η5-C5H5)(η5-C4H4N) ("azaferrocene"). Azaferrocene arises from decarbonylation of Fe(η5-C5H5)(CO)2(η1-pyrrole) in cyclohexane. 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:
Ferrocene, a precursor to iron nanoparticles, can be used as a catalyst for the production of carbon nanotubes. 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.
[1,1'-Bis(diphenylphosphino)ferrocene]palladium(II) dichloride is a palladium complex containing the bidentate ligand 1,1'-bis(diphenylphosphino)ferrocene (dppf), abbreviated as [(dppf)PdCl2]. This commercially available material can be prepared by reacting dppf with a suitable nitrile complex of palladium dichloride:
dppf + PdCl2(RCN)2 → (dppf)PdCl2 + 2 RCN (RCN = CH3CN or C6H5CN)The compound is popularly used for palladium-catalyzed coupling reactions, such as the Buchwald–Hartwig amination and the reductive homocoupling of aryl halides.1,1'-Bis(diphenylphosphino)ferrocene
1,1'-Bis(diphenylphosphino)ferrocene, commonly abbreviated dppf, is an organophosphorus compound commonly used as a ligand in homogeneous catalysis. It contains a ferrocene moiety in its backbone, and is related to other bridged diphosphines such as 1,2-bis(diphenylphosphino)ethane (dppe).Acetylferrocene
Acetylferrocene is the organoiron compound with the formula (C5H5)Fe(C5H4COMe). It consists of ferrocene substituted by an acetyl group on one of the cyclopentadienyl rings. It is an orange, air-stable solid that is soluble in organic solvents.Antiknock agent
An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs.
The mixture known as gasoline or petrol, when used in high compression internal combustion engines, has a tendency to knock (also called "pinging" or "pinking") and/or to ignite early before the correctly timed spark occurs (pre-ignition, refer to engine knocking).Cobaltocene
Cobaltocene, known also as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt", is an organocobalt compound with the formula Co(C5H5)2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovered shortly after ferrocene, the first metallocene. Due to the ease with which it reacts with oxygen, the compound must be handled and stored using air-free techniques.Decamethylferrocene
Decamethylferrocene is a sandwich compound. In terms of structure and bonding it resembles ferrocene, but with a methyl group on each of the carbons of the cyclopentadienyl (Cp) rings. It is a yellow crystalline solid that is a weak reductant.Ferrocene-containing dendrimers
Ferrocene-containing dendrimers are dendrimers that contain ferrocene substituents. Some ferrocene-containing dendrimers feature ferrocene cores and others do not. All feature with peripheral ferrocene groups.Ferrocenium tetrafluoroborate
Ferrocenium tetrafluoroborate is an organometallic compound with the formula [Fe(C5H5)2]BF4. This salt is composed of the cation [Fe(C5H5)2]+ and the tetrafluoroborate anion (BF−4). The related hexafluorophosphate is also a popular reagent with similar properties. The cation is often abbreviated Fc+ or Cp2Fe+. The salt is deep blue in color and paramagnetic.
Ferrocenium salts are sometimes used as one-electron oxidizing agents, and the reduced product, ferrocene, is inert and readily separated from ionic products. The ferrocene–ferrocenium couple is often used as a reference in electrochemistry. The standard potential of ferrocene-ferrocenium is 0.400 V vs. the normal hydrogen electrode (NHE) and is often assumed to be invariant between different solvents.Ferrocenophanes
Ferrocenophanes, also called ansa ferrocenes (from ansa: handle in greek), are organometallic compounds which are derived from ferrocene. They are a subset of ansa-metallocenes in which the metal is iron. In this compound class, the cyclopentadienyl ligands of the ferrocene are connected by one or more bridging groups. This hinders the rotation of the cyclopentadienyl ligands against one another and the reactivity of the complex with respect to the parent compound ferrocene is altered. A variety of organic and inorganic groups is suitable as bridging group.Hapticity
Hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated (otherwise the κ-notation is used). In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity (not hapticity), and the κ-notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.Iron
Iron is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal in the first transition series. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Its abundance in rocky planets like Earth is due to its abundant production by fusion in high-mass stars, where it is the last element to be produced with release of energy before the violent collapse of a supernova, which scatters the iron into space.
Like the other group 8 elements, ruthenium and osmium, iron exists in a wide range of oxidation states, −2 to +7, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen and water. Fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.
Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is relatively soft, but is unobtainable by smelting because it is significantly hardened and strengthened by impurities, in particular carbon, from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, which has a high carbon content. Further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and iron alloys formed with other metals (alloy steels) are by far the most common industrial metals because they have a great range of desirable properties and iron-bearing rock is abundant.
Iron chemical compounds have many uses. Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the halogens and the chalcogens. Among its organometallic compounds is ferrocene, the first sandwich compound discovered.
Iron plays an important role in biology, forming complexes with molecular oxygen in hemoglobin and myoglobin; these two compounds are common oxygen-handling proteins in vertebrates (hemoglobin for oxygen transport, and myoglobin for oxygen storage). Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Iron is distributed throughout the human body, and is especially abundant in hemoglobin. Total iron content of the adult human body is approximately 3.8 grams in males and 2.3 grams in females. Iron is a critical element in the metabolism of hundreds of proteins and enzymes involved in diverse body functions, such as oxygen transport, DNA synthesis, and cell growth.Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions (C5H−5, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.
Some metallocenes consist of metal plus two cyclooctatetraenide anions (C8H2−8, abbreviated cot2−), namely the lanthanocenes and the actinocenes (uranocene and others).
Metallocenes are a subset of a broader class of compounds called sandwich compounds .
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromatically stabilized. Here they are shown in a staggered conformation.Organoiron chemistry
Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. Iron adopts oxidation states from Fe(−II) through to Fe(VII). Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.Organouranium chemistry
Organouranium chemistry is the science exploring the properties, structure and reactivity of organouranium compounds, which are organometallic compounds containing a carbon to uranium chemical bond. The field is of some importance to the nuclear industry and of theoretical interest in organometallic chemistry.
The development of organouranium compounds started in World War II when the Manhattan Project required volatile uranium compounds for 235U/238U isotope separation. For example, Henry Gilman attempted to synthesize compounds like tetramethyluranium and others worked on uranium metal carbonyls but none of the efforts met success due to organouranium instability. After the discovery of ferrocene in 1951, Todd Reynolds and Geoffrey Wilkinson in 1956 synthesized the uranium metallocene Cp3UCl from sodium cyclopentadienide and uranium tetrachloride as a stable but extremely air-sensitive compound. In it the U-Cl bond is an ionic bond and the uranium bonds with the three cyclopentadienyl ligands are covalent of the type found in sandwich compounds with involvement of the uranium 5f atomic orbitals.
Ernst Otto Fischer in 1962 discovered tetracyclopentadienyluranium Cp4U by reaction of NaCp with UCl4 (6% yield) as a compound stable in air as a solid but not in solution. A zero molecular dipole moment and IR spectroscopy revealed that it was also a sandwich compound with uranium in a tetrahedral molecular geometry. In 1970, Fischer added Cp3U to the list of known organouranium compounds by reduction of Cp4U with elemental uranium.
In 1968, the group of Andrew Streitwieser prepared the stable but pyrophoric compound uranocene (COT)2U which has an atom of uranium sandwiched between two cyclooctatetraenide anions (D8h molecular symmetry). The uranium f orbitals interact substantially with the aromatic rings just as the d-orbitals in ferrocene interact with the Cp ligands. Uranocene differs from ferrocene because its HOMO and LUMO are centered on the metal and not on the rings and all reactions thus involve the metal often resulting in ligand - metal cleavage.
Uranocenes show ease of reduction of U(IV) compounds to U(III) compounds; otherwise they are fairly unreactive. A close relative that does have sufficient reactivity, obtained by reaction of uranocene with uranium borohydride is the half-sandwich compound (COT)U(BH4)2 discovered in 1983 by the group of M.J. Ephritikhine. Compounds of this type react in many different ways, for instance alkylation at uranium with organolithium reagents or conversion to hybrid sandwich compounds.
Other organouranium compounds are inverted uranocenes with a COT ligand in between two uranium atoms or uranium sandwich compounds with pentalenide ligands instead of COT ligands.Polyferrocenes
Polyferrocenes are polymers containing ferrocene units. Ferrocene offers many advantages over pure hydrocarbons when used as a building block of macromolecular chemistry. The variety of possible substitutions at the ferrocene parent body results in a multitude of accessible polymers with interesting electronic and photonic properties. Many polyferrocenes are relatively easily accessible. Poly(1,1'-ferrocene-silane) can be prepared by ring-opening polymerization and has a variety of interesting properties, such as a high refractive index or semiconductor properties. Ring-opening polymerization usually leads to polymers containing ferrocene in the backbone. Besides the latter motif, ferrocene can be attached to the backbone as pendant unit as well.Reference electrode
A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each participant of the redox reaction.There are many ways reference electrodes are used. The simplest is when the reference electrode is used as a half cell to build an electrochemical cell. This allows the potential of the other half cell to be determined. An accurate and practical method to measure an electrode's potential in isolation (absolute electrode potential) has yet to be developed.Rhodocene
Rhodocene, formally known as bis(η5-cyclopentadienyl)rhodium(II), is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures (−196 °C). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.The history of organometallic chemistry includes the 19th-century discoveries of Zeise's salt and nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with existing chemical bonding models. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts. The study of organometallic species including these ultimately led to the development of new bonding models that explained their formation and stability. Work on sandwich compounds, including the rhodocenium-rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and tris(acetylacetonato)rhodium(III); numerous other approaches have since been reported, including gas-phase redox transmetalation and using half-sandwich precursors. Octaphenylrhodocene (a derivative with eight phenyl groups attached) was the first substituted rhodocene to be isolated at room temperature, though it decomposes rapidly in air. X-ray crystallography confirmed that octaphenylrhodocene has a sandwich structure with a staggered conformation. Unlike cobaltocene, which has become a useful one-electron reducing agent in research, no rhodocene derivative yet discovered is stable enough for such applications.
Biomedical researchers have examined the applications of rhodium compounds and their derivatives in medicine and reported one potential application for a rhodocene derivative as a radiopharmaceutical to treat small cancers. Rhodocene derivatives are used to synthesise linked metallocenes so that metal–metal interactions can be studied; potential applications of these derivatives include molecular electronics and research into the mechanisms of catalysis. The value of rhodocenes tends to be in the insights they provide into the bonding and dynamics of novel chemical systems, rather than their applications.Ruthenocene
Ruthenocene is an organoruthenium compound with the formula (C5H5)2Ru. This pale yellow, volatile solid is classified as a sandwich compound and more specifically, as a metallocene.Sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives (for example Cn(CH3)n) and heterocyclic derivatives (for example BCnHn+1). Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the metallocenes.
The term sandwich compound was introduced in organometallic nomenclature in the mid-1950s in a report by J. D. Dunitz, L. E. Orgel and R. A. Rich, who confirmed the structure of ferrocene by X-ray crystallography. The correct structure had been proposed several years previously by Robert Burns Woodward and, separately, by Ernst Otto Fischer. The structure helped explain puzzles about ferrocene's conformers, the molecule features an iron atom sandwiched between two parallel cyclopentadienyl rings. This result further demonstrated the power of X-ray crystallography and accelerated the growth of organometallic chemistry.