Linkage Isomerism

Bond isomerism occurs with binary ligands that can coordinate in a variety of ways. The most famous cases involve monodentate ligands: SCN-/NCS- and NO2-/ONO-. The only difference is which atoms bind the molecular ligand to the central ion. The ligand must have more than one donor atom, but can only bind the ion in one position. For example, an ion (NO2-) is a ligand that can bind to the central atom through nitrogen or oxygen atom, but not through both oxygen and nitrogen, in this case, it is called polydentate instead of multi-toothed. ligand. The names used to designate modifying ligands have also been changed. For example, an ion (NO2-) is called a nitro group when bound to an N atom and a nitroso group when bound to an O atom.

Ligands 

Ligands are the ions or molecules that form the central atom/ion during a coordination entity. These ions could also be simple ions like Cl– or small molecules like NH3 or H2O. They’ll even be larger molecules like H2NCH2CH2NH2 or N(CH2CH2NH2)3 or macromolecules like proteins.

Ambidentate Ligand

Ligands with two different potential sites for metal binding produce two different binding isomers. Examples; NH2CSNH2, SCN, NO2,, S2O3, NH2CONH2, SO3, (CH3)2S. A ligand must satisfy three conditions to be considered a dual ligand. these are:

• Two potential sites
• Single tooth
• Energy difference

Different Types of Ligands 

Unidentate Ligands

In a lot of cases, just one atom within the ligand is sure to be the metal ion.The examples of unidentate ligands are H2O, NH3 or Cl–.

Bidentate Ligands

Ligands that bind to the metal ion through two donor atoms are bidentate. The examples are H2NCH2CH2NH2 (ethane-1,2-diamine) and C2O42- (oxalate).

Polydentate Ligands

A polydentate ligand can bind to a metal ion through several donor atoms. EDTA4 is an ion of ethylenediaminetetraacetate (N(CH2CH2NH2)3). Through two nitrogen atoms and four oxygen atoms, EDTA binds to a central metal ion.

Typical Ligands Producing Linkage Isomers

• Thiocyanate, SCN−  isothiocyanate, NCS−
• Nitrite, NO2−
• Selenocyanate, SeCN− isoselenocyanate, NCSe−
• Sulfite, SO32−

Examples of Linkage Isomers

[(NH3)5Co-SCN]2+ is violet, and [(NH3)5Co-SCN]2+ is orange is an example of linkage isomer. Due to S versus O adhesion on the methyl groups of DMSO, linkage isomerism is observed in the NMR spectrum of the S-bonded to the N-bonded isomer within the complex (dimethylsulfoxide)ruthenium(II). For linkage isomerism, the lowercase Greek letter, kappa, precedes the kappa notation where the atom shortens to the metal; for instance, NO2− is represented as nitrite-κ-N and nitrite-κ-O, replacing the old system of trivial names like nitro and nitroso.

History of Linkage Isomers 

There are two detachable linkage isomers of the cationic cobalt complex. Nitrogen binds the nitro ligand within the yellow isomer. One oxygen atom is attached to the nitrite in the red linkage isomer. Usually, the O-bonded isomer is written as [Co(NH3)5(ONO)]2+. Despite the existence of the isomers being known since the late 1800s, the structural difference was not explained until 1907. As a result of UV irradiation, the red isomer converts to the yellow isomer. The formation of the nitro isomer (Co-NO2) from the nitrite isomer (Co-ONO) occurs by an intramolecular rearrangement. The first reported example of linkage isomerism had the formula [Co(NH3)5(NO2)]Cl2.

Why Does Linkage Isomerism Take Place?

All solvents and reagents were obtained from commercial sources and used as received unless otherwise stated. The solvents tetrahydrofuran (THF), ether (Et2O), toluene, pentane, and dichloromethane were skilled solvent purification columns (Glass Contour, Laguna, CA). Dichloromethane and acetonitrile were dried over CaH2 and then distilled under vacuum before use. THF was dried over sodium/benzophenone before use. Acetone was dried over activated 3 Å molecular sieves and distilled under a vacuum before use. Purified solvents were stored during a nitrogen-filled glovebox over either activated 3 Å molecular sieves or CaH2 and filtered through a 0.45 μm PTFE syringe filter immediately before use.

Conclusion

Isomers are nothing but compounds that possess a similar molecular formula but distinct chemical structures. It has a property between a pair of molecules called isomerism. A molecule is an isomer of another molecule. Thus, the two isomers will have different properties. As two types, there are two types of structural isomers, namely coordinate isomerism and bonding isomerism. There are two types of stereoisomers namely geometric isomers and optical isomers. In linkage isomerism, one ligand is linked to the central atom through different atoms in the ligand. This is known as linkage isomerism. An example of this separation of isomerism is given by the ions [Co(NO2)(NH3)5]2+ and [Co(NO2)(NH3)5]2+, in which the anionic ligand is bonded to the cobalt atom by nitrogen or by oxygen. Another example of this separation of isomerism is given by the pair of ions [Co(CN)5(NCS)]3* and [Co(CN)5(SCN)]3*.