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User:Barras/Ferrocene
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(contracted; show full) 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 [[Aromaticity|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 Fe<sup>2+</sup>, the complex attains an [[18-electron rule|18-electron]] configuration.

==Synthesis and handling properties==
The first reported
<ref>{{cite journal|year=1951|last1=Kealy|first1=T. J.|last2=Pauson|first2=P. L.|journal=[[Nature (journal)|Nature]]|volume=168|pages=1039|doi=10.1038/1681039b0|issue=4285|title=A New Type of Organo-Iron Compound|bibcode = 1951Natur.168.1039K }}</ref> synthesis of ferrocene used the [[Grignard reaction|Grignard reagent]] cyclopentadienyl magnesium bromide, which can be prepared by reacting [[cyclopentadiene]] with magnesium and [[bromoethane]] in [[anhydrous]] [[benzene]]. [[Iron(II) chloride]] is then suspended in anhydrous [[diethyl ether]] and added to the Grignard reagent. An idealized equation for this synthesis is:

:4&nbsp;C<sub>5</sub>H<sub>5</sub>MgBr  +  2&nbsp;FeCl<sub>2</sub>   →   2&nbsp;Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>  +  4&nbsp;MgBrCl

Numerous other syntheses have been reported, including the direct reaction of [[gas]]-phase cyclopentadiene with metallic iron<ref>{{cite journal|doi=10.1021/ja01636a080|title=Bis-cyclopentadienyl Compounds of Nickel and Cobalt|year=1954|last1=Wilkinson|first1=G.|authorlink1=Geoffrey Wilkinson|last2=Pauson|first2=P. L.|last3=Cotton|first3=F. A.|authorlink3=F. Albert Cotton|journal=[[J. Am. Chem. Soc.]]|volume=76|pages=1970|issue=7}}</ref> at 350&nbsp;°C or with [[iron pentacarbonyl]].<ref>{{cite journal|doi=10.1002/978-0-470-16602-4.ch1|year=1959|last1=Wilkinson|first1=G.|authorlink1=Geoffrey Wilkinson|last2=Cotton|first2=F. A.|authorlink2=F. Albert Cotton|title=Cyclopentadienyl and Arene Metal Compounds|journal=Prog. Inorg. Chem.|volume=1|pages=1–124|isbn=978-0-470-16602-4}}</ref>

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

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

More efficient preparative methods are generally a modification of the original [[transmetalation]] sequence using either commercially available [[sodium cyclopentadienide]]<ref name=orgsyn>{{OrgSynth|title = Ferrocene|author = [[Geoffrey Wilkinson]]|collvol = 4|collvolpages = 473|year = 1963|prep = cv4p0473}}</ref> or freshly [[dicyclopentadiene|cracked]] cyclopentadiene deprotonated with [[potassium hydroxide]]<ref>{{cite book|last=Jolly|first= W. L.|title= The Synthesis and Characterization of Inorganic Compounds |publisher=Prentice-Hall |location=New Jersey |date=1970}}</ref> and reacted with anhydrous iron(II) chloride in ethereal solvents:

:2&nbsp;NaC<sub>5</sub>H<sub>5</sub> + FeCl<sub>2</sub> → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;NaCl

:FeCl<sub>2</sub>·4H<sub>2</sub>O + 2&nbsp;C<sub>5</sub>H<sub>6</sub> + 2&nbsp;KOH → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;KCl + 6&nbsp;H<sub>2</sub>O syntheses of ferrocene were nearly simultaneous.  Pauson and Kealy synthesised ferrocene using [[iron(III) chloride]] and a [[Grignard reaction|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]].<ref>{{cite journal|year=1951|last1=Kealy|first1=T. J.|last2=Pauson|first2=P. L.|journal=[[Nature (journal)|Nature]]|volume=168|pages=1039|doi=10.1038/1681039b0|issue=4285|title=A New Type of Organo-Iron Compound|bibcode = 1951Natur.168.1039K }}</ref>  An iron(III) salt was chosen as they sought to couple the cyclopentadienyl [[moiety (chemistry)|moieties]] to form dihydrofulvalene and then fullvalene, but ferrocene was formed instead as the oxidative formation of dihydrofulvalene also produced iron(II) by reduction, which in turn reacts with the Grignard.

:[[File:Kealy_Ferrocen_Synthese.svg|800px]]

[[File:Miller Ferrocen Synthese.svg|thumb|right|300px|The Miller ''et al.''<ref name = Miller /> approach to ferrocene]]
The other early synthesis of ferrocene was by Miller ''et al.'',<ref name = Miller>{{cite journal|journal = J. Chem. Soc.|last1 = Miller|first1 = S. A.|first2 = J. A.|last2 = Tebboth|first3 = J. F.|last3 = Tremaine|title = Dicyclopentadienyliron|year = 1952|volume = |page = 632–635|doi = 10.1039/JR9520000632}}</ref> who reacted metallic iron directly with [[gas]]-phase cyclopentadiene at elevated temperature.<ref>{{cite journal|doi=10.1021/ja01636a080|title=Bis-cyclopentadienyl Compounds of Nickel and Cobalt|year=1954|last1=Wilkinson|first1=G.|authorlink1=Geoffrey Wilkinson|last2=Pauson|first2=P. L.|last3=Cotton|first3=F. A.|authorlink3=F. Albert Cotton|journal=[[J. Am. Chem. Soc.]]|volume=76|pages=1970|issue=7}}</ref>  An approach using [[iron pentacarbonyl]] was also reported.<ref>{{cite journal|doi=10.1002/978-0-470-16602-4.ch1|year=1959|last1=Wilkinson|first1=G.|authorlink1=Geoffrey Wilkinson|last2=Cotton|first2=F. A.|authorlink2=F. Albert Cotton|title=Cyclopentadienyl and Arene Metal Compounds|journal=Prog. Inorg. Chem.|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>(g) → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 5&nbsp;CO(g) + H<sub>2</sub>(g)

More efficient preparative methods are generally a modification of the original [[transmetalation]] sequence using either commercially available [[sodium cyclopentadienide]]<ref name=orgsyn>{{OrgSynth|title = Ferrocene|author = [[Geoffrey Wilkinson]]|collvol = 4|collvolpages = 473|year = 1963|prep = cv4p0473}}</ref> or freshly [[dicyclopentadiene|cracked]] cyclopentadiene deprotonated with [[potassium hydroxide]]<ref>{{cite book|last=Jolly|first= W. L.|title= The Synthesis and Characterization of Inorganic Compounds |publisher=Prentice-Hall |location=New Jersey |date=1970}}</ref> and reacted with anhydrous iron(II) chloride in ethereal solvents.  A modern modification of the Grignard approach is also known:

:2&nbsp;NaC<sub>5</sub>H<sub>5</sub> + FeCl<sub>2</sub> → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;NaCl

:FeCl<sub>2</sub>·4H<sub>2</sub>O + 2&nbsp;C<sub>5</sub>H<sub>6</sub> + 2&nbsp;KOH → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;KCl + 6&nbsp;H<sub>2</sub>O

:2&nbsp;C<sub>5</sub>H<sub>5</sub>MgBr + FeCl<sub>2</sub> → Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;MgBrCl

Even some [[amine]] bases can be used for the deprotonation, though the reaction proceeds more slowly than when using stronger bases:<ref name=orgsyn/>

(contracted; show full)
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[[Category:Organoiron compounds]]
[[Category:Metallocenes]]
[[Category:Antiknock agents]]
[[Category:Sandwich compounds]]
[[Category:Cyclopentadienyl complexes]]