Editing Ferrocene

{{Short description|Organometallic compound: Fe(II) sandwiched between two cyclopentadienyl rings}}
{{chembox
| Verifiedfields = changed
| Watchedfields  = changed
| 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<ref>{{cite book |author=[[International Union of Pure and Applied Chemistry]] |date=2014 |title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 |publisher=[[Royal Society of Chemistry|The Royal Society of Chemistry]] |pages=1041 |doi=10.1039/9781849733069 |isbn=978-0-85404-182-4}}</ref>
| OtherNames     = {{ubl|Dicyclopentadienyl iron|Bis(''η''<sup>5</sup>-cyclopentadienyl)iron|Iron(II) cyclopentadienide}}
| Section1       = {{Chembox Identifiers
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 7329
|InChIKey = KTWOOEGAPBSYNW-UHFFFAOYAZ
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/2C5H5.Fe/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = KTWOOEGAPBSYNW-UHFFFAOYSA-N
|CASNo_Ref = {{cascite|correct|CAS}}
|CASNo = 102-54-5
|PubChem = 11985121
|ChEBI_Ref = {{ebicite|changed|EBI}}
|ChEBI = 30672
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = U96PKG90JQ
|SMILES = [CH-]1C=CC=C1.[CH-]1C=CC=C1.[Fe+2]
|Jmol   = None
|InChI = 1/2C5H5.Fe/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
}}
| Section2       = {{Chembox Properties
|Formula = C<sub>10</sub>H<sub>10</sub>Fe
|MolarMass = 186.04 g/mol
|Appearance = light orange powder
|Odor = [[camphor]]-like
|Density = 1.107 g/cm<sup>3</sup> (0 °C)<br>1.490 g/cm<sup>3</sup> (20 °C)<ref>{{cite web|url=http://www.chemicalbook.com/ProductMSDSDetailCB1414721_EN.htm|title=Ferrocene (102-54-5) | work = ChemicalBook |access-date=3 February 2010}}</ref>
|MeltingPtC = 172.5
|MeltingPt_ref = <ref>{{RubberBible86th|page=3.258|name-list-style=vanc}}</ref>
|BoilingPtC = 249
|Solubility = Insoluble in water, soluble in most organic solvents
|LogP =2.04050<ref name="chemsrc">{{Cite web|url=https://www.chemsrc.com/en/cas/102-54-5_311894.html|title=FERROCENE Material safety data sheet | work = ChemSrc }}</ref>
}}
| Section3       = {{Chembox Structure
|PointGroup = D<sub>5h</sub> (eclipsed)<br>D<sub>5d</sub> (staggered)
|MolShape = Sandwich structure with iron centre
}}
| Section4       = {{Chembox Hazards
|NFPA-F = 2 
|NFPA-H = 3
|NFPA-R = 1
|NFPA_ref = <ref>{{cite web|title=Ferrocene MSDS|website=ScienceLab|url=http://www.sciencelab.com/msds.php?msdsId=9924047|access-date=2015-11-25|archive-url=https://web.archive.org/web/20151212034538/http://www.sciencelab.com/msds.php?msdsId=9924047|archive-date=2015-12-12|url-status=dead}}</ref>
|PEL = TWA 15 mg/m<sup>3</sup> (total) TWA 5 mg/m<sup>3</sup> (resp)<ref name=PGCH>{{PGCH|0205}}</ref>
|IDLH = N.D.<ref name=PGCH/>
|REL = TWA 10 mg/m<sup>3</sup> (total) TWA 5 mg/m<sup>3</sup> (resp)<ref name=PGCH/>
}}
| Section5       = {{Chembox Related
|OtherCompounds = {{ubl|[[Metallocene]]|[[f-block metallocene]]|[[Actinocene]]|[[Actinocene#Characterised actinocenes|Thorocene]]|[[Actinocene#Characterised actinocenes|Protactinocene]]|[[Uranocene]]|[[Neptunocene]]|[[Plutonocene]]|[[Zirconocene]]|[[Vanadocene]]|[[Chromocene]]|[[Bis(benzene)chromium]]|[[Manganocene]]|[[Nickelocene]]|[[Cobaltocene]]|[[Ruthenocene]]|[[Rhodocene]]|[[Osmocene]]|[[Hassium#Experimental chemistry|Hassocene]]|[[Stannocene]]|[[Plumbocene]]}}
}}
}}
<!-- |Correct|https://doi.org/10.1016/j.jorganchem.2012.04.009 ?-->

'''Ferrocene''' is an [[organometallic chemistry|organometallic compound]] with the formula {{chem2|Fe(C5H5)2}}. The molecule is a [[Cyclopentadienyl complex|complex]] consisting of two [[Cyclopentadienyl anion|cyclopentadienyl]] rings sandwiching a central [[iron]] atom. It is an orange solid with a camphor-like odor that [[Sublimation (phase transition)|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&nbsp;°C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium [[cation]] {{chem2|Fe(C5H5)2(+)}}.<ref name="werner2012" /> Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and {{chem2|Fc+}} respectively.

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 [[Structural analog|analogues]], known as [[metallocene]]s, 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 [[Role of chance in scientific discoveries|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.<ref name=werner2012/><ref name = Pauson2001 />

The second time was around 1950, when Samuel A. Miller, John A. Tebboth, and John F. Tremaine, researchers at [[BOC (company)|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&nbsp;°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.<ref name=werner2012/><ref name=miller/><ref name=laszloRmon/> Kealy and Pauson were later provided with a sample by Miller ''et al.'', who confirmed that the products were the same compound.<ref name = Pauson2001 />

In 1951, [[Peter L. Pauson]] and [[Thomas J. Kealy]] at [[Duquesne University]] attempted to prepare [[fulvalene]] ({{chem2|(C5H4)2}}) by oxidative dimerization of [[cyclopentadiene]] ({{chem2|C5H6}}). To that end, they reacted the [[Grignard reagent|Grignard]] compound [[cyclopentadienyl magnesium bromide]] in [[diethyl ether]] with [[iron(III) chloride|ferric chloride]] as an oxidizer.<ref name=werner2012/> However, instead of the expected fulvalene, they obtained a light orange powder of "remarkable stability", with the formula {{chem2|C10H10Fe}}.<ref name = Pauson2001 /><ref name=pauson/>

===Determining the structure===
[[File:Ferrocene kealy.svg|thumb|left|Pauson and Kealy's original (incorrect) notion of ferrocene's molecular structure.<ref name=pauson/>]]
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.<ref name=werner2012/> 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.<ref name=laszloRmon/><ref name=federman/>

The structure was deduced and reported independently by three groups in 1952.<ref name=werner2008/> [[Robert Burns Woodward]], [[Geoffrey Wilkinson]], et al. observed that the compound was diamagnetic and nonpolar.<ref name="wilk52" />  A few months later they described its reactions as being typical of aromatic compounds such as [[benzene]].<ref>{{Cite journal |last1=Woodward |first1=R. B. |last2=Rosenblum |first2=M. |last3=Whiting |first3=M. C. |date=1952-06-02 |title=A New Aromatic System |url=https://pubs.acs.org/doi/abs/10.1021/ja01133a543 |journal=Journal of the American Chemical Society |language=en |volume=74 |issue=13 |pages=3458–3459 |doi=10.1021/ja01133a543 |issn=0002-7863}}</ref> The name ferrocene was coined by Mark Whiting, a postdoc with Woodward.<ref>{{Cite web |last=Sutton |first=Mike |date=2023-09-27 |title=Fifty years since the ferrocene furore |url=https://www.chemistryworld.com/features/fifty-years-since-the-ferrocene-furore/4018098.article |access-date=2024-11-02 |website=Chemistry World |language=en}}</ref> [[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.<ref name="fischer" /> Fischer described the structure as ''Doppelkegelstruktur'' ("double-cone structure"), although the term "sandwich" came to be preferred by British and American chemists.<ref name="okuda" /> Philip Frank Eiland and Raymond Pepinsky confirmed the structure through [[X-ray crystallography]] and later by [[Nuclear magnetic resonance|NMR]] spectroscopy.<ref name="laszloRmon" /><ref name="eiland52" /><ref name="dunitz53" /><ref name="dunitz56" />

The "sandwich" structure of ferrocene was shockingly novel and led to intensive theoretical studies. Application of [[molecular orbital theory]] with the assumption of a Fe<sup>2+</sup> centre between two [[cyclopentadienide]] anions {{chem2|C5H5(-)}} resulted in the successful [[Dewar–Chatt–Duncanson model]], allowing correct prediction of the geometry of the molecule as well as explaining its remarkable stability.<ref name=mingos/><ref name=mehr/>

===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 compound]]s".<ref>{{Cite web |url= http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/ |title= The Nobel Prize in Chemistry 1973 |publisher= [[Nobel Foundation]] |access-date= 12 September 2010}}</ref>

==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]]), {{chem2|Fe(2+)[C5H5(-)]2}}.

Each ring has six π-electrons, which makes them [[Aromaticity|aromatic]] according to [[Hückel's rule]]. These π-electrons are then shared with the metal via covalent bonding. Since Fe<sup>2+</sup> has six ''d''-electrons, the complex attains an [[18-electron rule|18-electron]] configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as {{chem2|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&nbsp;Å, and all Fe–C bond distances are 2.04&nbsp;Å. The Cp rings rotate with a low barrier about the Cp<sub>(centroid)</sub>–Fe–Cp<sub>(centroid)</sub> axis, as observed by measurements on substituted derivatives of ferrocene using <sup>1</sup>H and <sup>13</sup>C [[nuclear magnetic resonance]] spectroscopy. For example, methylferrocene (CH<sub>3</sub>C<sub>5</sub>H<sub>4</sub>FeC<sub>5</sub>H<sub>5</sub>) exhibits a singlet for the C<sub>5</sub>H<sub>5</sub> ring.<ref>{{cite journal | vauthors = Abel EW, Long NJ, Orrell KG, Osborne AG, Šik V |title = Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes |journal = [[Journal of Organometallic Chemistry|J. Org. Chem.]] |year = 1991 |volume = 403 |issue=1–2 |pages = 195–208 |doi = 10.1016/0022-328X(91)83100-I}}</ref>

From room temperature down to 164&nbsp;K, [[X-ray crystallography]] yields the monoclinic space group; the cyclopentadienide rings are a staggered conformation, resulting in a centrosymmetric molecule, with [[symmetry group]] D<sub>5d</sub>.<ref name=eiland52/> However, below 110&nbsp;K, ferrocene crystallizes in an orthorhombic crystal lattice in which the Cp rings are ordered and eclipsed, so that the molecule has symmetry group D<sub>5h</sub>.<ref name=seiler/> In the gas phase, [[electron diffraction]]<ref name=haal68/> and computational studies<ref name=coriani/> show that the Cp rings are eclipsed. While ferrocene has no permanent dipole moment at room temperature, between 172.8 and 163.5&nbsp;K the molecule exhibits an "incommensurate modulation", breaking the D<sub>5</sub> symmetry and acquiring an electric dipole.<ref>{{Cite journal |last1=Katrusiak |first1=Andrzej |last2=Rusek |first2=Michalina |last3=Dušek |first3=Michal |last4=Petříček |first4=Václav |last5=Szafrański |first5=Marek |date=2023-04-06 |title=Dipole-Moment Modulation in New Incommensurate Ferrocene |journal=The Journal of Physical Chemistry Letters |language=en |volume=14 |issue=13 |pages=3111–3119 |doi=10.1021/acs.jpclett.3c00215 |issn=1948-7185 |pmc=10084461 |pmid=36951481}}</ref>

In solution, eclipsed D<sub>5h</sub> ferrocene was determined to dominate over the staggered D<sub>5d</sub> conformer, as suggested by both [[Fourier-transform infrared spectroscopy]] and [[Density functional theory|DFT]] calculations.<ref>{{Cite journal |last1=Wang |first1=Feng |last2=Mohammadi |first2=Narges |last3=Best |first3=Stephen P. |last4=Appadoo |first4=Dominique |last5=Chantler |first5=Christopher T. |title=Dominance of eclipsed ferrocene conformer in solutions revealed by the IR spectra between 400 and 500 cm-1 |url=https://linkinghub.elsevier.com/retrieve/pii/S0969806X21002401 |journal=Radiation Physics and Chemistry |year=2021 |language=en |volume=188 |pages=109590 |doi=10.1016/j.radphyschem.2021.109590|bibcode=2021RaPC..18809590W}}</ref>

==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.<ref name=pauson/> A [[redox|redox reaction]] produces iron(II) chloride. The formation of fulvalene (the intended outcome), does not occur.<ref name = Pauson2001>{{cite journal|title = Ferrocene&mdash;how it all began| vauthors = Pauson PL |author-link = Peter Pauson|journal = [[Journal of Organometallic Chemistry]]|volume = 637–639|year = 2001|pages = 3–6|doi = 10.1016/S0022-328X(01)01126-3}}</ref>
:[[File:Kealy and Pauson synthesis of ferrocene v2.jpg|600px]]

[[File:Miller Ferrocen Synthese.svg|thumb|300px|The Miller ''et al.''<ref name=miller/> approach to ferrocene|left]]
Another early synthesis of ferrocene was by Miller ''et al.'',<ref name=miller/> who treated metallic iron with [[gas]]eous cyclopentadiene at elevated temperature.<ref>{{cite journal | vauthors = Wilkinson G, Pauson PL, Cotton FA |author-link1=Geoffrey Wilkinson |author-link3=F. Albert Cotton |doi=10.1021/ja01636a080|title=Bis-cyclopentadienyl Compounds of Nickel and Cobalt|year=1954|journal=[[J. Am. Chem. Soc.]]|volume=76|pages=1970–1974|issue=7}}</ref> An approach using [[iron pentacarbonyl]] was also reported.<ref>{{cite book | vauthors = Wilkinson G, Cotton FA |author-link1=Geoffrey Wilkinson |author-link2=F. Albert Cotton |doi=10.1002/9780470166024.ch1|year=1959 |chapter=Cyclopentadienyl and Arene Metal Compounds|title=Progress in Inorganic Chemistry|volume=1|pages=1–124|isbn=978-0-470-16602-4}}</ref>

:Fe(CO)<sub>5</sub> + 2&nbsp;C<sub>5</sub>H<sub>6</sub> → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 5&nbsp;CO + H<sub>2</sub>

===Via alkali cyclopentadienide===
More efficient preparative methods are generally a modification of the original [[transmetalation]] sequence using either commercially available [[sodium cyclopentadienide]]<ref name = orgsyn /> or freshly [[dicyclopentadiene|cracked]] cyclopentadiene deprotonated with [[potassium hydroxide]]<ref>{{cite book| vauthors = Jolly WL |title= The Synthesis and Characterization of Inorganic Compounds |url=https://archive.org/details/synthesischaract0000joll|url-access=registration|publisher=Prentice-Hall |location=New Jersey |date=1970|isbn= 9780138799328}}</ref> and reacted with anhydrous iron(II) chloride in ethereal solvents.

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

*Using sodium cyclopentadienide: &nbsp; &nbsp; &nbsp; 2&nbsp;NaC<sub>5</sub>H<sub>5</sub> &nbsp; + &nbsp; FeCl<sub>2</sub> &nbsp; → &nbsp; Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> &nbsp; + &nbsp; 2&nbsp;NaCl
*Using freshly-cracked cyclopentadiene: &nbsp; &nbsp; FeCl<sub>2</sub>·4H<sub>2</sub>O &nbsp; + &nbsp; 2&nbsp;C<sub>5</sub>H<sub>6</sub> &nbsp; + &nbsp; 2&nbsp;KOH &nbsp; → &nbsp; Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> &nbsp; + &nbsp; 2&nbsp;KCl &nbsp; + &nbsp; 6&nbsp;H<sub>2</sub>O
*Using an iron(II) salt with a Grignard reagent: &nbsp; &nbsp; 2&nbsp;C<sub>5</sub>H<sub>5</sub>MgBr &nbsp; + &nbsp; FeCl<sub>2</sub> &nbsp; → &nbsp; Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> &nbsp; + &nbsp; 2&nbsp;MgBrCl

Even some [[amine]] bases (such as [[diethylamine]]) can be used for the deprotonation, though the reaction proceeds more slowly than when using stronger bases:<ref name=orgsyn>{{OrgSynth|title = Ferrocene|authorlink = Geoffrey Wilkinson| vauthors = Wilkinson G |volume = 36|page = 31|year = 1956|doi = 10.15227/orgsyn.036.0031}}</ref>

:2&nbsp;C<sub>5</sub>H<sub>6</sub> &nbsp; + &nbsp; 2&nbsp;(CH<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>NH &nbsp; + &nbsp; FeCl<sub>2</sub> &nbsp; → &nbsp; Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> &nbsp; + &nbsp; 2&nbsp;(CH<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>NH<sub>2</sub>Cl

Direct transmetalation can also be used to prepare ferrocene from some other metallocenes, such as [[manganocene]]:<ref>{{cite journal | vauthors = Wilkinson G, Cotton FA, Birmingham JM |author-link1= Geoffrey Wilkinson |year= 1956|title= On manganese cyclopentadienide and some chemical reactions of neutral bis-cyclopentadienyl metal compounds|journal= [[J. Inorg. Nucl. Chem.]]|volume= 2|issue= 2|pages= 95–113|doi=10.1016/0022-1902(56)80004-3}}</ref>
:FeCl<sub>2</sub> &nbsp; + &nbsp; Mn(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> &nbsp; → &nbsp; MnCl<sub>2</sub> &nbsp; + &nbsp; Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>

==Properties==
[[File:Ferrocen.jpg|thumb|right|Crystals of ferrocene after purification by vacuum sublimation]]
Ferrocene is an [[air]]-stable orange solid with a camphor-like odor.<ref name=SC06/>    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&nbsp;°C.<ref name=SC06>{{cite book| vauthors = Solomons G, Craig F |title=Organic Chemistry |edition=9th |location=USA |publisher=John Wiley & Sons|date=2006}}</ref>

Ferrocene readily [[Sublimation (phase transition)|sublimes]], especially upon heating in a vacuum. Its vapor pressure is about 1 [[Pascal (unit)|Pa]] at 25&nbsp;°C, 10 Pa at 50&nbsp;°C, 100 Pa at 80&nbsp;°C, 1000 Pa at 116&nbsp;°C, and 10,000 Pa (nearly 0.1 [[atmosphere (unit)|atm]]) at 162&nbsp;°C.<ref>{{cite journal | vauthors = Monte MJ, Santos LM, Fulem M, Fonseca JM, Sousa CA |doi=10.1021/je050502y|title=New Static Apparatus and Vapor Pressure of Reference Materials: Naphthalene, Benzoic Acid, Benzophenone, and Ferrocene|year=2006 |journal=Journal of Chemical & Engineering Data |volume=51|page=757|issue=2}}</ref><ref name=fulem2013>{{cite journal | vauthors = Fulem M, Růžička K, Červinka C, Rocha MA, Santos LM, Berg RF | year = 2013 | title = Recommended vapor pressure and thermophysical data for ferrocene | journal = Journal of Chemical Thermodynamics | volume = 57 | pages = 530–540 | doi = 10.1016/j.jct.2012.07.023 }}</ref>

==Reactions==
[[Image:FcGen'l.png|upright=1.5|thumb|left|Important ferrocene reactions|alt=Formylation, acylation, mercuration, oxidation]]

===Aromatic substitution===
Ferrocene is an [[aromaticity|aromatic substance]].  [[Electrophiles]] typically [[substitution reaction|substitute onto]], rather than [[addition reaction|add to]], the cyclopentadienyl ligands. For example, a common undergraduate experiment performs [[Friedel-Crafts reaction#Friedel-Crafts acylation|Friedel-Crafts acylation]] with [[acetic anhydride]] and a [[phosphoric acid]] catalyst.  Just as this reagent mixture converts [[benzene]] to [[acetophenone]], it converts ferrocene to [[acetylferrocene]].<ref>{{cite journal| vauthors = Bozak RE |title = Acetylation of Ferrocene: A Chromatography Experiment for Elementary Organic Laboratory|journal = [[J. Chem. Educ.]]|year = 1966|volume = 43|issue = 2|page = 73|doi = 10.1021/ed043p73|bibcode = 1966JChEd..43...73B}}</ref> 

In the presence of [[aluminium chloride]], Me<sub>2</sub>NPCl<sub>2</sub> and ferrocene react to give ferrocenyl dichlorophosphine,<ref>{{cite journal |title = Ferrocene derivatives. 27. Ferrocenyldimethylphosphine | vauthors = Knox GR, Pauson PL, Willison D |author-link2=Peter Pauson |journal = Organometallics |volume = 11 |issue = 8 |pages = 2930–2933 |year = 1992 |doi = 10.1021/om00044a038 }}</ref> whereas treatment with [[dichlorophenylphosphine|phenyldichlorophosphine]] under similar conditions forms ''P'',''P''-diferrocenyl-''P''-phenyl phosphine.<ref>{{cite journal | vauthors = Sollott GP, Mertwoy HE, Portnoy S, Snead JL |title = Unsymmetrical Tertiary Phosphines of Ferrocene by Friedel–Crafts Reactions. I. Ferrocenylphenylphosphines |journal = [[J. Org. Chem.]] |year = 1963 |volume = 28 |pages = 1090–1092 |doi = 10.1021/jo01039a055 |issue = 4 }}</ref>  [[Vilsmeier-Haack reaction|Vilsmeier-Haack formylation]] using [[formanilide|formylanilide]] and [[phosphorus oxychloride]] gives [[ferrocenecarboxaldehyde]].<ref name=Rausch/>  

Unsubstituted ferrocene undergoes aromatic substitution more easily than benzene, because electrophiles can attack the metal ion before rearranging to the [[Wheland intermediate]].<ref name=GE1287>{{Greenwood&Earnshaw1st|p=1287}}</ref>  Thus ferrocene reacts with the weak electrophile [[Phosphorus pentasulfide|P<sub>4</sub>S<sub>10</sub>]] to form a diferrocenyl-dithiadiphosphetane disulfide.<ref>{{cite journal |title = 2,4-Diferrocenyl-1,3-dithiadiphosphetane 2,4-disulfide; structure and reactions with catechols and [PtCl<sub>2</sub>(PR<sub>3</sub>)<sub>2</sub>](R = Et or Bun) | vauthors = ((St J Foreman MR)), Alexandra MZ, Slawin AM, Woollins JD |journal = [[Dalton Transactions|J. Chem. Soc., Dalton Trans.]] |issue = 18 |year = 1996 |pages = 3653–3657 |doi = 10.1039/DT9960003653}}</ref> [[Mannich reaction|Mannich conditions]] suffice to iminylate ferrocene unto [[N,N-Dimethylaminomethylferrocene|N,N-dimethylaminomethylferrocene]].{{cn|date=May 2025}}  

Superacidic protonation does not complete aromatic substitution, but rather traps the unrearranged [[bent metallocene|bent]] intermediate [[hydrido]] salt, [Cp<sub>2</sub>FeH]PF<sub>6</sub>.<ref>{{cite journal | vauthors = Malischewski M, Seppelt K, Sutter J, Heinemann FW, Dittrich B, Meyer K | title = Protonation of Ferrocene: A Low-Temperature X-ray Diffraction Study of [Cp<sub>2</sub> FeH](PF<sub>6</sub> ) Reveals an Iron-Bound Hydrido Ligand | journal = Angewandte Chemie | volume = 56 | issue = 43 | pages = 13372–13376 | date = October 2017 | pmid = 28834022 | doi = 10.1002/anie.201704854}}</ref>  Strongly oxidizing electrophiles, such as [[halogen]]s and [[nitric acid]], neither rearrange to a Wheland intermediate nor coordinate to iron, instead generating ferrocenium salts (see {{slink||Redox chemistry}}).<ref name=GE1287/>  

In accordance with [[Jemmis mno rules|cluster compound theory]], ferrocene's rings behave as a single delocalized &pi; system.  Electronic perturbations to one ring propagate to the other.   For example, introduction of a [[Electrophilic aromatic directing groups|deactivating]] aldehyde group on one ring inhibits formylation of the other ring as well.<ref name=Rausch>{{cite journal |doi=10.1139/v63-182 |title=Metallocene Chemistry—A Decade of Progress |date=1963 |last1=Rausch |first1=M. D. |journal=Canadian Journal of Chemistry |volume=41 |issue=5 |pages=1289–1314 }}</ref>

===Metallation===
Ferrocene readily metallates.  Ferrocene reacts with [[butyllithium]] to give [[1,1'-Dilithioferrocene|1,1′-dilithioferrocene]], which is a versatile [[nucleophile]]. In combination with butyllithiium, [[Tert-Butyllithium|''tert''-butyllithium]] produces monolithioferrocene.<ref>{{cite journal |author=Busacca |first1=Carl A. |last2=Eriksson |first2=Magnus C. |last3=Haddad |first3=Nizar |last4=Han |first4=Z. Steve |last5=Lorenz |first5=Jon C. |last6=Qu |first6=Bo |last7=Zeng |first7=Xingzhong |last8=Senanayake |first8=Chris H. |year=2013 |title=Practical Synthesis of Di-tert-Butyl-Phosphinoferrocene |journal=Organic Syntheses |volume=90 |page=316 |doi=10.15227/orgsyn.090.0316 |doi-access=free}}</ref>  Likewise ferrocene [[mercuration|mercurates]] to give ferrocendiyl dimercuriacetate.<ref name=GE1288>{{Greenwood&Earnshaw1st|p=1288}}</ref>  

Further reaction gives the [[nitro compound|nitro]], halo-, and borono derivatives.<ref name=GE1288/>

=== Redox chemistry ===
[[File:Biferrocene.svg|thumb|left|132px|The one-electron oxidized derivative of [[biferrocene]] has attracted much research attention.]]
Ferrocene undergoes a one-electron oxidation at around 0.4&nbsp;V versus a [[saturated calomel electrode]] (SCE), becoming '''ferrocenium'''.<ref name=federman/> This reversible oxidation has been used as standard in electrochemistry as Fc<sup>+</sup>/Fc&nbsp;= 0.64&nbsp;V versus the [[standard hydrogen electrode]],<ref>{{cite journal | vauthors = Cardona CM, Li W, Kaifer AE, Stockdale D, Bazan GC | title = Electrochemical considerations for determining absolute frontier orbital energy levels of conjugated polymers for solar cell applications | journal = Advanced Materials | volume = 23 | issue = 20 | pages = 2367–2371 | date = May 2011 | pmid = 21462372 | doi = 10.1002/adma.201004554 | bibcode = 2011AdM....23.2367C | s2cid = 40766788 }}</ref> however other values have been reported.<ref>{{citation|surname1=Vitaly V Pavlishchuk, Anthony W Addison|periodical=Inorganica Chimica Acta|title=Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C|volume=298|issue=1|at=pp.&nbsp;97–102|date=January 2000 |language=German |doi=10.1016/S0020-1693(99)00407-7|url=https://linkinghub.elsevier.com/retrieve/pii/S0020169399004077|access-date=2022-07-26
}}</ref><ref>{{cite journal | vauthors = Mygind JB, Deissler NH, Li S, Fu X, Kibsgaard J, Chorkendorff I | title = Hydrogen Oxidation Beyond Water: In Search of Proton Mediation Pathways | journal = ACS Electrochemistry | volume = X | issue = X | pages = X-X | date = 1 March 2025 | publisher = American Chemical Society | doi = 10.1021/acselectrochem.5c00009 | url = https://doi.org/10.1021/acselectrochem.5c00009 }}</ref> [[Ferrocenium tetrafluoroborate]] is a common reagent.<ref>{{cite journal | vauthors = Connelly NG, Geiger WE | title = Chemical Redox Agents for Organometallic Chemistry | journal = Chemical Reviews | volume = 96 | issue = 2 | pages = 877–910 | date = March 1996 | pmid = 11848774 | doi = 10.1021/cr940053x }}</ref> The remarkably reversible oxidation-reduction behaviour has been extensively used to control electron-transfer processes in electrochemical<ref>{{cite journal | vauthors = Sirbu D, Turta C, Gibson EA, Benniston AC | title = The ferrocene effect: enhanced electrocatalytic hydrogen production using meso-tetraferrocenyl porphyrin palladium(II) and copper(II) complexes | journal = Dalton Transactions | volume = 44 | issue = 33 | pages = 14646–14655 | date = September 2015 | pmid = 26213204 | doi = 10.1039/C5DT02191J | url = https://eprint.ncl.ac.uk/fulltext.aspx?url=214473/2DD72927-25BC-4809-8636-5FDB014D7CB0.pdf&pub_id=214473 }}</ref><ref>{{cite journal | vauthors = Lennox AJ, Nutting JE, Stahl SS | title = Selective electrochemical generation of benzylic radicals enabled by ferrocene-based electron-transfer mediators | journal = Chemical Science | volume = 9 | issue = 2 | pages = 356–361 | date = January 2018 | pmid = 29732109 | pmc = 5909123 | doi = 10.1039/C7SC04032F | doi-access = free }}</ref> and photochemical<ref>{{Cite journal| vauthors = Dannenberg JJ, Richards JH |date=1965-04-01|title=Photosensitization by Ferrocene. Photochemistry of Higher Electronic Excited States|journal=Journal of the American Chemical Society|volume=87|issue=7|pages=1626–1627|doi=10.1021/ja01085a048|issn=0002-7863}}</ref><ref>{{Cite journal| vauthors = Sirbu D, Turta C, Benniston AC, Abou-Chahine F, Lemmetyinen H, Tkachenko NV, Wood C, Gibson E | display-authors = 6 |date=2014-05-23|title=Synthesis and properties of a meso- tris–ferrocene appended zinc(II) porphyrin and a critical evaluation of its dye sensitised solar cell (DSSC) performance |journal=RSC Advances|language=en|volume=4|issue=43|pages=22733–22742|doi=10.1039/C4RA03105A|bibcode=2014RSCAd...422733S |issn=2046-2069| url = https://eprint.ncl.ac.uk/fulltext.aspx?url=199647/CED25CA5-111E-4500-8188-DCFC5D4D7190.pdf&pub_id=199647}}</ref> systems. 

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 [[Cathode|cathodic]] direction (more negative). Thus, [[decamethylferrocene]] is much more easily oxidised than ferrocene and can even be oxidised to the corresponding dication.<ref>{{cite journal | vauthors = Malischewski M, Adelhardt M, Sutter J, Meyer K, Seppelt K | title = Isolation and structural and electronic characterization of salts of the decamethylferrocene dication | journal = Science | volume = 353 | issue = 6300 | pages = 678–682 | date = August 2016 | pmid = 27516596 | doi = 10.1126/science.aaf6362 | s2cid = 43385610 | bibcode = 2016Sci...353..678M }}</ref> Ferrocene is often used as an [[internal standard]] for calibrating redox potentials in non-aqueous [[electrochemistry]].

==Stereochemistry of substituted ferrocenes==
[[Image:Planar chiral ferrocene derivative.svg|thumb|right|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&nbsp;– for example [CpFe(EtC<sub>5</sub>H<sub>3</sub>Me)]. This [[planar chirality]] arises despite no single atom being a [[stereocenter|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 [[Alcohol (chemistry)|alcohol]]s.<ref>{{cite journal | vauthors = Ruble JC, Latham HA, Fu GC |year= 1997|title= Effective Kinetic Resolution of Secondary Alcohols with a Planar-Chiral Analogue of 4-(dimethylamino)pyridine. Use of the Fe(C<sub>5</sub>Ph<sub>5</sub>) Group in Asymmetric Catalysis|journal= [[J. Am. Chem. Soc.]]|volume= 119|issue= 6|pages= 1492–1493|doi= 10.1021/ja963835b}}</ref>
Several approaches have been developed to asymmetrically 1,1′-functionalise the ferrocene.<ref>{{cite journal | vauthors = Atkinson RC, Gibson VC, Long NJ | title = The syntheses and catalytic applications of unsymmetrical ferrocene ligands | journal = Chemical Society Reviews | volume = 33 | issue = 5 | pages = 313–328 | date = June 2004 | pmid = 15272371 | doi = 10.1039/B316819K }}</ref>

==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 drug|pharmaceutical candidate]]s), [[robustness]] ([[anti-knock|anti-knock formulation]]s, [[precursor (chemistry)|precursor]]s to materials), and redox (reagents and redox standards).

===Ligand scaffolds===
Chiral ferrocenyl [[phosphine]]s 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 [[diphosphines|diphosphine]] [[1,1'-bis(diphenylphosphino)ferrocene|1,1′-bis(diphenylphosphino)ferrocene]] (dppf) is a valued ligand for [[palladium]]-[[coupling reaction]]s and [[Josiphos ligand]] is useful for hydrogenation catalysis.<ref name="H-U. Blaser 2002">{{cite journal | title = Solvias Josiphos ligands: From discovery to technical applications | doi = 10.1023/a:1013832630565 | volume = 19 | year = 2002 | journal = Topics in Catalysis | pages = 3–16 | vauthors = Blaser HU | s2cid = 95738043 }}</ref> They are named after the technician who made the first one, Josi Puleo.<ref>{{cite book | vauthors = Zhou QL |title=Privileged Chiral Ligands and Catalysts |date=2011 |publisher=Wiley-VCH Verlag |location=Weinheim |isbn=978-3-527-63521-4}}</ref><ref name=Stepnicka>{{cite book| vauthors = Stepnicka P |title=Ferrocenes: Ligands, Materials and Biomolecules|publisher=J. Wiley |location=Hoboken, NJ|date=2008 |isbn=978-0-470-03585-6}}</ref>

[[File:Josiphos.png|thumb|Josiphos ligand.<ref name="H-U. Blaser 2002"/>]]

===Fuel additives===
Ferrocene and its derivatives are [[antiknock agent]]s used in the fuel for [[petrol engine]]s. They are safer than previously used [[tetraethyllead]].<ref>{{cite conference | vauthors = Bennett J | conference = International Conference on Automotive Technology | location = Istanbul | date = 26 November 2004 |url=http://www.osd.org.tr/14.pdf |title=Application of fuel additives |url-status=dead |archive-url=https://web.archive.org/web/20060505193757/http://www.osd.org.tr/14.pdf |archive-date=2006-05-05 }}</ref> Petrol additive solutions containing ferrocene can be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol.<ref>{{cite patent|country=US|number=4104036|title=Iron-containing motor fuel compositions and method for using same |inventor = Chao TS | assign1 = Atlantic Richfield Co |pubdate= 1978-08-01}}</ref> 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.<ref>{{cite patent | inventor = Dewey FM | assign1 = U.S. Air Force | title = Ferrocene Polyglycols | country = US | number = 3598850 | fdate = June 11, 1969 | gdate = Aug. 10, 1971 | url = https://patentimages.storage.googleapis.com/6f/2a/1c/dad6147ea46bcb/US3598850.pdf | postscript = .}}</ref>

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.<ref>{{cite patent | inventor = Kerley RV | assign1 = Ethyl Corporation | title = Coal Combustion Process and Composition | country = US | number = 3927992 | fdate = November 23, 1971 | gdate = December 23, 1975 | url = https://patentimages.storage.googleapis.com/0d/03/57/c94e635d15e1fb/US3927992.pdf | postscript = . }}</ref>

===Pharmaceuticals===
[[File:Ferroceron.svg|thumb|Ferrocerone]]
Ferrocene derivatives have been investigated as drugs,<ref>{{cite journal | vauthors = van Staveren DR, Metzler-Nolte N | title = Bioorganometallic chemistry of ferrocene | journal = Chemical Reviews | volume = 104 | issue = 12 | pages = 5931–5985 | date = December 2004 | pmid = 15584693 | doi = 10.1021/cr0101510 }}</ref> with one compound {{ill|ferrocerone|ru|Ферроцерон}} approved for use in the USSR in the 1970s as an [[iron supplement]], though it is no longer marketed today.<ref>{{cite journal | vauthors = Ong YC, Gasser G | title = Organometallic compounds in drug discovery: Past, present and future | journal = Drug Discovery Today: Technologies | volume = 37 | pages = 117–124 | date = December 2020 | pmid = 34895650 | doi = 10.1016/j.ddtec.2019.06.001 | s2cid = 198268304 | url = https://hal.archives-ouvertes.fr/hal-02169801/file/Organometallic%20Compounds%20in%20Drug%20Discovery%20Past%2C%20Present%20and%20futur.pdf }}</ref> Only one drug has entered clinical trials in recent years, [[Ferroquine]] (7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine), an [[antimalarial]],<ref name="BiotNosten2011">{{cite journal | vauthors = Biot C, Nosten F, Fraisse L, Ter-Minassian D, Khalife J, Dive D | title = The antimalarial ferroquine: from bench to clinic | journal = Parasite | volume = 18 | issue = 3 | pages = 207–214 | date = August 2011 | pmid = 21894260 | pmc = 3671469 | doi = 10.1051/parasite/2011183207 }} {{open access}}</ref><ref>{{cite journal | vauthors = Roux C, Biot C | title = Ferrocene-based antimalarials | journal = Future Medicinal Chemistry | volume = 4 | issue = 6 | pages = 783–797 | date = April 2012 | pmid = 22530641 | doi = 10.4155/fmc.12.26 }}</ref><ref>{{cite journal | vauthors = Wani WA, Jameel E, Baig U, Mumtazuddin S, Hun LT | title = Ferroquine and its derivatives: new generation of antimalarial agents | journal = European Journal of Medicinal Chemistry | volume = 101 | pages = 534–551 | date = August 2015 | pmid = 26188909 | doi = 10.1016/j.ejmech.2015.07.009 | pmc = 7115395 }}</ref> which has reached Phase IIb trials.<ref>{{cite journal | vauthors = Adoke Y, Zoleko-Manego R, Ouoba S, Tiono AB, Kaguthi G, Bonzela JE, Duong TT, Nahum A, Bouyou-Akotet M, Ogutu B, Ouedraogo A, Macintyre F, Jessel A, Laurijssens B, Cherkaoui-Rbati MH, Cantalloube C, Marrast AC, Bejuit R, White D, Wells TN, Wartha F, Leroy D, Kibuuka A, Mombo-Ngoma G, Ouattara D, Mugenya I, Phuc BQ, Bohissou F, Mawili-Mboumba DP, Olewe F, Soulama I, Tinto H | display-authors = 6 | title = A randomized, double-blind, phase 2b study to investigate the efficacy, safety, tolerability and pharmacokinetics of a single-dose regimen of ferroquine with artefenomel in adults and children with uncomplicated Plasmodium falciparum malaria | journal = Malaria Journal | volume = 20 | issue = 1 | pages = 222 | date = May 2021 | pmid = 34011358 | doi = 10.1186/s12936-021-03749-4 | pmc = 8135182 | doi-access = free }}</ref> Ferrocene-containing polymer-based drug delivery systems have been investigated.<ref>{{Cite journal| vauthors = Gu H, Mu S, Qiu G, Liu X, Zhang L, Yuan Y, Astruc D |date=June 2018|title=Redox-stimuli-responsive drug delivery systems with supramolecular ferrocenyl-containing polymers for controlled release|journal=Coordination Chemistry Reviews|volume=364|pages=51–85|doi=10.1016/j.ccr.2018.03.013|s2cid=103022297 |issn=0010-8545}}</ref>

[[Image:Ferroquine.png|thumb|220 px|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]].<ref name=":0">{{Cite journal| vauthors = Ornelas C |s2cid=56521492|title=Application of ferrocene and its derivatives in cancer research|journal=New Journal of Chemistry|volume=35|issue=10|pages=1973|doi=10.1039/c1nj20172g|year=2011}}</ref> Some ferrocenium salts exhibit anticancer activity, but no compound has seen evaluation in the clinic.<ref name=Babin>{{cite journal | vauthors = Babin VN, Belousov YA, Borisov VI, Gumenyuk VV, Nekrasov YS, Ostrovskaya LA, Sviridova IK, Sergeeva NS, Simenel AA, Snegur LV | display-authors = 6 | year = 2014 | title = Ferrocenes as potential anticancer drugs. Facts and hypotheses | journal = Russ. Chem. Bull. | volume = 63 | issue = 11| pages = 2405–2422 | doi = 10.1007/s11172-014-0756-7 | s2cid = 94618726 }}</ref> 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.<ref>{{cite patent | inventor = Yong J, Lu C | assign1 = Xiamen Institute of Rare Earth Materials | title = Ferrocene Derivative, Preparation Method and Use Thereof. | country = US | number = 9738673 | fdate = November 29, 2016 | gdate = August 22, 2017 | url = https://patentimages.storage.googleapis.com/dd/6e/d6/9fd8e3c5c96b67/US9738673.pdf | postscript = . }}</ref> An experimental drug was reported which is a ferrocenyl version of [[tamoxifen]].<ref name = top2003 /> The idea is that the tamoxifen will bind to the [[estrogen]] binding sites, resulting in cytotoxicity.<ref name=top2003>{{cite journal | vauthors = Top S, Vessières A, Leclercq G, Quivy J, Tang J, Vaissermann J, Huché M, Jaouen G | display-authors = 6 | title = Synthesis, biochemical properties and molecular modelling studies of organometallic specific estrogen receptor modulators (SERMs), the ferrocifens and hydroxyferrocifens: evidence for an antiproliferative effect of hydroxyferrocifens on both hormone-dependent and hormone-independent breast cancer cell lines | journal = Chemistry: A European Journal | volume = 9 | issue = 21 | pages = 5223–5236 | date = November 2003 | pmid = 14613131 | doi = 10.1002/chem.200305024 }}</ref><ref>{{cite journal|journal=[[Chemical and Engineering News]]|date=16 September 2002| title= The Bio Side of Organometallics| vauthors = Dagani R | volume = 80| issue= 37| pages = 23–29| url=http://pubs.acs.org/cen/science/8037/8037sci1.html|doi=10.1021/cen-v080n037.p023}}</ref>

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]].<ref>{{Cite web|url=https://www.rocketmotorparts.com/index.aspx?pageid=1577809&prodid=15759228|title=Ferrocene Burn Rate Catalyst|website=www.rocketmotorparts.com|access-date=2020-01-13}}</ref>

==Derivatives and variations==
Ferrocene analogues can be prepared with variants of cyclopentadienyl. For example, bis[[indene|indenyliron]] and bisfluorenyliron.<ref name=Stepnicka/>

[[Image:FcVarietyPack.png|400px|center|Various ferrocene derivatives where cyclopentadienyl has been replaced by related ligands]]

Carbon atoms can be replaced by heteroatoms as illustrated by Fe(''η''<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)(''η''<sup>5</sup>-P<sub>5</sub>) and Fe(''η''<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(''η''<sup>5</sup>-C<sub>4</sub>H<sub>4</sub>N) ("[[azaferrocene]]"). Azaferrocene arises from decarbonylation of Fe(''η''<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(CO)<sub>2</sub>(''η''<sup>1</sup>-pyrrole) in [[cyclohexane]].<ref>{{cite journal|doi=10.1016/0022-328X(90)85359-7|title=An improved photochemical synthesis of azaferrocene|year=1990| vauthors = Zakrzewski J, Giannotti C |journal=[[Journal of Organometallic Chemistry|J. Organomet. Chem.]]|volume=388|issue=1–2|pages=175–179}}</ref> This compound on boiling under [[reflux]] in [[benzene]] is converted to ferrocene.<ref>{{cite journal|doi=10.1021/ic00133a006|title=Chemistry of some ''η''<sup>5</sup>-pyrrolyl- and ''η''<sup>1</sup>-''N''-pyrrolyliron complexes |year=1982 | vauthors = Efraty A, Jubran N, Goldman A |journal=Inorg. Chem.|volume=21|pages=868–873|issue=3}}</ref>

The bis(benzene)iron(II) cation, isoelectronic with [[bis(benzene)chromium]], is unstable against nucleophilic attack, and decomposes "instantaneously" in [[acetonitrile]]. It can be observed, however, in metastable [[nitromethane]] solution.<ref>{{cite journal|doi=10.1016/0022-328x(89)88096-9|journal=Journal of Organometallic Chemistry|volume=359|year=1989|p=333|publisher=Elsevier Seqouia|location=Netherlands|title=A spectroscopic study of bis(&eta;<sup>6</sup>-arene)iron(II) salts|first1=S.|last1=Abdul-Rahman|first2=A.|last2=Houlton|first3=R.&nbsp;M.&nbsp;G.|last3=Roberts|first4=J.|last4=Silver|orig-date=27 May 1988}}</ref>  

Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand,<ref>{{cite journal | vauthors = Yu Y, Bond AD, Leonard PW, Vollhardt KP, Whitener GD | title = Syntheses, structures, and reactivity of radial oligocyclopentadienyl metal complexes: penta(ferrocenyl)cyclopentadienyl and congeners | journal = Angewandte Chemie | volume = 45 | issue = 11 | pages = 1794–1799 | date = March 2006 | pmid = 16470902 | doi = 10.1002/anie.200504047 }}</ref> features a cyclopentadienyl anion derivatized with five ferrocene substituents.
[[Image:Penta(ferrocenyl)cyclopentadienyl.png|500px|center|Penta(ferrocenyl)cyclopentadienyl ligand]]

[[Image:Hexaferrocenylbenzene-3D-sticks.png|200px|thumb|right|Structure of hexaferrocenylbenzene]]

In '''hexaferrocenylbenzene''', C<sub>6</sub>[(''η''<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)Fe(''η''<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)]<sub>6</sub>, all six positions on a [[benzene]] molecule have ferrocenyl substituents ('''R''').<ref name = "hexaferrocenylbenzene">{{cite journal | vauthors = Yu Y, Bond AD, Leonard PW, Lorenz UJ, Timofeeva TV, Vollhardt KP, Whitener GD, Yakovenko AA | display-authors = 6 | title = Hexaferrocenylbenzene | journal = Chemical Communications | issue = 24 | pages = 2572–2574 | date = June 2006 | pmid = 16779481 | doi = 10.1039/b604844g | url = https://zenodo.org/record/896672 }}</ref> [[X-ray diffraction]] analysis of this compound confirms that the cyclopentadienyl ligands are not co-planar with the benzene core but have alternating [[dihedral angle]]s 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 [[pyramidalization|pyramidalized]]. Also, the benzene core has a [[chair conformation]] with dihedral angles of 14° and displays [[bond length]] alternation between 142.7&nbsp;[[picometer|pm]] and 141.1&nbsp;pm, both indications of steric crowding of the substituents.

The synthesis of hexaferrocenylbenzene has been reported using [[Negishi coupling]] of [[hexaiodobenzene]] and diferrocenylzinc, using [[tris(dibenzylideneacetone)dipalladium(0)]] as [[catalyst]], in [[tetrahydrofuran]]:<ref name = "hexaferrocenylbenzene" />
:[[Image:Hexaferrocenylbenzene.png|400px|Hexaferrocenylbenzene synthesis by Negishi coupling]]
The [[yield (chemistry)|yield]] is only 4%, which is further evidence consistent with substantial [[steric strain|steric]] crowding around the arene core.

{{clear}}

===Materials chemistry===
[[File:Wettability of a silica surface with a bound ferrocene-substituted polymer.jpg|left|thumb|500px|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.<ref name = Pietschnig />]] 
Ferrocene, a precursor to iron nanoparticles, can be used as a catalyst for the production of carbon nanotubes.<ref>{{cite journal| vauthors = Conroy D, Moisala A, Cardoso S, Windle A, Davidson J |journal=[[Chemical Engineering Science|Chem. Eng. Sci.]]|year=2010|volume=65|pages=2965–2977|doi=10.1016/j.ces.2010.01.019|title=Carbon nanotube reactor: Ferrocene decomposition, iron particle growth, nanotube aggregation and scale-up|issue=10}}</ref> [[Vinylferrocene]] can be converted to (polyvinylferrocene, PVFc), a ferrocenyl version of [[polystyrene]] (the phenyl groups are replaced with ferrocenyl groups). Another [[Polyferrocenes|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 [[polysiloxane]]s, [[polyphosphazene]]s, and poly[[phosphinoborane]]s, (&ndash;PH(R)&ndash;BH<sub>2</sub>&ndash;)<sub>''n''</sub>, and the resulting materials exhibit unusual physical and electronic properties relating to the ferrocene / ferrocinium redox couple.<ref name = Pietschnig>{{cite journal | vauthors = Pietschnig R | title = Polymers with pendant ferrocenes | journal = Chemical Society Reviews | volume = 45 | issue = 19 | pages = 5216–5231 | date = October 2016 | pmid = 27156979 | doi = 10.1039/C6CS00196C | doi-access = free }}</ref> 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.<ref name = Pietschnig /><ref>{{cite journal| vauthors = Elbert J, Gallei M, Rüttiger C, Brunsen A, Didzoleit H, Stühn B, Rehahn M |journal = [[Organometallics]]|year = 2013|volume = 32|issue = 20|pages = 5873–5878|title = Ferrocene Polymers for Switchable Surface Wettability|doi = 10.1021/om400468p}}</ref>

== See also ==
* [[Josiphos ligands]]

== References ==
<references>
<ref name="pauson">{{cite journal| vauthors = Kealy TJ, Pauson PL |author-link2 = Peter Pauson|title = A New Type of Organo-Iron Compound|journal = [[Nature (journal)|Nature]]|year = 1951|volume = 168|pages = 1039–1040|doi = 10.1038/1681039b0 |issue = 4285|bibcode = 1951Natur.168.1039K|s2cid = 4181383}}</ref>
<ref name="miller">{{cite journal| vauthors = Miller SA, Tebboth JA, Tremaine JF |journal= [[Journal of the Chemical Society|J. Chem. Soc.]]|year=1952| pages= 632–635| title=114. Dicyclopentadienyliron |doi=10.1039/JR9520000632}}</ref>
<ref name="wilk52">{{cite journal |author-link1=Geoffrey Wilkinson |author-link4=Robert Burns Woodward |vauthors=Wilkinson G, Rosenblum M, Whiting MC, Woodward RB |date=1952-03-24 |title=The structure of iron ''bis''-cyclopentadienyl |journal=[[Journal of the American Chemical Society|J. Am. Chem. Soc.]] |volume=74 |issue=8 |pages=2125–2126 |doi=10.1021/ja01128a527}}</ref>
<ref name="fischer">{{cite journal | vauthors = Fischer EO, Pfab W |author-link1= Ernst Otto Fischer |title = Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels|trans-title=On the crystal structure of the bis-cyclopentadienyl compounds of divalent iron, cobalt and nickel |journal = Zeitschrift für Naturforschung B |year = 1952 |volume = 7 |issue = 7|pages = 377–379 |doi =10.1515/znb-1952-0701|doi-access = free}}</ref>
<ref name="eiland52">{{Cite journal | vauthors = Eiland PF, Pepinsky R |year= 1952 |title= X-ray Examination of Iron Biscyclopentadienyl |journal= [[Journal of the American Chemical Society|J. Am. Chem. Soc.]] |volume= 74 |issue= 19 |page= 4971 |doi= 10.1021/ja01139a527}}</ref>
<ref name="dunitz53">{{cite journal| vauthors = Dunitz JD, Orgel LE |title=Bis-Cyclopentadienyl&nbsp;– A Molecular Sandwich|journal= [[Nature (journal)|Nature]] |year=1953| volume=171 |pages= 121–122|doi = 10.1038/171121a0|issue=4342|bibcode = 1953Natur.171..121D |s2cid= 4263761}}</ref>
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<ref name="haal68">{{cite journal | vauthors = Haaland A, Nilsson JE | year = 1968 | title = The Determination of Barriers to Internal Rotation by Means of Electron Diffraction. Ferrocene and Ruthenocene | journal = [[Acta Chemica Scandinavica|Acta Chem. Scand.]] | volume = 22 | pages = 2653–2670 | doi = 10.3891/acta.chem.scand.22-2653 | doi-access = free}}</ref>
<ref name="seiler">{{Cite journal| vauthors = Seiler P, Dunitz JD |year=1982|title=Low-temperature crystallization of orthorhombic ferrocene: structure analysis at 98 K|journal=[[Acta Crystallographica Section B]]|volume=38|issue=6|pages=1741–1745|doi=10.1107/s0567740882007080|issn=0567-7408}}</ref>
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<ref name="laszloRmon">{{cite journal | vauthors = Laszlo P, Hoffmann R | title = Ferrocene: Ironclad History or Rashomon Tale? | journal = Angewandte Chemie | volume = 39 | issue = 1 | pages = 123–124 | date = January 2000 | pmid = 10649350 | doi = 10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z | author-link2 = Roald Hoffman}}</ref>
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<ref name="federman">{{Cite journal | vauthors = Federman Neto A, Pelegrino AC, Darin VA |year= 2004 |title= Ferrocene: 50 Years of Transition Metal Organometallic Chemistry — From Organic and Inorganic to Supramolecular Chemistry (Abstract) |journal= Trends in Organometallic Chemistry |volume= 4 |pages= 147–169 |url= http://www.researchtrends.net/tia/abstract.asp?in=0&vn=4&tid=9&aid=870&pub=2002&type=3 |publisher= [[Research Trends]]}}</ref>
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<ref name="okuda">{{Cite journal| vauthors = Okuda J |date=2016-12-28|title=Ferrocene - 65 Years After|journal=European Journal of Inorganic Chemistry|volume=2017|issue=2|pages=217–219|doi=10.1002/ejic.201601323|issn=1434-1948|doi-access=free}}</ref>
</references>

== External links ==
{{Commons category}}
*[http://www.periodicvideos.com/videos/mv_ferrocene.htm Ferrocene] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
*[https://www.cdc.gov/niosh/npg/npgd0205.html NIOSH Pocket Guide to Chemical Hazards] (Centers for Disease Control and Prevention)

{{Iron compounds}}
{{Cyclopentadiene complexes}}
{{Authority control}}

[[Category:Ferrocenes| ]]
[[Category:Antiknock agents]]
[[Category:Sandwich compounds]]
[[Category:Cyclopentadienyl complexes]]
[[Category:Substances discovered in the 1950s]]