Wikipedia:WikiProject Chemicals/Chembox validation/VerifiedDataSandbox and Rhodocene: Difference between pages

{{short description|Organometallic chemical compound}}

{{Chembox

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 464382001

| ImageFile = Rhodocene-2D-skeletal.png

| ImageFile1 = Rhodocene-3D-balls.png

| ImageSize = 100px

| IUPACName = Rhodocene

| OtherNames = {{ubl|bis(cyclopentadienyl)rhodium(II)|dicyclopentadienylrhodium|rhodium(II) cyclopentadienide}}

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

| CASNo = 12318-21-7

| PubChem = 3082022

| Jmol = [cH-]1cccc1.[Rh+2].[cH-]1cccc1<!-- altered from SMILES to show correct -->

| C=10 | H=10 | Rh=1

| MeltingPtC = 174

| MeltingPt_notes = with decomposition (dimer)<ref name="El_Murr_1979" />

| Solubility ={{ubl| somewhat soluble in [[dichloromethane]] (dimer)<ref name="El_Murr_1979" />|soluble in [[acetonitrile]]<ref name="El_Murr_1979" />}}

| OtherCompounds = [[ferrocene]], [[cobaltocene]], [[iridocene]], [[bis(benzene)chromium]]}}

}}

'''Rhodocene''' is a [[chemical compound]] with the formula {{chem2|[Rh(C5H5)2]}}. Each molecule contains an atom of [[rhodium]] bound between two planar [[aromaticity|aromatic]] systems of five [[carbon]] atoms known as [[cyclopentadienyl]] rings in a [[sandwich compound|sandwich]] arrangement. It is an [[Organometallic chemistry|organometallic compound]] as it has ([[hapticity|haptic]]) [[covalent bond|covalent]] rhodium–carbon bonds.<ref name="Crabtree"/> The {{chem2|[Rh(C5H5)2]}} [[radical (chemistry)|radical]] is found above {{cvt|150|°C}} or when trapped by cooling to [[liquid nitrogen]] temperatures ({{cvt|−196|°C|F|sigfig=3|disp=sqbr}}). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a [[Dimer (chemistry)|dimer]], a yellow solid.<ref name="El_Murr_1979" /><ref name="Fischer_1966"/><ref name="Keller_1967"/>

The history of [[organometallic chemistry]] includes the 19th-century discoveries of [[Zeise's salt]]<ref name="Zeise discovery"/><ref name="Zeise review"/><ref name="DCW 1"/> and [[nickel tetracarbonyl]].<ref name="Crabtree"/> These compounds posed a challenge to chemists as the compounds did not fit with existing [[chemical bonding]] models. A further challenge arose with the discovery of [[ferrocene]],<ref name="Hoffman"/> the iron analogue of rhodocene and the first of the class of compounds now known as [[metallocene]]s.<ref name="FerroceneHistory"/> Ferrocene was found to be unusually [[chemical stability|chemically stable]],<ref name="Pauson_Kealy"/> as were analogous chemical structures including rhodocenium, the unipositive [[cation]] of rhodocene{{#tag:ref|The 18-valence electron cation {{chem2|[Rh(C5H5)2]+}} is called the rhodocenium cation in some journal articles<ref name="El_Murr_1979" /> and the rhodicinium cation in others.<ref name="JACS_1953" /> The former spelling appears more common in more recent literature and so is adopted in this article, but both formulations refer to the same chemical species.|name=cenium-cinium|group=Note}} and its [[cobalt]] and [[iridium]] counterparts.<ref name="JACS_1953"/> The study of organometallic species including these ultimately led to the development of new bonding models that explained their formation and stability.<ref name="Dewar-Chatt-Duncanson"/><ref name="ferrocene bonding"/> Work on sandwich compounds, including the rhodocenium-rhodocene system, earned [[Geoffrey Wilkinson]] and [[Ernst Otto Fischer]] the 1973 [[Nobel Prize in Chemistry|Nobel Prize for Chemistry]].<ref name="Nobel Prize"/><ref name="New Scientist Nobel Prize"/>

Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and [[Metal acetylacetonates|tris(acetylacetonato)rhodium(III)]];<ref name="JACS_1953" /> numerous other approaches have since been reported, including gas-phase redox [[transmetalation]]<ref name="JACS_1982"/> and using [[Half sandwich compound|half-sandwich]] precursors.<ref name="He_PhD"/> Octaphenylrhodocene (a derivative with eight [[phenyl group]]s attached) was the first substituted rhodocene to be isolated at room temperature, though it decomposes rapidly in air. [[X-ray crystallography]] confirmed that octaphenylrhodocene has a sandwich structure with a [[staggered conformation]].<ref name="Collins_1995"/> Unlike cobaltocene, which has become a useful one-electron [[reducing agent]] in research,<ref name="cobaltocenium"/> no rhodocene derivative yet discovered is stable enough for such applications.

[[Biomedical research]]ers have examined the applications of rhodium compounds and their derivatives in medicine<ref name="Rh in medicine"/> and reported one potential application for a rhodocene derivative as a [[radiopharmaceutical]] to treat small [[cancer]]s.<ref name="Wenzel"/><ref name="organ distrib"/> Rhodocene derivatives are used to synthesise linked metallocenes so that metal–metal interactions can be studied;<ref name="linked metallocenes"/> potential applications of these derivatives include [[molecular electronics]] and research into the mechanisms of [[catalysis]].<ref name="Wagner_2006"/>

==History==

[[File:Zeise's-salt-anion-from-xtal-3D-SF.png|thumb|left|[[Space-filling model]] of {{chem2|[(\h{2}C2H4)PtCl3]−}}, the [[anion]] of Zeise's salt, based on X-ray crystallographic data<ref name="Zeise anion 1"/><ref name="Zeise anion 2"/>]]

Discoveries in [[organometallic chemistry]] have led to important insights into [[chemical bonding]]. [[Zeise's salt]], {{chem2|K[PtCl3(C2H4)]·H2O}}, was reported in 1831<ref name="Zeise discovery" /> and [[Ludwig Mond|Mond's]] discovery of [[nickel tetracarbonyl]] ({{chem2|Ni(CO)4}}) occurred in 1888.<ref name="leigh-2002"/> Each contained a bond between a metal centre and small molecule, [[ethylene]] in the case of Zeise's salt and [[carbon monoxide]] in the case of nickel tetracarbonyl.<ref name="Zeise review" /> The [[space-filling model]] of the anion of Zeise's salt (image at left)<ref name="Zeise anion 1"/><ref name="Zeise anion 2"/> shows direct bonding between the [[platinum]] metal centre (shown in blue) and the carbon atoms (shown in black) of the ethylene [[ligand]]; such metal–carbon bonds are the defining characteristic of [[organometallic compound|organometallic species]]. Bonding models were unable to explain the nature of such metal–alkene bonds until the [[Dewar–Chatt–Duncanson model]] was proposed in the 1950s.<ref name="Dewar-Chatt-Duncanson"/><ref name="DCW 1"/><ref name="astruc-2007"/> The original formulation covered only metal–alkene bonds<ref name="leigh-2002"/> but the model was expanded over time to cover systems like [[metal carbonyl]]s (including {{chem2|[Ni(CO)4]}}) where [[π backbonding]] is important.<ref name=astruc-2007/>

{{multiple image

| direction = vertical

| width1 = 150

| image1 =Fulvalen.png

| caption1 = [[Fulvalene]], which Pauson and Kealy sought to prepare

| width3 = 90

| image3 =Ferrocene.svg

| caption3 = The [[structural formula]] of ferrocene

| width2 = 150

| image2 =Ferrocene_kealy.svg

| caption2 = The (incorrect) structure for [[ferrocene]] that Pauson and Kealy proposed

[[Ferrocene]], {{chem2|[Fe(C5H5)2]}}, was first synthesised in 1951 during an attempt to prepare the [[fulvalene]] ({{chem|C|10|H|8}}) by oxidative dimerization of [[cyclopentadiene]]; the resultant product was found to have [[molecular formula]] {{chem|C|10|H|10|Fe}} and reported to exhibit "remarkable stability".<ref name="Pauson_Kealy" /> The discovery sparked substantial interest in the field of organometallic chemistry,<ref name="Hoffman"/><ref name="FerroceneHistory" /> in part because the structure proposed by [[Peter Pauson|Pauson]] and Kealy was inconsistent with then-existing bonding models and did not explain its unexpected stability. Consequently, the initial challenge was to definitively determine the structure of ferrocene in the hope that its bonding and properties would then be understood. The sandwich structure was deduced and reported independently by three groups in 1952: [[Robert Burns Woodward]] and [[Geoffrey Wilkinson]] investigated the reactivity in order to determine the structure<ref name=wilkinson-1952/> and demonstrated that ferrocene undergoes similar reactions to a typical aromatic molecule (such as [[benzene]]),<ref name=werner-2008/> [[Ernst Otto Fischer]] deduced the sandwich structure and also began synthesising other [[metallocene]]s including [[cobaltocene]];<ref name="EOFischer"/> Eiland and Pepinsky provided [[X-ray crystallography|X-ray crystallographic]] confirmation of the sandwich structure.<ref name=eiland-1952/> Applying [[valence bond theory]] to ferrocene by considering an {{chem|Fe|2+}} centre and two cyclopentadienide anions (C₅H₅⁻), which are known to be [[aromaticity|aromatic]] according to [[Hückel's rule]] and hence highly stable, allowed correct prediction of the geometry of the molecule. Once [[molecular orbital theory]] was successfully applied, the reasons for ferrocene's remarkable stability became clear.<ref name="ferrocene bonding"/>

The properties of cobaltocene reported by Wilkinson and Fischer demonstrated that the unipositive cobalticinium cation {{chem2|[Co(C5H5)2]+}} exhibited stability similar to that of ferrocene itself. This observation is not unexpected given that the cobalticinium cation and ferrocene are [[isoelectronic]], although the bonding was not understood at the time. Nevertheless, the observation led Wilkinson and [[F. Albert Cotton]] to attempt the synthesis of rhodocenium<ref name=cenium-cinium group=Note/> and iridocenium [[salt (chemistry)|salts]].<ref name="JACS_1953" /> They reported the synthesis of numerous rhodocenium salts, including those containing the [[bromide|tribromide]] ({{chem2|[Rh(C5H5)2]Br3}}), [[perchlorate]] ({{chem2|[Rh(C5H5)2]ClO4}}), and [[Reinecke's salt|reineckate]] ({{chem2|[Rh(C5H5)2] [Cr(NCS)4(NH3)2]·H2O}}) anions, and found that the addition of dipicrylamine produced a compound of composition {{chem2|[Rh(C5H5)2] [N(C6H2N3O6)2]}}.<ref name="JACS_1953" /> In each case, the rhodocenium cation was found to possess high stability. Wilkinson and Fischer went on to share the 1973 [[Nobel Prize in Chemistry|Nobel Prize]] for Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, so called [[sandwich compound]]s".<ref name="Nobel Prize"/><ref name="New Scientist Nobel Prize"/>

The stability of metallocenes can be directly compared by looking at the [[reduction potential]]s of the one-electron [[reduction (chemistry)|reduction]] of the unipositive cation. The following data are presented relative to the [[saturated calomel electrode]] (SCE) in [[acetonitrile]]:

:{{chem2|[Fe(C5H5)2]+}} / {{chem2|[Fe(C5H5)2]}} +0.38&nbsp;V<ref name="ferrocenium redox couple"/>

:{{chem2|[Co(C5H5)2]+}} / {{chem2|[Co(C5H5)2]}} −0.94&nbsp;V<ref name="El_Murr_1979" />

:{{chem2|[Rh(C5H5)2]+}} / {{chem2|[Rh(C5H5)2]}} −1.41&nbsp;V<ref name="El_Murr_1979" />

These data clearly indicate the stability of neutral ferrocene and the cobaltocenium and rhodocenium cations. Rhodocene is ca. 500&nbsp;mV more reducing than cobaltocene, indicating that it is more readily oxidised and hence less stable.<ref name="El_Murr_1979" /> An earlier [[Polarography|polarographic]] investigation of rhodocenium perchlorate at neutral [[pH]] showed a cathodic wave peak at −1.53&nbsp;V (versus SCE) at the [[dropping mercury electrode]], corresponding to the formation rhodocene in solution, but the researchers were unable to isolate the neutral product from solution. In the same study, attempts to detect [[iridocene]] by exposing iridocenium salts to oxidising conditions were unsuccessful even at elevated pH. These data are consistent with rhodocene being highly unstable and may indicate that iridocene is even more unstable still.<ref name="JACS_1953" />

==Speciation==

The [[18-electron rule]] is the equivalent of the [[octet rule]] in [[main group]] chemistry and provides a useful guide for predicting the stability of [[organometallic compound]]s.<ref name="Kotz et al."/> It predicts that organometallic species "in which the sum of the metal valence electrons plus the electrons donated by the ligand groups total 18 are likely to be stable."<ref name="Kotz et al." /> This helps to explain the unusually high stability observed for ferrocene<ref name="Pauson_Kealy" /> and for the cobalticinium and rhodocenium cations<ref name="EOFischer" /> – all three species have [[Analog (chemistry)|analogous]] geometries and are [[isoelectronic]] 18-valence electron structures. The instability of rhodocene and cobaltocene is also understandable in terms of the 18-electron rule, in that both are 19-valence electron structures; this explains early difficulties in isolating rhodocene from rhodocenium solutions.<ref name="JACS_1953" /> The chemistry of rhodocene is dominated by the drive to attain an 18-electron configuration.<ref name="Kotz et al."/>

Rhodocene exists as {{chem2|[Rh(C5H5)2]}}, a [[paramagnetic]] 19-valence electron [[radical (chemistry)|radical]] [[monomer]] only at or below {{cvt|−196|°C}} ([[liquid nitrogen]] temperatures) or above {{cvt|150|°C}} in the [[gas phase]].<ref name="El_Murr_1979" /><ref name="Fischer_1966" /><ref name="Keller_1967" /> It is this monomeric form that displays the typical [[staggered conformation|staggered]] [[metallocene]] sandwich structure. At room temperature ({{cvt|25|°C|F|disp=sqbr}}), the lifetime of the monomeric form in [[acetonitrile]] is less than two seconds;<ref name="El_Murr_1979" /> and rhodocene forms {{chem2|[Rh(C5H5)2]2}}, a [[diamagnetic]] 18-valence electron [[bridge (chemical)|bridged]] [[Dimer (chemistry)|dimeric]] ''ansa''-metallocene structure.<ref name="Progress in IC"/> [[Electron spin resonance]] (ESR), [[nuclear magnetic resonance]] (NMR) and [[infrared spectroscopy|infrared spectroscopic]] (IR) measurements point to the presence of an [[chemical equilibrium|equilibrium]] interconverting the monomeric and dimeric forms.<ref name="Keller_1967"/> ESR evidence confirms that the monomer possesses a high order [[axis of symmetry]] (C_''n'', ''n''&nbsp;>&nbsp;2) with a [[mirror plane]] (σ) perpendicular to it as [[Molecular symmetry#Symmetry concepts|symmetry elements]]; this experimentally demonstrates that the monomer does possess the typical sandwich structure of a metallocene<ref name="Fischer_1966" />{{#tag:ref|The presence of a mirror plane perpendicular to the {{chem|C|5}} ring centroid–metal–ring centroid axis of symmetry suggests an eclipsed rather than a staggered conformation. Free rotation of cyclopentadienyl ligands about this axis is common in metallocenes – in ferrocene, the energy barrier to rotation is ~5&nbsp;kJ&nbsp;mol⁻¹.<ref name="ferrocene bonding" /> Consequently, there would be both staggered and eclipsed rhodocene monomer molecules co-existing, and rapidly interconverting, in the solution. It is only in the solid state that a definitive assignment of staggered or eclipsed conformation is meaningful.|group=Note}} although the interpretation of the ESR data has been questioned.<ref name="Progress in IC" /> The decomposition pathway of the monomer has also been studied by [[mass spectrometry]].<ref name="Mass spec"/> The dimerisation is a [[redox]] process; the dimer is a rhodium(I) species and the monomer has a rhodium(II) centre.{{#tag:ref|In the rhodocene dimer, the joined cyclopentadiene rings are shown with the H atoms in the [[endo-exo isomerism|"endo"]] position (i.e. the H's are inside, the other half of the ligands are on the outside). Although this is not based on crystal structure data, it does follow the illustrations provided by El Murr ''et al.''<ref name="El_Murr_1979" /> and by Fischer and Wawersik<ref name="Fischer_1966" /> in their discussion of the ¹H NMR data they collected. The paper by Collins ''et al.'',<ref name="Collins_1995" /> shows the H atoms in the "exo" position.|group=Note}} [[Rhodium]] typically occupies [[oxidation state]]s +I or +III in its stable compounds.<ref name="precious metals"/>

[[File:Rhodocene dimerisation.svg|Temperature-controlled equilibrium between rhodocene and its dimer]]

This dimerisation process has the overall effect of decreasing the [[electron counting|electron count]] around the rhodium centre from 19 to 18. This occurs because the [[oxidative coupling]] of the two cyclopentadienyl ligands produces a new ligand with lower [[hapticity]] and which donates fewer electrons to the metal centre. The term hapticity is used to indicate the "number of carbon (or other) atoms through which [a ligand] binds (''n'')"<ref name=hill-2002/> to a metal centre and is symbolised as η^''n''. For example, the ethylene ligand in Zeise's salt is bound to the platinum centre through both carbon atoms, and it hence formally has the formula {{chem2|K[PtCl3(\h{2}C2H4)]·H2O}}.<ref name="Zeise review"/> The carbonyl ligands in nickel tetracarbonyl are each bound through only a carbon atom and are hence described as monohapto ligands, but η¹-notations are typically omitted in formulae. The cyclopentadienyl ligands in many [[metallocene]] and [[Sandwich compound#Half-sandwich compounds|half-sandwich compounds]] are pentahapto ligands, hence the formula {{chem2|[Rh(\h{5}C5H5)2]}} for the rhodocene monomer. In the rhodocene dimer, the coupled cyclopentadienyl ligands are 4-electron tetrahapto donors to each rhodium(I) metal centre, in contrast to the 6-electron{{#tag:ref|There are two distinct approaches to [[electron counting]], based on either radical species or ionic species. Using the radical approach, a rhodium centre has 9 electrons irrespective of its oxidation states and a cyclopentadienyl ligand is a 5 electron donor. Using the ionic approach, the cyclopentadienyl ligand is a 6 electron donor and the electron count of the rhodium centre depends on its oxidation state – rhodium(I) is an 8 electron centre, rhodium(II) is a 7 electron centre, and rhodium(III) is a 6 electron centre. The two approaches generally reach the same conclusions but it is important to be consistent in using only one or the other.|group=Note}} pentahapto cyclopentadienyl donors. The increased stability of the 18-valence electron rhodium(I) dimer species as compared to the 19-valence electron rhodium(II) monomer likely explains why the monomer is only detected under extreme conditions.<ref name="El_Murr_1979"/><ref name="Keller_1967" />

[[File:Protonated rhodocene.svg|thumb|right|{{chem2|[(\h{5}C5H5)Rh(\h{4}C5H6)]}}, an 18-valence electron mixed hapticity rhodocene derivative<ref name="Fischer_1966" /> that can form when the rhodocene monomer is generated in [[protic solvent|protic solutions]]]]

Cotton and Wilkinson demonstrated<ref name="JACS_1953" /> that the 18-valence electron rhodium(III) rhodocenium cation {{chem2|[Rh(\h{5}C5H5)2]+}} can be reduced in aqueous solution to the monomeric form; they were unable to isolate the neutral product as not only can it dimerise, the rhodium(II) radical monomer can also spontaneously form the mixed-hapticity stable rhodium(I) species {{chem2|[(\h{5}C5H5)Rh(\h{4}C5H6)]}}.<ref name="Fischer_1966" /> The differences between rhodocene and this derivative are found in two areas:

# One of the bound cyclopentadienyl ligands has formally gained a hydrogen atom to become cyclopentadiene, which remains bound to the metal centre but now as a 4-electron η⁴- donor.

# The rhodium(II) metal centre has been reduced to rhodium(I).

These two changes make the derivative an 18-valence electron species. Fischer and colleagues hypothesised that the formation of this rhodocene derivative might occur in separate protonation and reduction steps, but published no evidence to support this suggestion.<ref name="Fischer_1966" /> (η⁴-Cyclopentadiene)(η⁵-cyclopentadienyl)rhodium(I), the resulting compound, is an unusual organometallic complex in that it has both a cyclopentadienyl anion and cyclopentadiene itself as ligands. It has been shown that this compound can also be prepared by [[sodium borohydride]] reduction of a rhodocenium solution in aqueous [[ethanol]]; the researchers who made this discovery characterised the product as biscyclopentadienylrhodium hydride.<ref name=green-1959/>

Fischer and co-workers also studied the chemistry of iridocene, the third transition series analogue of rhodocene and cobaltocene, finding the chemistry of rhodocene and iridocene are generally similar. The synthesis of numerous iridocenium salts including the tribromide and [[hexafluorophosphate]] have been described.<ref name="Keller_1967" /> Just as with rhodocene, iridocene dimerises at room temperature but a monomer form can be detected at low temperatures and in gas phase and IR, NMR, and ESR measurements indicate a chemical equilibrium is present and confirm the sandwich structure of the iridocene monomer.<ref name="Fischer_1966" /><ref name="Keller_1967" /> The complex {{chem2|[(\h{5}C5H5)Ir(\h{4}C5H6)]}}, the analogue of rhodocene derivative reported by Fischer,<ref name="Fischer_1966" /> has also been studied and demonstrates properties consistent with a greater degree of π-backbonding in iridium(I) systems than is found in the analogous cobalt(I) or rhodium(I) cases.<ref name=sazek-1991/>

==Synthesis==

Rhodocenium salts were first reported<ref name="JACS_1953"/> within two years of the discovery of ferrocene.<ref name="Pauson_Kealy"/> These salts were prepared by reacting the [[carbanion]] [[Grignard reaction|Grignard reagent]] cyclopentadienylmagnesium bromide ({{chem|C|5|H|5|Mg|Br}}) with [[Metal acetylacetonates|tris(acetylacetonato)rhodium(III)]] (Rh(acac)₃). More recently, gas-phase rhodocenium cations have been generated by a [[redox]] [[transmetalation]] reaction of rhodium(I) ions with ferrocene or [[nickelocene]].<ref name="JACS_1982"/>

:{{chem2|Rh+ + [(\h{5}C5H5)2M] → M + [(\h{5}C5H5)2Rh]+}} M = Ni or Fe

Modern [[microwave chemistry|microwave synthetic methods]] have also been reported.<ref name=baghurst-1990/> Rhodocenium hexafluorophosphate forms after reaction of cyclopentadiene and [[rhodium(III) chloride|rhodium(III) chloride hydrate]] in [[methanol]] following [[work-up (chemistry)|work-up]] with methanolic [[ammonium hexafluorophosphate]]; the reaction [[yield (chemistry)|yield]] exceeds 60% with only 30 seconds of exposure to [[microwave radiation]].<ref name=baghurst-1989/>

:<chem>{RhCl3.\mathit{x}H2O} + {2C5H6} + NH4PF6 -> {[(\eta^5-C5H5)2Rh]PF6}(v) + {2HCl} + {NH4Cl} + \mathit{x}H2O</chem>

Rhodocene itself is then formed by reduction of rhodocenium salts with molten [[sodium]].<ref name="Fischer_1966"/> If a rhodocenium containing melt is treated with sodium or potassium metals and then [[sublimation (phase transition)|sublimed]] onto a liquid nitrogen-cooled cold finger, a black polycrystalline material results.<ref name="Progress in IC" /> Warming this material to room temperature produces a yellow solid which has been confirmed as the rhodocene dimer. A similar method can be used to prepare the iridocene dimer.<ref name="Progress in IC" />

==Substituted rhodocenes and rhodocenium salts==

===The [(η⁵-C₅^''t''Bu₃H₂)Rh(η⁵-C₅H₅)]⁺ cation===

Novel approaches to synthesising substituted cyclopentadienyl complexes have been developed using substituted vinylcyclopropene starting materials.<ref name="vinylcyclopropene_1997"/><ref name="vinylcyclopropene_1998"/><ref name=hughes-1999/> Ring-enlarging [[vinylcyclopropane rearrangement]] reactions to produce cyclopentenes are well known<ref name=goldschmidt-1988/> and serve as precedent for vinylcyclopropenes [[rearrangement reaction|rearranging]] to cyclopentadienes. The [(η⁵-C₅[[tert-butyl|^''t''Bu]]₃H₂)Rh(η⁵-C₅H₅)]⁺ cation has been generated by a reaction sequence beginning with addition of the chlorobisethylenerhodium(I) dimer, [(η²-C₂H₄)₂Rh(μ-Cl)]₂, to 1,2,3-tri-''tert''-butyl-3-vinyl-1-cyclopropene followed by reaction with [[thallium cyclopentadienide]]:<ref name="vinylcyclopropene_1997" /><ref name="vinylcyclopropene_1998" />

:[[File:Synthesis of cation of ((C5tBu3H2)Rh(C5H5))BF4.svg|800px|Synthesis of the 1,2,3-tri-''tert''-butylrhodocenium cation from 1,2,3-tri-''tert''-butyl-3-vinyl-1-cyclopropene]]

[[File:1,2,3-tri-tert-butylrhodocenium.png|thumb|right|275px|Adapted from Donovan-Merkert et al.<ref name="vinylcyclopropene_1998" /> crystal structure determination, a representation of the cation from the [(η⁵-C₅^''t''Bu₃H₂)Rh(η⁵-C₅H₅)]BF₄ salt showing the carbon atom numbering. The near-vertical line contained within the plane of symmetry ({{fontcolor|purple|in purple}}) joins the metal centre to the [[centroid]]s of the cyclopentadienyl ligands. Hydrogen atoms are omitted for clarity.]]

The 18-valence electron rhodium(III) pentadienediyl species generated by this reaction demonstrates again the instability of the rhodocene moiety, in that it can be refluxed in toluene for months without 1,2,3-tri-''tert''-butylrhodocene forming but in oxidising conditions the 1,2,3-tri-''tert''-butylrhodocenium cation forms rapidly.<ref name="vinylcyclopropene_1997" /> Cyclic voltammetry has been used to investigate this and similar processes in detail.<ref name="vinylcyclopropene_1997" /><ref name="vinylcyclopropene_1998" /> The mechanism of the reaction has been shown to involve a loss of one electron from the pentadienediyl ligand followed by a fast rearrangement (with loss of a hydrogen atom) to form the 1,2,3-tri-''tert''-butylrhodocenium cation.<ref name="vinylcyclopropene_1998" /> Both the [[tetrafluoroborate]] and hexafluorophosphate salts of this cation have been structurally characterised by X-ray crystallography.<ref name="vinylcyclopropene_1998" />

[(η⁵-C₅^''t''Bu₃H₂)Rh(η⁵-C₅H₅)]BF₄ forms a colourless [[centrosymmetric]] [[monoclinic crystal system|monoclinic]] crystal belonging to the ''P''2₁/c [[space group]], and with a [[density]] of 1.486&nbsp;g&nbsp;cm⁻³.<ref name="vinylcyclopropene_1998" /> Looking at the [[molecular graphics|ORTEP]] diagram of the structure of the cation (at right), it is evident that it possesses the typical geometry expected of a rhodocene or rhodocenium cation. The two cyclopentadienyl rings are close to parallel (the [[centroid]]–Rh–centroid angle is 177.2°) and the rhodium centre is slightly closer to the substituted cyclopentadienyl ring (Rh–centroid distances are 1.819&nbsp;[[Ångström|Å]] and 1.795&nbsp;Å), an observation attributed to the greater inductive effect of the [[tert-butyl|''tert''-butyl]] groups on the substituted ligand.<ref name="vinylcyclopropene_1998" /> The ORTEP diagram shows that the cation adopts an eclipsed conformation in the solid state. The crystal structure of the hexafluorophosphate salt shows three crystallographically independent cations, one eclipsed, one staggered, and one which is rotationally disordered.<ref name="vinylcyclopropene_1998" /> This suggests that the conformation adopted is dependent on the anion present and also that the energy barrier to rotation is low – in ferrocene, the rotational energy barrier is known to be ~5&nbsp;kJ&nbsp;mol⁻¹ in both solution and gas phase.<ref name="ferrocene bonding" />

[[File:Structure of cation of ((C5tBu3H2)Rh(C5H5))BF4.png|thumb|650px|centre|Selected bond lengths (Å) (left) and bond angles (°) (right) for the unsubstituted (top) and substituted (bottom) cyclopentadienyl ligands in the cation from the salt [(η⁵-C₅^''t''Bu₃H₂)Rh(η⁵-C₅H₅)]BF₄.<ref name="vinylcyclopropene_1998" />]]

The diagram above shows the rhodium–carbon ({{fontcolor|red|in red,}} inside pentagons on the left) and carbon–carbon ({{fontcolor|blue|in blue,}} outside pentagons on the left) bond distances for both ligands, along with the bond angles ({{fontcolor|green|in green,}} inside pentagons on the right) within each cyclopentadienyl ring. The atom labels used are the same as those shown in the crystal structure above. Within the unsubstituted cyclopentadienyl ligand, the carbon–carbon bond lengths vary between 1.35&nbsp;Å and 1.40&nbsp;Å and the internal bond angles vary between 107° and 109°. For comparison, the internal angle at each vertex of a [[regular pentagon]] is 108°. The rhodium–carbon bond lengths vary between 2.16&nbsp;Å and 2.18&nbsp;Å.<ref name="vinylcyclopropene_1998" /> These results are consistent with η⁵-coordination of the ligand to the metal centre. In the case of the substituted cyclopentadienyl ligand, there is somewhat greater variation: carbon–carbon bond lengths vary between 1.39&nbsp;Å and 1.48&nbsp;Å, the internal bond angles vary between 106° and 111°, and the rhodium–carbon bond lengths vary between 2.14&nbsp;Å and 2.20&nbsp;Å. The greater variation in the substituted ligand is attributed to the distortions necessary to relieve the steric strain imposed by neighbouring ''tert''-butyl substituents; despite these variations, the data demonstrate that the substituted cyclopentadienyl is also η⁵-coordinated.<ref name="vinylcyclopropene_1998" />

The stability of metallocenes changes with ring substitution. Comparing the reduction potentials of the cobaltocenium and decamethylcobaltocenium cations shows that the decamethyl species is ca. 600&nbsp;mV more reducing than its parent metallocene,<ref name="cobaltocenium"/> a situation also observed in the ferrocene<ref name="decamethylferrocenium"/> and rhodocene systems.<ref name="decamethylrhodocenium"/> The following data are presented relative to the ferrocenium / ferrocene [[redox couple]]:<ref name=gagne-1980/>

{| class="wikitable"

! [[Half-reaction]] !! E° (V)

|-

| [Fe(C₅H₅)₂]⁺ + e⁻ ⇌ [Fe(C₅H₅)₂] || 0 (by definition)

|-

| [Fe(C₅Me₅)₂]⁺ + e⁻ ⇌ [Fe(C₅Me₅)₂] || −0.59<ref name="decamethylferrocenium"/>

|-

| [Co(C₅H₅)₂]⁺ + e⁻ ⇌ [Co(C₅H₅)₂] || −1.33<ref name="cobaltocenium" />

|-

| [Co(C₅Me₅)₂]⁺ + e⁻ ⇌ [Co(C₅Me₅)₂] || −1.94<ref name="cobaltocenium" />

|-

| [Rh(C₅H₅)₂]⁺ + e⁻ ⇌ [Rh(C₅H₅)₂] || −1.79<ref name="El_Murr_1979" /> †

|-

| [Rh(C₅Me₅)₂]⁺ + e⁻ ⇌ [Rh(C₅Me₅)₂] || −2.38<ref name="decamethylrhodocenium"/>

|-

| [(C₅[[tert-butyl|^''t''Bu]]₃H₂)Rh(C₅H₅)]⁺ + e⁻ ⇌ [(C₅[[tert-butyl|^''t''Bu]]₃H₂)Rh(C₅H₅)] || −1.83<ref name="vinylcyclopropene_1998" />

|-

| [(C₅^''t''Bu₃H₂)Rh(C₅Me₅)]⁺ + e⁻ ⇌ [(C₅^''t''Bu₃H₂)Rh(C₅Me₅)] || −2.03 <ref name="vinylcyclopropene_1998" />

|-

| [(C₅H₅Ir(C₅Me₅)]⁺ + e⁻ ⇌ [(C₅H₅Ir(C₅Me₅)] || −2.41<ref name="decamethyliridocenium"/> †

|-

| [Ir(C₅Me₅)₂]⁺ + e⁻ ⇌ [Ir(C₅Me₅)₂] || −2.65<ref name="decamethyliridocenium"/> †

|-

| † after correcting by 0.38 V<ref name="ferrocenium redox couple" /> for the different standard

|}

The differences in reduction potentials are attributed in the cobaltocenium system to the inductive effect of the alkyl groups,<ref name="cobaltocenium" /> further stabilising the 18-valence electron species. A similar effect is seen in the rhodocenium data shown above, again consistent with inductive effects.<ref name="vinylcyclopropene_1998" /> In the substituted iridocenium system, cyclic voltammetry investigations shows irreversible reductions at temperatures as low as −60&nbsp;°C;<ref name="decamethyliridocenium" /> by comparison, the reduction of the corresponding rhodocenes is quasi-reversible at room temperature and fully reversible at −35&nbsp;°C.<ref name="decamethylrhodocenium" /> The irreversibility of the substituted iridocenium reductions is attributed to the extremely rapid dimerisation of the resulting 19-valence electron species, which further illustrates that iridocenes are less stable than their corresponding rhodocenes.<ref name="decamethyliridocenium" />

===Pentasubstituted cyclopentadienyl ligands===

The body of knowledge concerning compounds with penta-substituted cyclopentadienyl ligands is extensive, with organometallic [[complex (chemistry)|complexes]] of the [[Pentamethylcyclopentadiene#References|pentamethylcyclopentadienyl]] and pentaphenylcyclopentadienyl ligands being well-known.<ref name=okuda-1992/> Substitutions on the cyclopentadienyl rings of rhodocenes and rhodocenium salts produce compounds of higher stability as they allow for the increased delocalisation of positive charge or [[electron density]] and also provide [[steric hindrance]] against other species approaching the metal centre.<ref name="Mass spec"/> Various mono- and di-substituted rhodocenium species are known, but substantial stabilisation is not achieved without greater substitutions.<ref name="Mass spec"/> Known highly substituted rhodocenium salts include decamethylrhodocenium hexafluorophosphate [(η⁵-C₅[[methyl|Me]]₅)₂Rh]PF₆,<ref name=koelle-1991/> decaisopropylrhodocenium hexafluorophosphate [(η⁵-C₅[[isopropyl|^''i''Pr]]₅)₂Rh]PF₆,<ref name="Buchholz_1994"/> and octaphenylrhodocenium hexafluorophosphate [(η⁵-C₅[[phenyl|Ph]]₄H)₂Rh]PF₆.<ref name="Collins_1995" />{{#tag:ref|There are common abbreviations used for molecular fragments in chemical species: "Me" stands for the [[methyl]] group, —CH₃; "^''i''Pr" stands for the [[isopropyl|''iso''-propyl]] group, —CH(CH₃)₂; "Ph" stands for the [[phenyl]] group, —C₆H₅; "^''t''Bu" stands for the [[tert-butyl|''tert''-butyl]] group, —C(CH₃)₃.|group=Note}} Decamethylrhodocenium tetrafluoroborate can be synthesised from the tris(acetone) complex [(η⁵-C₅Me₅)Rh(Me₂CO)₃](BF₄)₂ by reaction with [[pentamethylcyclopentadiene]], and the analogous iridium synthesis is also known.<ref name="tris(acetone)"/> Decaisopropylrhodicnium hexafluorophosphate was synthesised in [[1,2-dimethoxyethane]] ([[solvent]]) in an unusual [[one-pot synthesis]] that involves the formation of 20 [[carbon-carbon bond#Synthesis|carbon–carbon bonds]]:<ref name="Buchholz_1994" />

:[[File:Decaisopropylrhodicinium hexafluorophosphate synthesis.svg|One-pot synthesis of decaisopropylrhodocenium hexafluorophosphate from decamethylrhodocenium hexafluorophosphate]]

In a similar reaction, pentaisopropylrhodocenium hexafluorophosphate [(η⁵-C₅^''i''Pr₅)Rh(η⁵-C₅H₅)]PF₆ can be synthesised from pentamethylrhodocenium hexafluorophosphate [(η⁵-C₅Me₅)Rh(η⁵-C₅H₅)]PF₆ in 80% yield.<ref name="Buchholz_1994" /> These reactions demonstrate that the acidity of the methyl hydrogens in a pentamethylcyclopentadienyl complex can be considerably increased by the presence of the metal centre. Mechanistically, the reaction proceeds with [[potassium hydroxide]] deprotonating a methyl group and the resulting carbanion undergoing [[nucleophilic substitution]] with [[methyl iodide]] to form a new carbon–carbon bond.<ref name="Buchholz_1994" />

The compounds pentaphenylrhodocenium [[tetrafluoroborate]] [(η⁵-C₅Ph₅)Rh(η⁵-C₅H₅)]BF₄, and pentamethylpentaphenylrhodocenium tetrafluoroborate [(η⁵-C₅Ph₅)Rh(η⁵-C₅Me₅)]BF₄ have also been reported. They demonstrate that rhodium sandwich compounds can be prepared from half-sandwich precursors. For example, in an approach broadly similar to the tris(acetone) synthesis of decamethylrhodocenium tetrafluoroborate,<ref name="tris(acetone)" /> pentaphenylrhodocenium tetrafluoroborate has been synthesised from the tris([[acetonitrile]]) salt [(η⁵-C₅Ph₅)Rh(CH₃CN)₃](BF₄)₂ by reaction with [[sodium cyclopentadienide]]:<ref name="He_PhD"/>

:[(η⁵-C₅Ph₅)Rh(MeCN)₃](BF₄)₂ + NaC₅H₅ → [(η⁵-C₅Ph₅)Rh(η⁵-C₅H₅)]BF₄ + NaBF₄ + 3 MeCN

[[File:Staggered ferrocene and eclipsed ruthenocene.svg|thumb|The [[staggered conformation]] of ferrocene, [[molecular symmetry#Point Group|D_5d symmetry]] (left), and the [[eclipsed conformation]] of [[ruthenocene]], [[molecular symmetry#Point Group|D_5h symmetry]] (right)<ref name="ferrocene bonding" />]]

Octaphenylrhodocene, [(η⁵-C₅Ph₄H)₂Rh], is the first rhodocene derivative to be isolated at room temperature. Its olive-green crystals decompose rapidly in solution, and within minutes in air, demonstrating a dramatically greater air sensitivity than the analogous [[cobalt]] complex, although it is significantly more stable than rhodocene itself. This difference is attributed to the relatively lower stability of the rhodium(II) state as compared to the cobalt(II) state.<ref name="Collins_1995" /><ref name="precious metals" /> The reduction potential for the [(η⁵-C₅Ph₄H)₂Rh]⁺ cation (measured in [[dimethylformamide]] relative the ferrocenium / ferrocene couple) is −1.44&nbsp;V, consistent with the greater thermodynamic stabilisation of the rhodocene by the C₅HPh₄ ligand compared with the C₅H₅ or C₅Me₅ ligands.<ref name="Collins_1995" /> Cobaltocene is a useful one-electron [[reducing agent]] in the research laboratory as it is soluble in [[non-polar]] organic solvents,<ref name="cobaltocenium"/> and its redox couple is sufficiently well behaved that it may be used as an [[internal standard]] in [[cyclic voltammetry]].<ref name=stojanovic-1993/> No substituted rhodocene yet prepared has demonstrated sufficient stability to be used in a similar way.

The synthesis of octaphenylrhodocene proceeds in three steps, with a [[diglyme]] reflux followed by workup with [[hexafluorophosphoric acid]], then a [[sodium amalgam]] reduction in [[tetrahydrofuran]]:<ref name="Collins_1995"/>

:Rh(acac)₃ + 2 KC₅Ph₄H → [(η⁵-C₅Ph₄H)₂Rh]⁺ + 2 K⁺ + 3 acac⁻

:[(η⁵-C₅Ph₄H)₂Rh]⁺ + 3 acac⁻ + 3 HPF₆ → [(η⁵-C₅Ph₄H)₂Rh]PF₆ + 3 [[acetylacetone|Hacac]] + 2 PF₆⁻

:[(η⁵-C₅Ph₄H)₂Rh]PF₆ + Na/Hg → [(η⁵-C₅Ph₄H)₂Rh] + NaPF₆

The [[X-ray crystallography|crystal structure]] of octaphenylrhodocene shows a staggered conformation<ref name="Collins_1995" /> (similar to that of ferrocene, and in contrast to the [[eclipsed conformation]] of [[ruthenocene]]).<ref name="ferrocene bonding" /> The rhodium–centroid distance is 1.904&nbsp;Å and the rhodium–carbon bond lengths average 2.26&nbsp;Å; the carbon–carbon bond lengths average 1.44&nbsp;Å.<ref name="Collins_1995" /> These distances are all similar to those found in the 1,2,3-tri-''tert''-butylrhodocenium cation described above, with the one difference that the effective size of the rhodium centre appears larger, an observation consistent with the expanded ionic radius of rhodium(II) compared with rhodium(III).<ref name="vinylcyclopropene_1998" />

==Applications==

===Biomedical use of a derivative===

[[File:Haloperidol.svg|thumb|300px|The [[molecular structure]] of [[haloperidol]], a conventional [[Typical antipsychotic|antipsychotic]] pharmaceutical. The fluorophenyl group is at the left hand edge of the structure shown]]

There has been extensive research into [[metallopharmaceutical]]s,<ref name=clarke-1999/><ref name=jones-2007/> including discussion of rhodium compounds in medicine.<ref name="Rh in medicine"/> A substantial body of research has examined using metallocene derivatives of [[ruthenium]]<ref name=clarke-2002/> and iron<ref name=fouda-2007/> as metallopharmaceuticals. One area of such research has utilised metallocenes in place of the fluorophenyl group in [[haloperidol]],<ref name="Wenzel"/> which is a [[pharmaceutical]] classified as a [[typical antipsychotic]]. The ferrocenyl–haloperidol compound investigated has structure (C₅H₅)Fe(C₅H₄)–C(=O)–(CH₂)₃–N(CH₂CH₂)₂C(OH)–C₆H₄Cl and can be converted to the ruthenium analog via a transmetalation reaction. Using the [[radioactive]] [[isotope]] [[ruthenium-103|¹⁰³Ru]] produces a ruthenocenyl–haloperidol radiopharmaceutical with a high affinity for [[lung]] but not [[brain]] [[tissue (biology)|tissue]] in [[mice]] and [[rat]]s.<ref name="Wenzel" /> [[Beta-decay]] of ¹⁰³Ru produces the [[Nuclear isomer#Metastable isomers|metastable isotope]] [[rhodium-103m|^103''m''Rh]] in a rhodocenyl–haloperidol compound. This compound, like other rhodocene derivatives, has an unstable 19-valence electron configuration and rapidly oxidises to the expected cationic rhodocenium–haloperidol species.<ref name="Wenzel" /> The separation of the ruthenocenyl–haloperidol and the rhodocenium–haloperidol species and the distributions of each amongst bodily organs has been studied.<ref name="organ distrib"/> ^103''m''Rh has a [[half-life]] of 56&nbsp;min and emits a [[gamma ray]] of energy 39.8&nbsp;[[keV]], so the [[gamma-decay]] of the rhodium isotope should follow soon after the beta-decay of the ruthenium isotope. Beta- and gamma-emitting [[radionuclide]]s used medically include [[iodine-131|¹³¹I]], [[Radiopharmacology#Iron-59|⁵⁹Fe]], and [[Radiopharmacology#Calcium-47|⁴⁷Ca]], and ^103''m''Rh has been proposed for use in [[radiotherapy]] for small tumours.<ref name="Rh in medicine" />

===Metal–metal interactions in linked metallocenes===

[[File:Bi- and ter-metallocenes containing rhodocenyl groups.svg|thumb|450px|Structures of the hexafluorophosphate salts of rhodocenylferrocene, 1,1'-dirhodocenylferrocene, and 1-cobaltocenyl-1'-rhodocenylferrocene (from left to right), examples of bi- and ter-metallocenes<ref name="Chromatographia"/>]]

The original motivation for research investigations of the rhodocene system was to understand the nature of and bonding within the metallocene class of compounds. In more recent times, interest has been rekindled by the desire to explore and apply the metal–metal interactions that occur when metallocene systems are linked.<ref name="linked metallocenes"/> Potential applications for such systems include [[molecular electronics]],<ref name="Wagner_2006"/> semi-conducting (and possibly [[ferromagnetic]]) metallocene polymers (an example of a [[molecular wire]]),<ref name="linked metallocenes" /> and exploring the threshold between [[heterogeneous catalysis|heterogeneous]] and [[homogeneous catalysis]].<ref name="Wagner_2006" /> Examples of known bimetallocenes and termetallocenes that possess the rhodocenyl moiety include the hexafluorophosphate salts of rhodocenylferrocene, 1,1'-dirhodocenylferrocene, and 1-cobaltocenyl-1'-rhodocenylferrocene,<ref name="Chromatographia" /> each shown at right. Linked metallocenes can also be formed by introducing several metallocenyl substituents onto a single cyclopentadienyl ligand.<ref name="Wagner_2006" />

Structural studies of termetallocene systems have shown they typically adopt an "eclipsed double transoid" "crankshaft" geometry.<ref name="termetallocenes"/> Taking as an example the 1-cobaltocenyl-1'-rhodocenylferrocene cation shown above, this means that the cobaltocenyl and rhodocenyl moieties are eclipsed, and thus carbon atoms 1 and 1' on the central ferrocene core are as close to vertically aligned as is possible given the staggered conformation of the cyclopentadienyl rings within each metallocene unit. Viewed from side-on, this means termetallocenes resemble the down–up–down pattern of a [[crankshaft]].<ref name="termetallocenes" /> The synthesis of this termetallocene involves the combining of rhodocenium and cobaltocenium solutions with [[Ferrocene#Lithiation|1,1'-dilithioferrocene]]. This produces an uncharged intermediate with linked cyclopentadienyl–cyclopentadiene ligands whose bonding resembles that found in the rhodocene dimer. These ligands then react with the [[triphenylmethyl hexafluorophosphate|triphenylmethyl carbocation]] to generate the termetallocene salt, [(η⁵-C₅H₅)Rh(μ-η⁵:η⁵-C₅H₄–C₅H₄)Fe(μ-η⁵:η⁵-C₅H₄–C₅H₄)Co(η⁵-C₅H₅)](PF₆)₂. This synthetic pathway is illustrated below:<ref name="Chromatographia" /><ref name="termetallocenes" />

:[[File:Synthesis of 1-cobaltocenyl-1'-rhodocenylferrocene cation.svg|700px|Synthesis of the 1-cobaltocenyl-1'-rhodocenylferrocene cation, an example of a termetallocene]]

===Rhodocenium-containing polymers===

The first rhodocenium-containing side-chain polymers were prepared through controlled polymerization techniques such as [[reversible addition−fragmentation chain-transfer polymerization]] (RAFT) and [[ring-opening metathesis polymerisation]] (ROMP).<ref name="Yan-2015"/>

==Notes==

{{Reflist|group=Note}}

==References==

{{Reflist|30em|refs=

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<ref name="baghurst-1989">{{Cite journal |last1= Baghurst |first1= D. R. |last2= Mingos |first2= D. M. P. |author-link2= Michael Mingos |last3= Watson |first3= M. J. |year= 1989 |title= Application of Microwave Dielectric Loss Heating Effects for the Rapid and Convenient Synthesis of Organometallic Compounds |journal= [[Journal of Organometallic Chemistry]] |volume= 368 |issue= 3 |pages= C43–C45 |doi= 10.1016/0022-328X(89)85418-X}}</ref>

<ref name="baghurst-1990">{{Cite journal |last1= Baghurst |first1= D. R. |last2= Mingos |first2= D. M. P. |author-link2= Michael Mingos |year= 1990 |title= Design and Application of a Reflux Modification for the Synthesis of Organometallic Compounds Using Microwave Dielectric Loss Heating Effects |journal= [[Journal of Organometallic Chemistry]] |volume= 384 |issue= 3 |pages= C57–C60 |doi= 10.1016/0022-328X(90)87135-Z}}</ref>

<ref name="Buchholz_1994">{{Cite journal |last1= Buchholz |first1= D. |last2= Astruc |first2= D. |year= 1994 |title= The First Decaisopropylmetallocene – One-Pot Synthesis of [Rh(C₅''i''Pr₅)₂]PF₆ from [Rh(C₅Me₅)₂]PF₆ by Formation of 20 Carbon–Carbon Bonds |journal= [[Angewandte Chemie International Edition]] |volume= 33 |issue= 15–16 |pages= 1637–1639 |doi =10.1002/anie.199416371 }}</ref>

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<ref name="clarke-1999">{{cite book |author1= Clarke, M. J. |author2= Sadler, P. J. |year= 1999 |title= Metallopharmaceuticals: Diagnosis and therapy |location= Berlin |publisher= Springer |isbn= 3-540-65308-2}}</ref>

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<ref name="Crabtree">{{cite book |last= Crabtree |first= R. H. |author-link= Robert H. Crabtree |year= 2009 |title= The Organometallic Chemistry of the Transition Metals |edition= 5th |location= Hoboken, NJ |publisher= John Wiley and Sons |isbn= 978-0-470-25762-3 |page= 2 |url= https://books.google.com/books?id=WLb962AKlSEC&pg=PA2 |quote= An industrial application of transition metal organometallic chemistry appeared as early as the 1880s, when Ludwig Mond showed that nickel can be purified by using CO to pick up nickel in the form of gaseous Ni(CO)₄ that can easily be separated from solid impurities and later be thermally decomposed to give pure nickel.<p>... Recent work has shown the existence of a growing class of metalloenzymes having organometallic ligand environments – considered as the chemistry of metal ions having C-donor ligands such as CO or the methyl group</p>}}</ref>

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<ref name="decamethylrhodocenium">{{cite journal |last1= Gusev |first1=O. V. |last2= Denisovich |first2=L. I. |last3= Peterleitner |first3=M. G. |last4= Rubezhov|first4=A. Z. |first5=Nikolai A. |last5= Ustynyuk |last6= Maitlis |first6=P. M. |author-link6= Peter Maitlis |year= 1993 |title= Electrochemical Generation of 19- and 20-electron Rhodocenium Complexes and Their Properties |journal= [[Journal of Organometallic Chemistry]] |volume= 452 |issue= 1–2 |pages= 219–222 |doi= 10.1016/0022-328X(93)83193-Y}}</ref>

<ref name="decamethyliridocenium">{{cite journal |last1= Gusev |first1=O. V. |last2= Peterleitner |first2=M. G. |last3= Ievlev |first3=M. A. |last4= Kal'sin|first4=A. M. |last5= Petrovskii|first5=P. V. |last6= Denisovich |first6=L. I. |first7=Nikolai A. |last7= Ustynyuk |year= 1997 |title= Reduction of Iridocenium Salts [Ir(η⁵-C₅Me₅)(η⁵-L)]+ (L= C₅H₅, C₅Me₅, C₉H₇); Ligand-to-Ligand Dimerisation Induced by Electron Transfer |journal= [[Journal of Organometallic Chemistry]] |volume= 531 |issue= 1–2 |pages= 95–100 |doi= 10.1016/S0022-328X(96)06675-2}}</ref>

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