Cyclopentadiene

CYCLOPENTADIENE AND SUBSTITUTED CYCLOPENTADIENE 1.INTRODUCTION: FEATURES: Cyclopentadiene is a chemical compound with formula C5H6 .It is a colourless liquid at room temperature and has a strong and unpleasant odour. It spontaneously undergoes Diels-Alder reaction at room temperature to form dimer dicyclopentadiene , this reaction is reversible at elevated temperature via retro Diels-Alder reaction. Molecular formula Molar Mass Appearance Density Solubility in water structure

C5H6 66.10 gm/mol Colourless liquid (at room temp) 0.81 gm/cm3 ,liquid Insoluble planar

STRUCTURE AND BONDING: Cp is planar cyclic molecule . The anionic Cp molecule is an aromatic compound with 6π electrons . Due to stability of anionic form of Cp is acidic with Pka = 16 which is unusual for hydrocarbons.The anionic Cp reacts with many chemical species . The anion acts as a weak nucleophile and it forms numerous cyclopentadiene complexes, such as metallocenes. Cp ligands can form η1 , η3 and η5 complexes. The MO scheme for C5H5 group The five p orbitals on carbon give rise o five molecular orbitals for C5H5- group. Important overlaps are ψ1 with dz2 , ψ2 and ψ3 with the dxz and dyz orbitals ψ4 and ψ5 do not interact very strongly with metal orbitals and Cp compound is therefore not a good π acceptor .This and the anionic charge on Cp means that Cp compound are generally basic , and presence of Cp encourages back donations from metal to other \ligands present.

1

MO scheme:

1

2 3

4 5

Diels –Alder Reaction : The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkenes, commonly termed the dienophile, to form a substituted cyclohexene system Cyclopentadiene undergoes Diels –Alder reaction to form its dimer bridged

2

2 This reaction is reversible and dicylopentadiene when heated over 1000C , it produces cyclopentadiene via Retro Diels Alder reaction. This is also called cracking of Cp2. 100 0C

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PENTAMETHYLCYCLOPENTADIENE: 1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H, which is often denoted as Cp*, in contrast to Cp it does not dimerize. The 1,2,3,4,5-pentamethylcyclopentadienyl ligand is a complexing agent of great utility in main group, transition metal, and f-element organomettallic chemistry. The electron density of this ligand is greater than compared to unsubstituted cyclopentadienyl ligands, and generally imparts enhanced solubility and crystallizability to a variety of metal complexes. As an ancilliary ligand, it increases thermal stability in a number of metal hydrocarbyl systems. It is a bulky ligand so its steric effects are also considerable and important, for actinides it leads to coordinative unsaturation by restricting the number of cyclopentadienyl groups that can bee bound to the metal ion. Pentamethylcyclopentadiene is commercially available. It was first prepared from tiglaldehyde via 2,3,4,5-tetramethylcyclopent-2enone. Alternatively 2-butenyllithium adds to ethylacetate followed by acidcatalyzed dehydrocyclization: MeCH=C(Li)Me + MeC(O)OEt → (MeCH=C(Me))2C(OLi)Me + LIOEt (MeCH=C(Me))2C(OLi)Me + H+ → Cp*H + H2O + Li+ 3

Preparation of Cp* : A.

Li

Br +

CH3CH=C

Et2O

2Li

CH3CH=C

CH3

+

LiBr

CH3 OH

Li Et2O

CH3CH=C

+

H2O

O

CH3

OH

C7H7SO3H B.

-H2O

COMPARISON BETWEEN Cp AND Cp*: Cp* is sterically more bulky and electron density on this is also greater than Cp. Being more electron-rich, Cp* is a stronger donor and is less easily removed from the metal. Consequently Cp* complexes exhibit increased thermal stability. Magnanocene and Rhenocenes demonstrate the effect of methyl substitution in its properties. Whereas Cp2Mn cannot be reduced and monomeric Cp2Re has only been detected in low-temperature matrices, both decamethylmagnanocene and decamethylrhenocene form stable 18 VE anions.

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CYCLOPENTADIENYL COMPLEXES The most important complexes of Cp is metallocenes,the metals bonded to two cyclopentadienyls Cp in a pentahapto mode form the metallocenes. In many other families of compounds , the metal is bonded to only one of Cp or derivatives. Common examples of this category include ternary families containing, besides the Cp type ligand, carbonyls, phosphines, nitrosyls , hydrides, oxo imido methyl, etc. Some common examples :

Fe

Co(CO)2

CO

OC Mo

Mo

CO Fe

OC CO

CO

Me3P

PMe3 PMe3

O N Cr

Cr

W ON

Cl

Cl Cl

Cl

5

NO

N O

Re O

Ta O

H

O

H H

H

Ir

Ta Me

N

Me Me

Me

t-Bu

EXAMPLE OF COMLEXES OF Cp AND Cp*(above)

Cyclopentadiene is a good spectator ligand for a whole series of complexes CpMLn, where we want chemistry to occur at MLn group or other ligand groups. The one very important complex family based on Cp are metallocenes (e.g.ferrocene) , other classic families are [MCp(CO)3]2,M=Cr,Mo,or W,[NiCp(CO)]2,mononuclear [VCp(CO)4],[MCp(CO)3],M=Mn,Tc, or Re, [MCp(CO)2] ,M=Co,Rh,or Ir and tertiary nitrosyl complexes. With neutral Cp complexes such as [VCp(CO)4],[MnCp(CO)3], it is possible to achieve electrophilic substitution on the Cp ring similar to those known with ferrocene, which allows to functionalize complexes in order to branch them on polymers, electrodes, dendrimers and derivated silica for chromatography. Although most of the time Cp forms a pentahapto complex, but there are complexes of main group and transition metals containing one or several Cp ligands that are monohapto coordinated.

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METALLOCENES The first metallocenes discovered was ferrocene, the discovery of structure of this compound by Wilkinson, Woodword and fischer in 1954 has been a starting point for the boom of chemistry of π complexes. The prediction of sandwich structure and aromaticity in this compound and further research by Fischer and Wilkinson in the field of organomettallic chemistry resulted in both being awarded Nobel prize in 1973. The family of metallocenes and decamethylmetallocenes (MCp*2) now includes all transition and main group metals. These have got multiple applications in the field of materials and molecular engineering. Molecular fragments, modified electrodes for redox catalysis (titration of glucose in blood), polymers and dendrirtic electrochemical sensors for molecular recognition , antitumor drugs and paintings. Metallocenes , in a broad sense , have the composition [MCp2] and are known for all the transition metal and many main group and rare earth metals. Some common examples of metallocenes are [FeCp2], [VCp2], [CrCp2], [NiCp2], [NbCp2] .

STRUCTURE OF SOME METALLOCENES:

Fe

M

A general metallocene structure

ferrocene

Co

Cr

7

Chromrcene

Cobaltocene

Ni

Os

Nickelocene

Osmocene

STRCTURE, BONDING AND PROPERTIES: Ferrocene was the first metallocene which structure was deduced in 1954. The structure was revealed by two groups in Cambridge (USA) and Munich (Germany) based on experimental evidences: Fischer: “Double –cone structure” based on*X-ray structural analysis *Diamagnetism *Chemical behaviour

Wilkinson, Woodward : “sandwich structure” based on*IR spectroscopy *Diamagnetism *Dipole moment = 0

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Bonding in metallocenes: If we put two Cp groups and one metal together , we obtain the MO diagram for a metallocene (shown in figure). After this we have to look for symmetry of Cp orbitals and how they interact with the metal orbitals.

MO orbital diagram and interactions in ferrocene: In the given figure the interaction of Cp MO are described with that of iron in the process of formation of ferrocene. Crystal field splitting pattern for Cp is shown and molecular orbital formation in Cp and their interaction with orbitals of Fe and filling of electrons in molecular orbitals is also shown.

(Formation of nodes in Cyclopentadiene)

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4p

4

4s

5

3d

2 

1

Group orbitals

Fe

Fe

Bonding between metal atoms and Cp (using Fe as metal atom)

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The d orbital occupation pattern for some first-row mettalocenes: The splitting pattern for d orbitals is approximately same as for octahedral ML6. Theoretical magnetic moment (µ=(n(n+1))0.5), no of unpaired electrons , experimental magnetic moment in, and colour of neutral metallocenes is also given along with.

NEV

VCp2

CrCp2

15e

16e

MnCp2 17e

FeCp2

CoCp2

18e

19e

NiCp2 20e

e*1g (xy, yz)

 bond

a'1g z2

 bond 

e2g

backbonding

2 2 x -y ,xy

unpaired electrons

3

 B (theoretical) 3.87

2

5

0

1

2

2.83

5.92

0

1.73

2.93

 B (exper.)

3.84

3.20

5.81

0

1.76

2.86

Colour

purple

scarlet

brown

orange

purple

green

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Different methods for preparation of metallocenes: A cyclopentadienyl compound was as early as 1901 by Thiele in the reaction:

K

C5H6

benzene

K

C5H5

½ H2

Half a century later the discovery of this compound organomettallic compound again become the matter of focus with discovery of ferrocene and its properties. Two independent discoveries of ferrocene were reported – *Miller, Tebboth, Tremaine (1951) 300 0C 2C5H6 + Fe

(C5H5)2Fe + H2 Al, Mo oxides

The actual goal was synthesis of organic amine from dinitrogen and cyclopentadiene in the presence of iron powder. *Kealy, Pauson (1951) 3C5HMgBr

Fe(C5H5)2

FeCl3

½ C10H10

3MgBrCl

The actual aim of this synthesis was fulvalene (C 10H 8). Binary Cyclopentadienyl metal complexes: Preparation: 1. Metal salt + cyclopentadienyl reagent Dicyclopentadiene is cracked first in a retro diels-alder reaction to give monomeric C5H6. Cyclopentadiene is a weak acid (Pka=15) and can be deprotonated by strong bases or by alkali metals. NaCp is most easily commercially available reagent for the introduction of cyclopentadienyl reagent.

Cp2

180 0C

2Cp

Na

MCl2 + 2NaC5H5

2NaC5H5

C5H5)2M

H2

; M =V, Cr, Mn, Fe, Co

(Solvent=THF, DME, NH3) 12

Ni(acac)2

2C5H5MgBr

(C5H5)2Ni

2(acac)MgBr

NaC5H5 can behave as a ligand source as well as reducing agent also: CrCl3

(C5H5)2Cr

3NaC5H5

½ C10H10

3NaCl

In many cases, treatment of NaC5H5 with metal (4d, 5d) salts does not give Cp2M complexes, but rather Cp metal hydrides or complexes that bear σbonded Cp rings. TaCl5

ReCl5

NaCp

NaCp

NaCp

(5-Cp)2TaCl3

(5-Cp)2Ta(1-Cp)2

( 5-Cp)2ReH

2. Metal + Cyclopentadiene

M

MC5H5

C5H6

M + 2C5H6

500 0C

1/2H2

M=Li, Na, K

(C5H5)2M + H2

M=Mg, Fe

3. Metal salt + Cyclopentadiene In this process auxiliary base is needed if the basicity of the salt anion is in sufficient to deprotonate cyclopentadiene:

Tl2SO4 + 2C5H6 + 2OH

H2O

2C5H5Tl + 2H2O + SO42

FeCl2 + 2C5H6 + 2Et2NH

(C5H5)2Fe + 2[Et2NH2]ClIn

other cases a reducing agent is needed: RuCl3(H2O)x + 3C5H6 + 3/2 Zn

EtOH

13

(C5H5)2Ru + C5H8 + 3/2Zn2+

Some properties of metallocenes: Complex

Colour

(C5H5)2Ti

Green

(C5H5)2V (C5H5)2Nb

purple yellow

(C5H5)2 Cr (C5H5)2 Mo

scarlet black

(C5H5)2W (C5H5)2Mn

Yellow Green Brown

(C5H5)2Fe

orange

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(C5H5)2Co

Purple black Green

174

(C5H5)2Ni

Melting Miscellaneous 0 Point ( C ) 200 Dimeric with two µ-h bridges and a fulvalenediyl bridging ligand 167 Very air sensitive Dimeric with η1: η5-C5H4 bridges and terminal hydride ligands 17 3 Very air sensitive Several dinuclear isomers with fulvalenediyl or η1- η5 bridges and terminal hydride ligands, Diamagnetic Same as above 173

Air sensitive and readily hydrolyzed ; At 158 0C converted into pink form Air stable ; can be oxidized to blue green (C5H5)2Fe+ Air-sensitive, oxidation gives the air stable, yellow cation (C5H5)2Co+ Slow oxidation by air to the labile, orange cation (C5H5)2Ni+

173

CHEMICAL PROPERTIES OF METALLOCENES: 1.REDOX PROPERTIES:A key property of metallocene is their ability to exist in the form of various oxidation states with the sandwich structure and a variable no. Of d-electrons (no.of valence electrons between 14 to 20), even if the stability decreases as the NVE changes from 18. Each metallocene exists in various oxidation states which is usually not the case for other families inorganic or organomettallic compounds. Monocationic metallocenes have been isolated and their stability is modest when their NVE is different from 18, but dications are only isolable for decamethylmetallocenes that are much more robust. The ferrocenium cations are stable as various salts, but they are slightly air and light sensitive in solution. 14

these salts are very often used as mild monoelectronic oxidants, and the reduction of ferrocenium salt can be easily achieved using an aqueous solution of dithionite or TiCl3 or in organic solution using [FeCp*2] :

1. H2SO4 or HNO3 2. Aq. HPF6

Fe(Cp)2

[Fe(Cp)2]+ PF6-

3. Na2S2O3 or TiCl3

SELECTED REACTIONS OF METALLOCENES: Owing to its high redox stability, ferrocene shows good organic reactivity. Thus ferrocene , ruthenocene, and osmocene are susceptible to electrophilic substitution. Ferrocene reacts 3*106 times faster than benzene: E E H

E Fe

Fe

-HH

Fe

FRIEDAL-CRAFT ACYLATION:

The electrophile should not be an oxidizing agent, as substitution would then be suppressed by oxidation to the ferricinium ion [FeCp2]+ .

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H

H

H Fe

Fe

Fe

H

MANNICH REACTION (aminomethylation) : CH2N(CH3)2

HCHO/HN(CH3)2 Fe

Fe

HOAc

OXIDATION PROPERTIES OF METALLOCENE: Ferrocene is easily oxidisable complex and its oxidation can be done chemically as well as using oxidising agents (e.g. HNO3). -e(C5H5)2Fe 18 VE

E0 = +0.40V, MeCN, NBu4PF6 Vs SCE

[(C5H5)2Fe]+ 17VE

the ferrocene/ferricinium couple has been accepted as internal standard in organomettallic cyclovoltammetry. Ruthenocene and osmocene can also be oxidised, but corresponding metallicillium ions are not stble as monomers. Dimerization occurs or disproportionation reaction occurs to yield Cp2M and [Cp2M]2+. In contrast permethylated complex (C5Me5)2Ru forms a cation [(C5Me5)2Ru]+, which is stable at -30 0C.

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The metallocenes of cobalt and nickel have the tendency to attain 18 VE configurations:

Cobaltocene is a good one-electron reducing agent. It is soluble inmany polar and non-polar media, and it generally forms diamagnetic cation [Cp2Co]+. Cobalticinium salts are very stable to further oxidation; with strong oxidising agents, the dimethylcobalticinium ion gives carboxylic acid without cleavage of Cp-Co bonds:

Cobaltocene acts as a reducing agent towards alkyl halides:

Organic radicals add to cobaltocene. The η5-cyclopentadienyl ligand is thereby converted into an η4-cyclopentadienyl ligand with attendant charge from an 19 VE to an 18 VE configuration:

Ferrocene and its derivatives: Ferrocene undergoes many electrophilic reactions, more rapidly than benzene, although they are limited by oxidation reactions with electrophiles that are strong oxidants (H2SO4 or HNO3). Formylation and carboxylation gives only monofunctionlization, because the functional group strongly deactivates the ferrocenyl group. On the other hand, metallation and acylation reactions can be followed by an identical reaction on the other ring leading to 1, 1’-disubstituted 17

derivatives, because the deactivation of the second Cp ring by substituent is only modest. Metalation of ferrocene:

The metalation of ferrocene with organolithium reagents is hampered by problems with mono-, and multiple lithiation. The preparation of monolithium ferrocene is done using SnBu3 and further forming organostannylferrocene. 1,1’-dilithioferrocene is obtained by reaction in presence of TMEDA.

The dilithio product precipitates as adduct of TMEDA having a complicated structure and is used as such in further reactions.

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19

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APPLICATIONS OF FERROCENE: Ferrocene has got many application in various fields of chemistry such as catalysts, material science, drugs, polymers. It is used in materil science for assembling charge transfer complexes(decamethylferrocene) and thermotropic liquid crystals (example given below).

Ferrocenyl phosphines are used as ligand for catalysis. 1-1’-bis(diphenylpsosphino)ferrocene (dppf) is the best known ferrocenyl-based ligand in the catalysis of many classic reactions and its chemistry and uses are numerous.

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The redox property of ferrocene has been used to attech it to macrocycles (example given below), cryptands, calixarenes and other endo receptors for sensing, a area of chemistry developed by Paul Beer at oxford.

Ferrocenes are key components of polymers. Ferrocene containing polymers have also been synthesized using cationic, radical or romp polymerization. They are used to derivatize interalia electrodes, which can provide excellent redox sensors. E.g. ring-opening polymerization of ferrocenophenes:-

Ferrocenes as many other cyclopentadienyl-metal complexes have been studied as antitumor drugs or therapeutic agents. The redox activity of ferrocenyl group may also add synergistic effects on cytotoxicity.

“Hydroxyferrocifen” related to anticancer drug tamoxifen in which a ferrocenyl group has replaced a phenyl substituent with benefit on the cytotoxicity. 22

18 ELECTRON RULE The 18-electron rule is a way to predict comparative stabilities of mainly transition metal complexes. Not all the formulas we write down are stable in nature and also their relative stabilities vary depending upon many factors. Like in the case of simple compounds filled octet (noble gas structure) is a important notion, for example CH5 doesn’t exist in nature as it requires a pentavalent carbon atom. Stable compounds such as CH4, NH3, and H2O etc have noble gas octet. So in this case we may call carbon to be following an 8e rule. This is due to carbon using one s-orbital, and three p-orbital for its bonding. For filling these 4 orbitals 8 electrons are required and we may consider 4-electrons comes from four H-atom and rest 4-electrons comes from carbon atom. And here H can be considered as 1e donating ligand to carbon. The transition metals now have 1 ns, 3 np and 5 (n-1) d orbitals, we need 18-electron to fill these 9 molecular orbitals; some electrons to fill these orbitals come from metal itself and rest comes from ligands attached to the metal. Some of these 9 molecular orbitals are bonding, and some are non-bonding or antibonding. Only limited number of combinations of metal and ligands fulfil 18-electron criteria. 18-electron rule is followed by a majority of complexes (many exceptions also). The 18-electron electron structure brings a good stability for complexes. ELCTRON COUNTS FOR COMMON LIGANDS: There are two models for counting the electrons in a complex: ionic model and covalent model, both of which lead to exactly same results. They differ in the way that electrons are considered to be coming from ligand or from the metal. For example consider ferrocene: Ionic model Fe2 2C5H5

Covalent model Fe

6e

2C5H5

6*2= 12e 18e

23

8e 2*5= 10e 18e

The ways of assigning electrons are just models and moreover each bond between two dissimilar atoms have some ionic and some covalent characteristics. The covalent model is more appropriate for the majority of lowvalent transition metal complexes especially with unsaturated ligands. In the same manner ionic model is more suitable for high-valent complexes with N, O, or Cl ligands. Some common ligands with electron donor power: LIGANDS

Formal charge

CO (bridging or terminal) NO PPh3 µ-CH2 µ-CR2 H (terminal or bridging hydride) X (terminal) µ-X OR, SR (terminal) (bridging) NR2, PR2 (terminal) (bridging) Me or other alkyl Cp

0 0 0 0 0 -1 -1 -1 -1 -1 -1 -1 -1 -1

HAPTICITY: The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. In most of the cases it is number of ligand atoms bound to central metal atom. The Greek letter η (eta) is used to show hapticity of a ligand in formula. For example: Ferrocene - bis(η5-cyclopentadienyl)iron Uranocene - bis(η8-1,3,5,7-cyclooctatetraene)uranium Zeise's salt - K[PtCl3(η2-C2H4)].H2O 24

-

No. Of e donated 2 3 2 2 2 2 2 4 2 4 2 4 2 6

SOME EXAMPLES OF 18-ELECTRON RULE: Ionic model Ni2+

8e

2*C3H5-

8e 16e

Mo

6e

Covalent model

Ni

Ni

10e

2*C3H5

6e 16e

Mo

6e

Mo(C6H6)2 2*C6H6

12e 18e

2*C6H6

12e 18e

Fe

6e

Fe

8e

2C5H5

12e 18e

2C5H5

10e 18e

Mo4+

2e

Mo

6e

4*H

8e

4*H

4e

4*PR3

8e 18e

4*PR3

8e 18e

Co3+

6e

2*C5H5

12e

Fe

MoH4(PR3)4

Co

18e

Co

9e

2*C5H5

10e

positive charge -1e 18e

4 Fe2+

CN

6e

CN

NC

6*CN

Fe

12e 18e

NC

CN CN

25

Fe

8e

6*CN

6e

4 negative charge 4E 18e

To predict total no. Of M-M bond in a polynuclear molecule: This can be done in schematically as mentioned – 1. Determine total no. of electrons in the entire complex i.e. the no. of valence electrons of metal plus total no. of electrons from ligand, say it A. 2. Subtract this no. From n*18 where n is no. Of metals in the complex, say it B. 3. a) B divided by 2 will give the total no. Of M-M bonds in complex b) B divided by n will give number of electrons per metal.18e means no M-M bond, 17e means 1 M-M bond, 16e means 2 M-M bond and so on. Examples: Fe3(CO)12 total electrons= 48

total M-M bonds: 54-48=6/2=3 bonds per metal=48/3=16; 2 bonds

similarly for Co4(CO)12 total e =60; M-M bonds=12/2=6 bonds per metal=3 [Cp(CO)2Mo]2

total e =30; M-M bonds= 6/2=3; bonds per metal = 3;

(µ-Br)2[Mn(CO)4]2 total e =34; M-M bonds= 2/2=1; bonds per metal = 1;

THE ISOLOBAL PRINCIPLE The isolobal analogy was first proposed by Ronald Hoffman in order to understand chemical bonds in more better way. Let us take an example of an and , these two radical are very much similar in reactions and overall chemistry. Both of these radicals have a single electron in a σ orbital pointing in the direction opposite to that of substituents. Both dimerize rapidly and have same radical-type chemistry.

H

H H

CH3

Mn(CO)5

Hoffman called these two fragments isolobal. Two fragments are isolobal if they have same number of frontier orbitals, the same total number of electrons 26

in these orbitals and if the symmetry, energy and shape properties of these orbitals are close. The way to indicate isolobality of two fragments is the following:

H

H H

Mn(CO)5

CH3

Some important points about isolobality:  If a fragment is isolobal with one of two isolobal fragments, it is also isolobal with the other one. For example Cr(CO)5-, and Fe(CO)5+ are isoelectronic with ; thus they are isolobal with .  Since the main quantum number has only little influence on the shape of the frontier orbital and also are isolobal with .  Any 17-electron ML5 fragments such as Mn(PR3)5 is considered to be isolobal with .

 The divalent fragments Fe(CO)4 and CH2 (carbene) are isolobal.

-

The CH3+, CH , SiR2, and SnR2, fragments are thus isolobal with + Fe(CO)4, as well as Ru(CO)4, Co(CO)4 and Mn(CO)4 .

The interest in isolobal analogy mainly concerns its structural implications. The term isolobal is not only applied to fragments, but it is used also for compounds made of isolobal fragments.

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