Cyclopentadienyliron dicarbonyl dimer: Difference between revisions

|Watchedfields = changed

|verifiedrevid = 437237521

|Name = Cyclopentadienyliron dicarbonyl dimer

|ImageFile = Fp2SingleNoFeFe.png

|ImageFile1 = Trans-cyclopentadienyliron-dicarbonyl-dimer-from-xtal-2009-3D-balls.png

|ImageFile2 = Cis-cyclopentadienyliron-dicarbonyl-dimer-from-xtal-1970-3D-balls.png

|OtherNames = Bis(cyclopentadienyl)tetracarbonyl-diiron,<br /> Di(cyclopentadienyl)tetracarbonyl-diiron,<br /> Bis(dicarbonylcyclopentadienyliron)

|IUPACName = Di-μ-carbonyldicarbonylbis(η⁵-cyclopenta-2,4-dien-1-yl)diiron

|Section1 = {{Chembox Identifiers

|SMILES = c1ccc[cH-]1.[Fe]1(C#[O+])C(=O)[Fe](C#[O+])C(=O)1.c1ccc[cH-]1

|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

|ChemSpiderID = 24589716

|InChI = 1/2C5H5.4CO.2Fe/c2*1-2-4-5-3-1;4*1-2;;/h2*1-5H;;;;;;/q;;;;2*-1;2*+1/r2C6H5FeO.2CO/c2*8-5-7-6-3-1-2-4-6;2*1-2/h2*1-4,6H;;

|InChIKey = XJSJEKXJJGHIGW-FEMRPSMKAV

|StdInChI_Ref = {{stdinchicite|correct|chemspider}}

|StdInChI = 1S/2C5H5.4CO.2Fe/c2*1-2-4-5-3-1;4*1-2;;/h2*1-5H;;;;;;/q;;;;2*-1;2*+1

|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

|StdInChIKey = XJSJEKXJJGHIGW-UHFFFAOYSA-N

|CASNo_Ref = {{cascite|correct|CAS}}

|CASNo = 12154-95-9

|PubChem = 11110989

|EINECS = 235-276-3

}}

|Section2 = {{Chembox Properties

|Formula = [[Carbon|C]]₁₄[[Hydrogen|H]]₁₀[[Iron|Fe]]₂[[Oxygen|O]]₄

|MolarMass = 353.925 g/mol

|Appearance = Dark purple crystals

|Density = 1.77 g/cm³, solid

|Solubility = insoluble

|Solvent = other solvents

|SolubleOther = benzene, THF, chlorocarbons

|MeltingPtC = 194

|BoilingPt = decomposition

}}

|Section3 = {{Chembox Structure

|Coordination = distorted octahedral

|CrystalStruct = <!-- e.g. [[triclinic]], [[monoclinic]], [[orthorhombic]], [[hexagonal]], [[trigonal]], [[tetragonal]], [[cubic]], and mention "close packed" or similar. You may also cite what class it belongs to, e.g. [[Cadmium chloride#Crystal_structure|CdCl₂]] -->

|Dipole = 3.1&nbsp;[[Debye|D]] (benzene solution)

}}

|Section7 = {{Chembox Hazards

|MainHazards = CO source

|GHSPictograms = {{GHS02}}{{GHS06}}{{GHS07}}

|GHSSignalWord = Danger

|HPhrases = {{H-phrases|228|302|331|330}}

}}

|Section8 = {{Chembox Related

|OtherCompounds = [[Ferrocene|Fe(C₅H₅)₂]]<br />[[Iron pentacarbonyl|Fe(CO)₅]]

}}

'''Cyclopentadienyliron dicarbonyl dimer''' is an [[organometallic compound]] with the formula [(''η''⁵-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).<ref>{{cite book|doi=10.1002/047084289X.rb139|chapter=Bis(dicarbonylcyclopentadienyliron)|title=Encyclopedia of Reagents for Organic Synthesis|year=2001|last1=Kelly|first1=William J.|isbn=0471936235}}</ref>

In solution, Cp₂Fe₂(CO)₄ can be considered a dimeric [[Sandwich compound|half-sandwich]] complex. It exists in three isomeric forms: ''cis'', ''trans'', and an unbridged, open form. These isomeric forms are distinguished by the position of the ligands. The ''cis'' and ''trans'' isomers differ in the relative position of C₅H₅ (Cp) ligands. The ''cis'' and ''trans'' isomers have the formulation [(''η''⁵-C₅H₅)Fe(CO)(''μ''-CO)]₂, that is, two CO ligands are terminal whereas the other two CO ligands bridge between the iron atoms. The ''cis'' and ''trans'' isomers interconvert via the open isomer, which has no bridging ligands between iron atoms. Instead, it is formulated as (''η''⁵-C₅H₅)(OC)₂Fe−Fe(CO)₂(''η''⁵-C₅H₅) — the metals are held together by an iron–iron bond. At equilibrium, the ''cis'' and ''trans'' isomers are predominant.

:[[File:Fp2-isomerization.png|frameless|440x440px]]

In addition, the terminal and bridging carbonyls are known to undergo exchange: the ''trans'' isomer can undergo bridging–terminal CO ligand exchange through the open isomer, or through a twisting motion without going through the open form. In contrast, the bridging and terminal CO ligands of the ''cis'' isomer can only exchange via the open isomer.<ref name=":2">{{Cite journal|last1=Harris|first1=Daniel C.|last2=Rosenberg|first2=Edward|last3=Roberts|first3=John D.|date=1974|title=Carbon-13 nuclear magnetic resonance spectra and mechanism of bridge–terminal carbonyl exchange in di-''µ''-carbonyl-bis[carbonyl(''η''-cyclopentadienyl)iron](Fe–Fe) [{(''η''-C₅H₅)Fe(CO)₂}₂]; ''cd''-di-''µ''-carbonyl-''f''-carbonyl-''ae''-di(''η''-cyclopentadienyl)-''b''-(triethyl-phosphite)di-iron(Fe–Fe) [(''η''-C₅H₅)₂Fe₂(CO)₃P(OEt)₃], and some related complexes|journal=Journal of the Chemical Society: Dalton Transactions|language=en|issue=22|pages=2398–2403|doi=10.1039/DT9740002398|issn=0300-9246|url=https://authors.library.caltech.edu/12272/1/HARjcsdt74.pdf}}</ref>

In solution, the ''cis'', ''trans'', and open isomers interconvert rapidly at room temperature, making the molecular structure [[Fluxional molecule|fluxional]]. The fluxional process for cyclopentadienyliron dicarbonyl dimer is faster than the NMR time scale, so that only an averaged, single Cp signal is observed in the [[Proton nuclear magnetic resonance|¹H&nbsp;NMR]] spectrum at 25&nbsp;°C. Likewise, the [[Carbon-13 nuclear magnetic resonance|¹³C&nbsp;NMR]] spectrum exhibits one sharp CO signal above −10&nbsp;°C, while the Cp signal sharpens to one peak above 60&nbsp;°C. NMR studies indicate that the ''cis'' isomer is slightly more abundant than the ''trans'' isomer at room temperature, while the amount of the open form is small.<ref name=":2" /> The fluxional process is not fast enough to produce averaging in the [[infrared spectroscopy|IR spectrum]]. Thus, three absorptions are seen for each isomer. The bridging CO ligands appear at around 1780&nbsp;cm⁻¹ whereas the terminal CO ligands are observed at around 1980&nbsp;cm⁻¹.<ref name="Girolami">{{cite book |author1-link=Gregory S. Girolami |author3-link=Robert Angelici | last1= Girolami |first1=G. |last2=Rauchfuss |first2=T. |last3=Angelici |first3=R. | title = Synthesis and Technique in Inorganic Chemistry | edition = 3rd | publisher = University Science Books | location = Sausalito, CA | year = 1999 | pages = 171–180 | isbn = 978-0-935702-48-4}}</ref> The averaged structure of these isomers of Cp₂Fe₂(CO)₄ results in a [[Dipole#Molecular dipoles|dipole moment]] of 3.1&nbsp;[[Debye (unit)|D]] in [[benzene]].<ref>{{Cite journal|author1-link=F. Albert Cotton|last1=Cotton|first1=F. Albert|last2=Yagupsky|first2=G.|date=January 1967|title=Tautomeric changes in metal carbonyls. I. .pi.-Cyclopentadienyliron dicarbonyl dimer and .pi.-cyclopentadienyl-ruthenum dicarbonyl dimer|url=https://pubs.acs.org/doi/abs/10.1021/ic50047a005|journal=Inorganic Chemistry|language=en|volume=6|issue=1|pages=15–20|doi=10.1021/ic50047a005|issn=0020-1669}}</ref>

The solid-state molecular structure of both ''cis'' and ''trans'' isomers have been analyzed by [[X-ray diffraction|X-ray]] and [[neutron diffraction]]. The Fe–Fe separation and the Fe–C bond lengths are the same in the Fe₂C₂ rhomboids, an exactly planar Fe₂C₂ four-membered ring in the ''trans'' isomer versus a folded rhomboid in ''cis'' with an angle of 164°, and significant distortions in the Cp ring of the ''trans'' isomer reflecting different Cp orbital populations.<ref name=wilkinson>{{ cite book | title = Comprehensive Organometallic Chemistry | volume = 4 | editor-last = Wilkinson | editor-first = G. | editor-link = Geoffrey Wilkinson | publisher = Pergamon Press | location = New York | year = 1982 | pages = 513–613 | isbn = 978-0-08-025269-8 }}</ref> Although older textbooks show the two iron atoms bonded to each other, theoretical analyses indicate the absence of a direct Fe–Fe bond. This view is consistent with computations and X-ray crystallographic data that indicate a lack of significant electron density between the iron atoms.<ref>{{cite journal|first1=Jennifer C. |last1=Green |first2=Malcolm L. H. |last2=Green |first3=Gerard |last3=Parkin |date=2012 |title=The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds |journal=Chemical Communications |volume=2012 |issue=94 |pages=11481–11503 |doi=10.1039/c2cc35304k|pmid=23047247 }}</ref> However, Labinger offers a dissenting view, based primarily on chemical reactivity and spectroscopic data, arguing that electron density is not necessarily the best indication of the presence of a chemical bond. Moreover, without an Fe–Fe bond, the bridging carbonyls must be formally treated as an μ-X₂ ligand and μ-L ligand in order for the iron centers to satisfy the [[18-electron rule]]. This formalism is argued to give misleading implications with respect to the chemical and spectroscopic behavior of the carbonyl groups.<ref name=":1">{{Cite journal|last=Labinger|first=Jay A.|date=2015|title=Does cyclopentadienyl iron dicarbonyl dimer have a metal–metal bond? Who's asking?|journal=Inorganica Chimica Acta|series=Metal–Metal Bonded Compounds and Metal Clusters|volume=424|pages=14–19|doi=10.1016/j.ica.2014.04.022|issn=0020-1693}}</ref>

Cp₂Fe₂(CO)₄ was first prepared in 1955 at Harvard by [[Geoffrey Wilkinson]] using the same method employed today: the reaction of [[iron pentacarbonyl]] and [[dicyclopentadiene]].<ref name=":1" /><ref>{{cite journal|last1=Piper|first1= T. S.|last2= Cotton|first2= F. A.|last3= Wilkinson|first3= G.|title=Cyclopentadienyl–carbon monoxide and related compounds of some transitional metals|journal= Journal of Inorganic and Nuclear Chemistry|date= 1955 |volume=1 |issue= 3|pages=165–174 |doi=10.1016/0022-1902(55)80053-X}}</ref>

:2&nbsp;Fe(CO)₅ + C₁₀H₁₂ → (''η''⁵-C₅H₅)₂Fe₂(CO)₄ + 6&nbsp;CO + H₂

In this preparation, dicyclopentadiene [[Cracking (chemistry)|cracks]] to give cyclopentadiene, which reacts with [[Iron pentacarbonyl|Fe(CO)₅]] with loss of [[carbon monoxide|CO]]. Thereafter, the pathways for the photochemical and thermal routes differ subtly but both entail formation of a [[hydride]] intermediate.<ref name=wilkinson/> The method is used in the teaching laboratory.<ref name=Girolami/>

==Reactions==

Although of no major commercial value, Fp₂ is a workhorse in [[organometallic chemistry]] because it is inexpensive and FpX derivatives are rugged (X = halide, organyl).

==="Fp⁻" (FpNa and FpK)===

Reductive cleavage of [CpFe(CO)₂]₂ (formally an iron(I) complex) produces alkali metal derivatives formally derived from the cyclopentadienyliron dicarbonyl anion, [CpFe(CO)₂]⁻ or called Fp⁻ (formally iron(0)), which are assumed to exist as a [[tight ion pair]]. A typical reductant is sodium metal or [[sodium amalgam]];<ref>{{OrgSynth | collvol = 8 | collvolpages = 479 | year = 1988 | title = Vinylation of Enolates with a Vinyl Cation Equivalent: ''trans''-3-Methyl-2-Vinylcyclohexanone | last1= Chang|first1= T. C. T.|last2= Rosenblum|first2= M.|last3= Simms|first3= N. | volume = 66 | pages = 95 | prep = cv8p0479}}</ref> [[NaK]] alloy, [[potassium graphite]] (KC₈), and alkali metal trialkylborohydrides have been used. [CpFe(CO)₂]Na is a widely studied reagent since it is readily alkylated, acylated, or metalated by treatment with an appropriate [[electrophile]].<ref>{{ cite journal | last= King|first= B. | title = Applications of Metal Carbonyl Anions in the Synthesis of Unusual Organometallic Compounds | journal = Accounts of Chemical Research | year = 1970 | volume = 3 | issue = 12 | pages = 417–427 | doi = 10.1021/ar50036a004 }}</ref> It is an excellent S_N2 nucleophile, being one to two orders of magnitude more nucleophilic than thiophenolate, PhS^– when reacted with primary and secondary alkyl bromides.<ref>{{Cite journal|last1=Dessy|first1=Raymond E.|last2=Pohl|first2=Rudolph L.|last3=King|first3=R. Bruce|date=1966-11-01|title=Organometallic Electrochemistry. VII.1 The Nucleophilicities of Metallic and Metalloidal Anions Derived from Metals of Groups IV, V, VI, VII, and VIII|url=https://doi.org/10.1021/ja00974a015|journal=Journal of the American Chemical Society|volume=88|issue=22|pages=5121–5124|doi=10.1021/ja00974a015|issn=0002-7863}}</ref>

:[CpFe(CO)₂]₂ + 2&nbsp;Na → 2&nbsp;CpFe(CO)₂Na

:[CpFe(CO)₂]₂ + 2&nbsp;KBH(C₂H₅)₃ → 2&nbsp;CpFe(CO)₂K + H₂ + 2&nbsp;B(C₂H₅)₃

Treatment of NaFp with an alkyl [[halide]] (RX, X = Br, I) produces FeR(''η''⁵-C₅H₅)(CO)₂

Fp₂ can also be cleaved with alkali metals<ref>{{ cite journal | last1= Ellis|first1= J. E.|last2= Flom|first2= E. A. | title = The Chemistry of Metal Carbonyl Anions: III. Sodium-Potassium Alloy: An Efficient Reagent for the Production of Metal Carbonyl Anions | journal = [[Journal of Organometallic Chemistry]] | year = 1975 | volume = 99 | issue = 2 | pages = 263–268 | doi = 10.1016/S0022-328X(00)88455-7 }}</ref> and by [[electrochemical reduction]].<ref>{{ cite journal | last1= Dessy|first1= R. E.|last2= King|first2 =R. B.|last3= Waldrop|first3= M. | title = Organometallic Electrochemistry. V. The Transition Series | journal = [[Journal of the American Chemical Society]] | year = 1966 | volume = 88 | issue = 22 | pages = 5112–5117 | doi = 10.1021/ja00974a013 }}</ref><ref>{{ cite journal | last1= Dessy|first1= R. E.|last2= Weissman|first2= P. M.|last3= Pohl|first3= R. L. | title = Organometallic Electrochemistry. VI. Electrochemical Scission of Metal–Metal Bonds | journal = [[Journal of the American Chemical Society]] | year = 1966 | volume = 88 | issue = 22 | pages = 5117–5121 | doi = 10.1021/ja00974a014 }}</ref>

===FpX (X = Cl, Br, I)===

[[Halogen]]s oxidatively cleave [CpFe(CO)₂]₂ to give the Fe(II) species FpX (X = Cl, Br, I):

:[CpFe(CO)₂]₂ + X₂ → 2&nbsp;CpFe(CO)₂X

One example is [[cyclopentadienyliron dicarbonyl iodide]].

=== Fp(''η''²-alkene)⁺, Fp(''η''²-alkyne)⁺ and other "Fp⁺" ===

In the presence of halide anion acceptors such as [[aluminium bromide]] or [[silver tetrafluoroborate]], FpX compounds (X = halide) react with [[alkenes]], [[alkynes]], or neutral labile ligands (such as [[ether]]s and [[nitrile]]s) to afford Fp⁺ complexes.<ref>{{Cite book|title=Chemistry of Iron|last=Silver|first=J.|date=1993|publisher=Springer Netherlands|isbn=9789401121408|location=Dordrecht|oclc=840309324}}</ref> In another approach, salts of [Fp(isobutene)]⁺ are readily obtained by reaction of NaFp with [[methallyl chloride]] followed by protonolysis. This complex is a convenient and general precursor to other cationic Fp–alkene and Fp–alkyne complexes.<ref name=":0" /> The exchange process is facilitated by the loss of gaseous and bulky [[isobutene]].<ref>{{cite encyclopedia|chapter=Dicarbonyl(cyclopentadienyl)(isobutene)iron Tetrafluoroborate|first=Mark M.|last=Turnbull|title=Encyclopedia of Reagents for Organic Synthesis|encyclopedia=eEROS|year=2001|doi=10.1002/047084289X.rd080|isbn=0471936235}}</ref> Generally, less substituted alkenes bind more strongly and can displace more hindered alkene ligands. Alkene and alkyne complexes can also be prepared by heating a cationic ether or aqua complex, for example {{chem|[Fp([[Tetrahydrofuran|thf]])]|+|BF|4|−}}, with the alkene or alkyne.<ref>{{Cite book|title=Iron, Ruthenium and Osmium|date=1995|publisher=Elsevier Science|last1=Schriver|first=D. F.|last2=Bruce|first2=M. I.|last3=Wilkinson|first3=G.|isbn=978-0-08-096396-9|location=Kidlington|oclc=953660855}}</ref> {{chem|[FpL]|+|BF|4|−}} complexes can also be prepared by treatment of FpMe with HBF₄·[[diethyl ether|Et₂O]] in [[dichloromethane|CH₂Cl₂]] at −78&nbsp;°C, followed by addition of L.<ref>{{Cite journal|last1=Redlich|first1=Mark D.|last2=Mayer|first2=Michael F.|last3=Hossain|first3=M. Mahmun|date=2003|title=Iron Lewis Acid [(''η''⁵-C₅H₅)Fe(CO)₂(THF)]⁺ Catalyzed Organic Reactions|journal=Aldrichimica Acta|volume=36|pages=3–13}}</ref>

:[[File:Fp(isobutylene)cation.png|330 px]]

[[Alkene]]–Fp complexes can also be prepared from Fp anion indirectly. Thus, hydride abstraction from Fp–alkyl compounds using [[triphenylmethyl hexafluorophosphate]] affords [Fp(α-alkene)]⁺ complexes.

:FpNa + RCH₂CH₂I → FpCH₂CH₂R + NaI

:FpCH₂CH₂R + Ph₃CPF₆ → {{chem|[Fp(CH|2|{{=}}CHR)|+|]PF|6|−}} + Ph₃CH

Reaction of NaFp with an [[epoxide]] followed by acid-promoted dehydration also affords alkene complexes. Fp(alkene)⁺ are stable with respect to [[bromination]], [[hydrogenation]], and [[acetoxymercuration]], but the alkene is easily released with [[sodium iodide]] in [[acetone]] or by warming with [[acetonitrile]].<ref name=pearson>{{ cite book | last= Pearson|first= A. J. | title = Iron Compounds in Organic Synthesis | publisher = Academic Press | location = San Diego, CA | year = 1994 | pages = 22–35 | isbn = 978-0-12-548270-7 }}</ref>

:[[File:Fp(isobutylene)exchange.png|580 px]]

The alkene ligand in these cations is activated toward attack by [[nucleophile]]s, opening the way to a number of [[carbon–carbon bond]]-forming reactions. [[Nucleophilic addition]]s usually occur at the more substituted carbon. This [[regiochemistry]] is attributed to the greater positive [[charge density]] at this position. The [[Regioselectivity|regiocontrol]] is often modest. The addition of the nucleophile is completely [[stereoselective]], occurring ''anti'' to the Fp group. Analogous Fp(alkyne)⁺ complexes are also reported to undergo nucleophilic addition reactions by various carbon, nitrogen, and oxygen nucleophiles.<ref>{{Cite journal|last1=Akita|first1=Munetaka|last2=Kakuta|first2=Satoshi|last3=Sugimoto|first3=Shuichiro|last4=Terada|first4=Masako|last5=Tanaka|first5=Masako|last6=Moro-oka|first6=Yoshihiko|date=2001|title=Nucleophilic Addition to the ''η''²-Alkyne Ligand in [CpFe(CO)₂(''η''²-R−C⋮C−R)]⁺. Dependence of the Alkenyl Product Stereochemistry on the Basicity of the Nucleophile|journal=Organometallics|volume=20|issue=13|pages=2736–2750|doi=10.1021/om010095t|issn=0276-7333}}</ref>

:[[File:FpMalonateRxn.png|330 px|Addition of carbanion to [Fp(alkene)]⁺.]]

Fp(alkene)⁺ and Fp(alkyne)⁺ π-complexes are also quite acidic at the allylic and propargylic positions, respectively, and can be quantitatively deprotonated with amine bases like Et₃N to give neutral Fp–allyl and Fp–allenyl σ-complexes (eqn 1, shown for alkene complex).<ref name=":0">{{Cite journal|last1=Cutler|first1=A.|last2=Ehnholt|first2=D.|last3=Lennon|first3=P.|last4=Nicholas|first4=K.|last5=Marten|first5=David F.|last6=Madhavarao|first6=M.|last7=Raghu|first7=S.|last8=Rosan|first8=A.|last9=Rosenblum|first9=M.|date=1975|title=Chemistry of dicarbonyl .eta.5-cyclopentadienyliron complexes. General syntheses of monosubstituted ''η''²-olefin complexes and of 1-substituted ''η''¹-allyl complexes. Conformational effects on the course of deprotonation of (''η''²-olefin) cations|journal=Journal of the American Chemical Society|volume=97|issue=11|pages=3149–3157|doi=10.1021/ja00844a038|issn=0002-7863}}</ref>

[[File:Fp-stoichiometric-func.png|frameless|350x350px]]

Fp–allyl and Fp–allenyl react with cationic electrophiles '''E''' (such as [[Trimethyloxonium tetrafluoroborate|Me₃O⁺]], [[Carbocation|carbocations]], [[Oxocarbenium|oxocarbenium ions]]) to generate allylic and propargylic functionalization products, respectively (eqn 2, shown for allyliron).<ref name=":0" /> The related complex [Cp*Fe(CO)₂(thf)]⁺[BF₄]⁻ (Cp* = C₅Me₅) has been shown to catalyze propargylic, allylic, and allenic C−H functionalization by combining the deprotonation and electrophilic functionalization processes described above with facile exchange of the unsaturated hydrocarbon bound to the cationic iron center.<ref>{{Cite journal|last1=Wang|first1=Yidong|last2=Zhu|first2=Jin|last3=Durham|first3=Austin C.|last4=Lindberg|first4=Haley|last5=Wang|first5=Yi-Ming|date=2019|title=α-C–H Functionalization of π-Bonds Using Iron Complexes: Catalytic Hydroxyalkylation of Alkynes and Alkenes|journal=Journal of the American Chemical Society|volume=141|issue=50|pages=19594–19599|doi=10.1021/jacs.9b11716|pmid=31791121|s2cid=208611984|issn=0002-7863}}</ref>

η²-Allenyl complexes of Fp⁺ and substituted cyclopentadienyliron dicarbonyl cations have also been characterized, with X-ray crystallographic analysis showing substantial bending at the central allenic carbon (bond angle < 150°).<ref>{{Cite journal|last=Foxman|first=Bruce M.|date=1975-01-01|title=X-Ray molecular structure of dicarbonyl-η5-cyclopentadienyl-(η2-tetramethylallenyl)iron tetrafluoroborate. A sterically crowded allene complex|url=https://pubs.rsc.org/en/content/articlelanding/1975/c3/c39750000221|journal=Journal of the Chemical Society, Chemical Communications|language=en|issue=6|pages=221–222|doi=10.1039/C39750000221|issn=0022-4936}}</ref><ref>{{Cite journal|last1=Wang|first1=Yidong|last2=Scrivener|first2=Sarah G.|last3=Zuo|first3=Xiao-Dong|last4=Wang|first4=Ruihan|last5=Palermo|first5=Philip N.|last6=Murphy|first6=Ethan|last7=Durham|first7=Austin C.|last8=Wang|first8=Yi-Ming|date=2021-09-22|title=Iron-Catalyzed Contrasteric Functionalization of Allenic C(sp 2 )–H Bonds: Synthesis of α-Aminoalkyl 1,1-Disubstituted Allenes|journal=Journal of the American Chemical Society|language=en|volume=143|issue=37|pages=14998–15004|pmid=34491051 | doi=10.1021/jacs.1c07512 |pmc=8458257 |issn=0002-7863}}</ref>

Fp-based reagents have been developed for [[cyclopropanation]]s.<ref>{{ OrgSynth | last= Mattson|first1= M. N.|last2= O'Connor|first2= E. J.|last3= Helquist|first3= P. | title = Cyclopropanation Using an Iron-Containing Methylene Transfer Reagent: 1,1-Diphenylcyclopropane | volume = 70 | pages = 177 | collvol = 9 | collvolpages = 372 | year = 1992 | prep = cv9p0372 }}</ref> The key reagent is prepared from FpNa with a [[thioether]] and [[methyl iodide]], and has a good shelf-life, in contrast to typical [[Simmons-Smith intermediate]]s and [[diazoalkane]]s.

:FpNa + ClCH₂SCH₃ → FpCH₂SCH₃ + NaCl

:FpCH₂SCH₃ + CH₃I + NaBF₄ → FpCH₂S(CH₃)₂]BF₄ + NaI

Use of [FpCH₂S(CH₃)₂]BF₄ does not require specialized conditions.

:{{chem|Fp(CH|2|S|+|(CH|3|)|2|)BF|4|−}} + (Ph)₂C=CH₂ → 1,1-diphenylcyclopropane + …

[[Iron(III) chloride]] is added to destroy any byproduct.

Precursors to {{chem|Fp{{=}}CH|2|+}}, like FpCH₂OMe which is converted to the iron [[carbene]] upon protonation, have also been used as cyclopropanation reagents.<ref>{{Citation|last=Johnson|first=M. D.|chapter=Mononuclear Iron Compounds with ''η''¹-Hydrocarbon Ligands|date=1982|chapter-url=https://linkinghub.elsevier.com/retrieve/pii/B9780080465180000490|pages=331–376|publisher=Elsevier|language=en|doi=10.1016/b978-008046518-0.00049-0|isbn=978-0-08-046518-0|access-date=2019-12-11|title=Comprehensive Organometallic Chemistry}}</ref>

===Photochemical reaction===

Fp₂ exhibits [[photochemistry]].<ref>{{cite journal | last= Wrighton|first= M. | title = Photochemistry of Metal Carbonyls | journal = [[Chemical Reviews]] | year = 1974 | volume = 74 | issue = 4 | pages = 401–430 | doi = 10.1021/cr60290a001}}</ref> For example, upon [[ultraviolet|UV]] irradiation at 350&nbsp;nm, it is reduced by benzylnicotinamide|1-benzyl-1,4-dihydronicotinamide dimer, also known as (BNA)₂.<ref>{{cite journal | last1= Fukuzumi|first1= S.|last2= Ohkubo|first2= K.|last3= Fujitsuka|first3= M.|last4= Ito|first4= O.|last5= Teichmann|first5= M. C.|last6= Maisonhaute|first6= E.|last7= Amatore|first7= C. | title = Photochemical Generation of Cyclopentadienyliron Dicarbonyl Anion by a Nicotinamide Adenine Dinucleotide Dimer Analogue |journal = [[Inorganic Chemistry (journal)|Inorganic Chemistry]] | year = 2001 | volume = 40 | issue = 6 | pages = 1213–1219 | doi = 10.1021/ic0009627 |pmid= 11300821}}</ref>

:[[File:1-benzyl-1,4-dihydronicotinamide dimer.png|650px]]

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{{iron compounds}}

[[Category:Dimers (chemistry)]]

[[Category:Half sandwich compounds]]