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Linkage Isomerism

The study is concerned with the types of isomerism found in coordination chemistry referred as linkage isomerism. Linkage isomerism occurs whenever a given ligand can attach itself to the same central metal atom by bonding through either one of two different atoms within the ligand. For any true linkage isomerism the factors like geometrical configurations remain just as same.

In this section we will discuss the aspects that are inherent to linkage isomerism and which were confined to only known ligands exhibiting this type of isomerism in metal complexes of nitrile ion $NO_{2}^{-}$.


Define Linkage Isomerism

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Linkage isomerism occurs when it is possible to attach a ligand to the metal through different atoms. The two most common ligands with which linkage isomerism arises are thiocyanate $SCN^-$, and nitrile $NO_{2}^{-}$. If we look into the Lewis structure of the thiocyanate ion, it shows that there are lone pairs of electrons on sulphur and nitrogen.

The ligand can attach to a metal either through sulphur or S bonded thiocyanate and through nitrogen in case of N bonded thiocyanate. The nitrile ion can attach either at oxygen or at nitrogen. The former ones are called nitrito complexes and the latter as nitro complexes. 

Linkage Isomerism

Explain Linkage Isomerism

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In 1893 the isolation of nitro $(Co-NO_{2})$ and nitrito (Co-ONO) penta ammines of cobalt was carried out by Jorgensen and until recently Basolo and Hammaker also succeeded in extending nitro-nitrito linkage isomerism to include the penta ammines of rhodium, iridium and platinum. It’s observed that to form both thiocyanate (M-SCN) and iso-thiocyanate (M-SCN-M) complexes depending on the central metal atom used along with the bridged (M-SCN-M) bonding.

This coincides with the bonding strengths of the halides which has $F^{-}$ > $Cl^{-}$ > $Br^{-}$ > $I^{-}$ to $I^{-}$ > $Br^{-}$ > $Cl^{-}$ > $F^{-}$. The optical isomers of CFClBrI shows rotating the structure on the right by 180 degree and Cl atoms could get super imposed resulting in a structure in which I and Br atoms are inter changed does not look super imposed. 

Trans Isomer of Cobalt

‘trans’ isomer of $[Co (en)_{2} Cl_{2}]^{+}$

Cis Isomer of Cobalt

Mirror images of cis $[Co (en)_{2} Cl_{2}]^{+}$

Many of the linkage isomerism involve the ions of $CN^{-}$, $SCN^{-}$, and $NO_{2}^{-}$ as each of them forms so many complexes. One can predict the more stable linkage isomers on the basis of the non-polarizing (hard) or polarizing (soft) interaction principle which depends on the electronic character of the metal and the ligand.

In case of $SCN^-$ the sulphur atom is large and quite polarizing electron donor whereas the atom of nitrogen is smaller and non-polarizing donor. Linkage isomerism is seen far and less common in which the ligands are neutral molecules although such isomers are only possible theoretically. In case of pyrazine N oxide molecule the structure could coordinate to metal ions through either oxygen atom or the nitrogen. Compounds containing ligands such as these are usually found to contain the ligand bound in a particular way and hence the other isomer is unknown. 
Ligand Bound
Such ligands function as bridging ligands bonded to two metal ions simultaneously.

Linkage Isomerism in Coordination Compounds

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In coordination chemistry quite similar to organic chemistry, the synthases are complicated by molecular rearrangements. A molecular rearrangement is defined as transformation which results in a change connectivity without any change in atomic composition. We can take geometrical isomerism as an example of rearrangement of molecule.

The most important member of rearrangement is linkage isomerism where complexes containing ambidentate ligands or ligands with more than one donor site but get to bind in mono dentate manner. Linkage isomerism has also dealt with nitro-nitrite isomerization and thiocyanate and iso thiocyanate isomerization. Linkage isomerism which involves cyanate, methanesulphinate, terpyridyl, alizarin, dihydroxy-benzoate and acetone are examples of linkage isomerism.

Linkage Isomerism Cobalt Complexes

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The protonation of the cyanide ligand or the formation of [HCN-Cr (NO) $(H_{2}O)_{4}]_{2}^{+}]$ was found to have significantly lower rate of isomerisation as compared to the un-protonated form.

The kinetics of O-S linkage isomerism in the cobalt (II) sulphite complexes $[Co (OSO_{2}) (Tetren)]^{+}$ and $[Cr (OSO_{2})$ $(H_{2}O)_{5}(Tetren)]^{+}$ are found out for conversion where O- bonded complexes get converted to S bonded isomers. The kinetics of the reaction between $[Pt (CN)_{4}Cl(OH)]^{2-}$ and $HSO_{3}^{-}$ is found to go through these steps.
  • Oxygen bonded sulfito complex is formed
  • Intermediate complex undergoes a slow rate determining intra molecular linkage isomerisation to a more thermodynamically stable S bonded isomer $[Pt(CN)_{4}Cl(SO_{3}H)]^{2-}$ which gets reduced quickly to $[Pt(CN)_{4}]^{2-}$ and $HSO_{4}^{-}$ in a two electron inner sphere process.

The solvent dependent rate of isomerisation was found to increase in the order of

Tetrahydrofuran < acetone < chloroform < dichloromethane.

The isomerisation is not observed in methanol due to stabilisation of the fulminate by hydrogen bonding with the solvent. The cobalt based coordination compounds are special types of constitutional isomers.

In case of $[Co(NH_{3})_{5}(SO_{4})]Br$ a red compound, the sulphate ion is attached to cobalt ion and the bromide ion is in crystal lattice as the metal complex cation is countered by anion.

In case of $[Co(NH_{3})_{5}Br] SO_{4}$ which is a violet compound the sulphate ion is in crystal lattice with bromide ion attached to cobalt ion.
Both cations and anions can be metal complex ions which yields another type of isomer. The following copper platinum compounds.

More constitutional isomers are displayed by the following compounds:

$Co(NH_{3})_{5}(ONO)]Cl_{2}$ and $Co(NH_{3})_{5}(NO_{2})]Cl_{2}$.
  • n the first compound the nitrite ligand $NO_{2}^{-}$ bonds to the cobalt atom through an electron pair on an oxygen atom. This is red originally and slowly changes to second compound
  • The second one is yellow brown in colour and the nitrite ligand bonds to the cobalt atom through an electron pair on the nitrogen atom.

Linkage Isomerism Examples

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Amongst the examples, the linkage isomerism is very common in nitrosyl containing compounds and it’s close to 80% N-O linkage isomerism.

$\Rightarrow $ Cis and trans forms $[RuCl(en)_{2} NO]Cl_{2}$

$\Rightarrow $ Only trans $[RuCl(NO)Py_{4}]PF_{6}$ and $Mn(CO) (NO)_{3}$

Despite being a common occurrence the linkage isomerism of nitrosyl compounds occurs only at a low temperature and has a very low yields.
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