Ferrocene

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Ferrocene
Ferrocene.svg
Ferrocene-from-xtal-3D-balls.png
Ferrocene 3d model 2.png
Photo of Ferrocene (powdered).JPG
Names
Preferred IUPAC name
Ferrocene [1]
Other names
  • Dicyclopentadienyl iron
  • Bis(η5-cyclopentadienyl)iron
  • Iron(II) cyclopentadienide
Identifiers
ChEBI
ChemSpider
ECHA InfoCard 100.002.764 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=1S/2C5H5.Fe/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2 Yes check.svgY
    Key: KTWOOEGAPBSYNW-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/2C5H5.Fe/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
    Key: KTWOOEGAPBSYNW-UHFFFAOYAZ
  • [CH-]1C=CC=C1.[CH-]1C=CC=C1.[Fe+2]
Properties
C10H10Fe
Molar mass 186.04 g/mol
Appearancelight orange powder
Odor camphor-like
Density 1.107 g/cm3 (0 °C)
1.490 g/cm3 (20 °C) [2]
Melting point 172.5 °C (342.5 °F; 445.6 K) [3]
Boiling point 249 °C (480 °F; 522 K)
Insoluble in water, soluble in most organic solvents
log P 2.04050 [4]
Structure
D5h (eclipsed)
D5d (staggered)
Sandwich structure with iron centre
Hazards
NFPA 704 (fire diamond)
[5]
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
3
2
1
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp) [6]
REL (Recommended)
TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp) [6]
IDLH (Immediate danger)
N.D. [6]
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule is a complex consisting of two cyclopentadienyl rings sandwiching a central iron atom. It is an 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. [7] Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and Fc+ respectively.

Contents

The first reported synthesis of ferrocene was in 1951. Its unusual stability puzzled chemists, and required the development of new theory to explain its formation and bonding. The discovery of ferrocene and its many analogues, known as metallocenes, sparked excitement and led to a rapid growth in the discipline of organometallic chemistry. Geoffrey Wilkinson and Ernst Otto Fischer, both of whom worked on elucidating the structure of ferrocene, later shared the 1973 Nobel Prize in Chemistry for their work on organometallic sandwich compounds. Ferrocene itself has no large-scale applications, but has found more niche uses in catalysis, as a fuel additive, and as a tool in undergraduate education.

History

Discovery

Ferrocene was discovered by accident twice. 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 Eugene O. Brimm, shortly after reading Kealy and Pauson's article, and was found to consist of ferrocene. [7] [8]

The second time was around 1950, when Samuel A. Miller, John A. Tebboth, and John F. 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. [7] [9] [10] Kealy and Pauson were later provided with a sample by Miller et al., who confirmed that the products were the same compound. [8]

In 1951, Peter L. Pauson and Thomas J. Kealy at Duquesne University attempted to prepare 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. [7] However, instead of the expected fulvalene, they obtained a light orange powder of "remarkable stability", with the formula C10H10Fe. [8] [11]

Determining the structure

Pauson and Kealy's original (incorrect) notion of ferrocene's molecular structure. Ferrocene kealy.svg
Pauson and Kealy's original (incorrect) notion of ferrocene's molecular structure.

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. [7] 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. [10] [12]

The structure was deduced and reported independently by three groups in 1952. [13] Robert Burns Woodward, Geoffrey Wilkinson, et al. deduced observe that the compound was diamagnetic and nonpolar. [14] A few months later they described its reactions as being typical of aromatic compounds such as benzene. [15] The name ferrocene was coined by Mark Whiting, a postdoc with Woodward. [16] . Ernst Otto Fischer and Wolfgang Pfab also noted ferrocene's diamagneticity and high symmetry. They also synthesize nickelocene and cobaltocene and confirmed they had the same structure. [17] Fischer described the structure as Doppelkegelstruktur ("double-cone structure"), although the term "sandwich" came to be preferred by British and American chemists. [18] Philip Frank Eiland and Raymond Pepinsky confirmed the structure through X-ray crystallography and later by NMR spectroscopy. [10] [19] [20] [21]

The "sandwich" structure of ferrocene was shockingly novel and led to intensive theoretical studies. 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 as well as explaining its remarkable stability. [22] [23]

Impact

The discovery of ferrocene was considered so significant that Wilkinson and Fischer shared the 1973 Nobel Prize in Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, called sandwich compounds". [24]

Structure and bonding

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), Fe2+[C5H−5]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(η5-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. [19] However, below 110 K, ferrocene crystallizes in an orthorhombic crystal lattice in which the Cp rings are ordered and eclipsed, so that the molecule has symmetry group D5h. [25] In the gas phase, electron diffraction [26] and computational studies [27] show that the Cp rings are eclipsed. While ferrocene has no permanent dipole moment at room temperature, between 172.8 and 163.5 K the molecule exhibits an "incommensurate modulation", breaking the D5 symmetry and acquiring an electric dipole. [28]

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. [29]

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. [30]

Synthesis

Early methods

The first reported syntheses of ferrocene were nearly simultaneous. Pauson and Kealy synthesised ferrocene using iron(III) chloride and cyclopentadienyl magnesium bromide. [11] A redox reaction produces iron(II) chloride. The formation of fulvalene, the intended outcome does not occur. [8]

Kealy and Pauson synthesis of ferrocene v2.jpg


The Miller et al. approach to ferrocene Miller Ferrocen Synthese.svg
The Miller et al. approach to ferrocene

Another early synthesis of ferrocene was by Miller et al., [9] who treated metallic iron with gaseous cyclopentadiene at elevated temperature. [31] An approach using iron pentacarbonyl was also reported. [32]

Fe(CO)5 + 2 C5H6 → Fe(C5H5)2 + 5 CO + H2

Via alkali cyclopentadienide

More efficient preparative methods are generally a modification of the original transmetalation sequence using either commercially available sodium cyclopentadienide [33] or freshly cracked cyclopentadiene deprotonated with potassium hydroxide [34] and reacted with anhydrous iron(II) chloride in ethereal solvents.

Modern modifications of Pauson and Kealy's original Grignard approach are known:

Even some amine bases (such as diethylamine) can be used for the deprotonation, though the reaction proceeds more slowly than when using stronger bases: [33]

2 C5H6  +   2 (CH3CH2)2NH   +   FeCl2  →   Fe(C5H5)2  +   2 (CH3CH2)2NH2Cl

Direct transmetalation can also be used to prepare ferrocene from some other metallocenes, such as manganocene: [35]

FeCl2  +   Mn(C5H5)2  →   MnCl2  +   Fe(C5H5)2

Properties

Crystals of ferrocene after purification by vacuum sublimation Ferrocen.jpg
Crystals of ferrocene after purification by vacuum sublimation

Ferrocene is an air-stable orange solid with a camphor-like odor. As expected for a symmetric, uncharged species, ferrocene is soluble in normal organic solvents, such as benzene, but is insoluble in water. It is stable to temperatures as high as 400 °C. [36]

Ferrocene readily sublimes, especially upon heating in a vacuum. Its vapor pressure is about 1 Pa at 25 °C, 10 Pa at 50 °C, 100 Pa at 80 °C, 1000 Pa at 116 °C, and 10,000 Pa (nearly 0.1 atm) at 162 °C. [37] [38]

Reactions

With electrophiles

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.

Important reactions of ferrocene with electrophiles and other reagents FcGen'l.png
Important reactions of ferrocene with electrophiles and other reagents

Ferrocene can itself be oxidized to the ferrocenium cation (Fc+); the ferrocene/ferrocenium couple is often used as a reference in electrochemistry. [12]

It is an aromatic substance and undergoes substitution reactions rather than addition reactions on the cyclopentadienyl ligands. For example, Friedel-Crafts acylation of ferrocene with acetic anhydride yields acetylferrocene [39] just as acylation of benzene yields acetophenone under similar conditions. Vilsmeier-Haack reaction (formylation) using formylanilide and phosphorus oxychloride gives ferrocenecarboxaldehyde. Diformylation does not occur readily, showing the electronic communication between the two rings. [40]

Protonation of ferrocene allows isolation of [Cp2FeH]PF6. [41]

In the presence of aluminium chloride, Me2NPCl2 and ferrocene react to give ferrocenyl dichlorophosphine, [42] whereas treatment with phenyldichlorophosphine under similar conditions forms P,P-diferrocenyl-P-phenyl phosphine. [43]

Ferrocene reacts with P4S10 forms a diferrocenyl-dithiadiphosphetane disulfide. [44]

Lithiation

Ferrocene reacts with butyllithium to give 1,1′-dilithioferrocene, which is a versatile nucleophile. In combination with butyllithiium, tert-butyllithium produces monolithioferrocene. [45]

Redox chemistry

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, [46] however other values have been reported. [47] Ferrocenium tetrafluoroborate is a common reagent. [48] The remarkably reversible oxidation-reduction behaviour has been extensively used to control electron-transfer processes in electrochemical [49] [50] and photochemical [51] [52] systems.

The one-electron oxidized derivative of biferrocene has attracted much research attention. Biferrocene.svg
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. [53] Ferrocene is often used as an internal standard for calibrating redox potentials in non-aqueous electrochemistry.

Stereochemistry of substituted ferrocenes

A planar chiral ferrocene derivative Planar chiral ferrocene derivative.svg
A planar chiral ferrocene derivative

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. [54] Several approaches have been developed to asymmetrically 1,1′-functionalise the ferrocene. [55]

Applications of ferrocene and its derivatives

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).

Ligand scaffolds

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. [56] They are named after the technician who made the first one, Josi Puleo. [57] [58]

Josiphos ligand. Josiphos.png
Josiphos ligand.

Fuel additives

Ferrocene and its derivatives are antiknock agents used in the fuel for petrol engines. They are safer than previously used tetraethyllead. [59] Petrol additive solutions containing ferrocene can be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol. [60] 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. [61]

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. [62]

Pharmaceuticals

Ferrocerone Ferroceron.svg
Ferrocerone

Ferrocene derivatives have been investigated as drugs, [63] with one compound ferrocerone  [ ru ] approved for use in the USSR in the 1970s as an iron supplement, though it is no longer marketed today. [64] Only one drug has entered clinical trials in recent years, Ferroquine (7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine), an antimalarial, [65] [66] [67] which has reached Phase IIb trials. [68] Ferrocene-containing polymer-based drug delivery systems have been investigated. [69]

Ferroquine Ferroquine.png
Ferroquine

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. [70] Some ferrocenium salts exhibit anticancer activity, but no compound has seen evaluation in the clinic. [71] 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. [72] An experimental drug was reported which is a ferrocenyl version of tamoxifen. [73] The idea is that the tamoxifen will bind to the estrogen binding sites, resulting in cytotoxicity. [73] [74]

Ferrocifens are exploited for cancer applications by a French biotech, Feroscan, founded by Pr. Gerard Jaouen.

Solid rocket propellant

Ferrocene and related derivatives are used as powerful burn rate catalysts in ammonium perchlorate composite propellant. [75]

Derivatives and variations

Ferrocene analogues can be prepared with variants of cyclopentadienyl. For example, bisindenyliron and bisfluorenyliron. [58]

Various ferrocene derivatives where cyclopentadienyl has been replaced by related ligands FcVarietyPack.png
Various ferrocene derivatives where cyclopentadienyl has been replaced by related ligands

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. [76] This compound on boiling under reflux in benzene is converted to ferrocene. [77]

Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand, [78] features a cyclopentadienyl anion derivatized with five ferrocene substituents.

Penta(ferrocenyl)cyclopentadienyl ligand Penta(ferrocenyl)cyclopentadienyl.png
Penta(ferrocenyl)cyclopentadienyl ligand
Structure of hexaferrocenylbenzene Hexaferrocenylbenzene-3D-sticks.png
Structure of hexaferrocenylbenzene

In hexaferrocenylbenzene, C6[(η5-C5H4)Fe(η5-C5H5)]6, all six positions on a benzene molecule have ferrocenyl substituents (R). [79] 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: [79]

Hexaferrocenylbenzene.png

The yield is only 4%, which is further evidence consistent with substantial steric crowding around the arene core.

Materials chemistry

Strands of an uncharged ferrocene-substituted polymer are tethered to a hydrophobic silica surface. Oxidation of the ferrocenyl groups produces a hydrophilic surface due to electrostatic attractions between the resulting charges and the polar solvent. Wettability of a silica surface with a bound ferrocene-substituted polymer.jpg
Strands of an uncharged ferrocene-substituted polymer are tethered to a hydrophobic silica surface. Oxidation of the ferrocenyl groups produces a hydrophilic surface due to electrostatic attractions between the resulting charges and the polar solvent.

Ferrocene, a precursor to iron nanoparticles, can be used as a catalyst for the production of carbon nanotubes. [81] Vinylferrocene can be converted to (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. [80] 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. [80] [82]

See also

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5H−
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<span class="mw-page-title-main">Cyclopentadienyl complex</span> Coordination complex of a metal and cyclopentadienyl groups

A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups. Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) bonding mode. The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.

<span class="mw-page-title-main">Nickelocene</span> Chemical compound

Nickelocene is the organonickel compound with the formula Ni(η5-C5H5)2. Also known as bis(cyclopentadienyl)nickel or NiCp2, this bright green paramagnetic solid is of enduring academic interest, although it does not yet have any known practical applications.

<span class="mw-page-title-main">Cobaltocene</span> Chemical compound

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.

<span class="mw-page-title-main">Titanocene dichloride</span> Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

<span class="mw-page-title-main">Hapticity</span> Number of contiguous atoms in a ligand that bond to the central atom in a coordination complex

In coordination chemistry, 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. In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity, 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.

<span class="mw-page-title-main">Chromocene</span> Chemical compound

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In organometallic chemistry, a transition metal indenyl complex is a coordination compound that contains one or more indenyl ligands. The indenyl ligand is formally the anion derived from deprotonation of indene. The η5-indenyl ligand is related to the η5cyclopentadienyl anion (Cp), thus indenyl analogues of many cyclopentadienyl complexes are known. Indenyl ligands lack the 5-fold symmetry of Cp, so they exhibit more complicated geometries. Furthermore, some indenyl complexes also exist with only η3-bonding mode. The η5- and η3-bonding modes sometimes interconvert.

<span class="mw-page-title-main">Sodium cyclopentadienide</span> Chemical compound

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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. 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.

<span class="mw-page-title-main">Ferrocenium tetrafluoroborate</span> Chemical compound

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 ferrocenium 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 dependent on specific electrochemical conditions.

<span class="mw-page-title-main">Rhodocene</span> Organometallic chemical compound

Rhodocene 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 (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.

<span class="mw-page-title-main">Acetylferrocene</span> Chemical compound

Acetylferrocene is the organoiron compound with the formula (C5H5)Fe(C5H4COCH3). 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.

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Decamethylferrocene or bis(pentamethylcyclopentadienyl)iron(II) is a chemical compound with formula Fe(C5(CH3)5)2 or C20H30Fe. It is a sandwich compound, whose molecule has an iron(II) cation Fe2+ attached by coordination bonds between two pentamethylcyclopentadienyl anions (Cp*−, (CH3)5C−5). It can also be viewed as a derivative of ferrocene, with a methyl group replacing each hydrogen atom of its cyclopentadienyl rings. The name and formula are often abbreviated to DmFc, Me10Fc or FeCp*2.

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Vanadocene, bis(η5-cyclopentadienyl) vanadium, is the organometallic compound with the formula V(C5H5)2, commonly abbreviated Cp2V. It is a violet crystalline, paramagnetic solid. Vanadocene has relatively limited practical use, but it has been extensively studied.

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl dimer</span> Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl iodide</span> Chemical compound

Cyclopentadienyliron dicarbonyl iodide is an organoiron compound with the formula (C5H5)Fe(CO)2I. It is a dark brown solid that is soluble in common organic solvents. (C5H5)Fe(CO)2I, or FpI as it is often known, is an intermediate for the preparation of other organoiron compounds such as in ferraboranes.

<span class="mw-page-title-main">Biferrocene</span> Organometallic compound

Biferrocene is the organometallic compound with the formula [(C5H5)Fe(C5H4)]2. It is the product of the formal dehydrocoupling of ferrocene, analogous the relationship between biphenyl and benzene. It is an orange, air-stable solid that is soluble in nonpolar organic solvents.

Germyl, trihydridogermanate(1-), trihydrogermanide, trihydridogermyl or according to IUPAC Red Book: germanide is an anion containing germanium bounded with three hydrogens, with formula GeH−3. Germyl is the IUPAC term for the –GeH3 group. For less electropositive elements the bond can be considered covalent rather than ionic as "germanide" indicates. Germanide is the base for germane when it loses a proton.

A molecular electron-reservoir complex is one of a class of redox-active systems which can store and transfer electrons stoichiometrically or catalytically without decomposition. The concept of electron-reservoir complexes was introduced by the work of French chemist, Didier Astruc. From Astruc's discoveries, a whole family of thermally stable, neutral, 19-electron iron(I) organometallic complexes were isolated and characterized, and found to have applications in redox catalysis and electrocatalysis. The following page is a reflection of the prototypal electron-reservoir complexes discovered by Didier Astruc.

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