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The compounds with the same molecular formula but the different structural arrangement of ligands around the central metal atom are known as isomers. They can be two or more and have similar or different physical and chemical properties.
Coordination compounds like organic compounds show isomerism with few differences. Coordination complexes like square planar, octahedral and tetrahedral exhibit different types of isomerism.
The study of isomerism plays a vital role in analysing similar complexes’ different properties. Like cisplatin has a therapeutic role in cancer while transplatin is toxic. Although they both have the same molecular and structural formula, the only difference is the arrangements of ligands.
Types Of Isomerism:
Class I. Structural Isomerism:
These are the isomers that contain the same number of atoms but differ in bonding. They have the same molecular formula but different structural arrangements due to differences in the bonding patterns of similar atoms.
Ligands are similar in isomers, but they are differently bonded to the central metal atom. There are 4 types of structural isomerism-
Ionisation Isomerism
In this type of isomerism, interchange of ligands takes place between coordination and the ionisation sphere. They have the same molecular formula but give different ions in solution.
Example: Co(NH3)5BrSO4 This is the molecular formula that has 2 different ionisation isomers. [CoBr(NH3)5 SO4] and [Co SO4(NH3)5]Br
Linkage Isomerism
Linkage isomerism is shown by those coordination compounds that have ambidentate ligands; this type of isomerism occurs due to a change of Donor atom due to ambidentate ligands.
Example [Co(NO2)(NH3)5] Cl is a yellow coloured complex containing ligand nitrito-N whereas [Co(ONO)(NH3)5] Cl is a red coloured complex containing ligand nitrito-O.
Coordination Isomerism
Coordination isomerism occurs in those coordination compounds in which cation and anion are both present as a complex. It takes place due to the interchange of ligands between two complexes.
Examples [Co(NH3)6] [Cr(CN)6] and [Cr(NH3)6] [Co(CN)6] show coordination isomerism.
Hydration or Solvate Isomerism
Hydration isomerism is present in those coordination complexes in which the number of water molecules is different; water molecules are present in coordination and the ionisation sphere. Isomerism is a type of ionisation isomerism known as solvent isomerism.
Example [Cr(H2O)6].Cl3 is violet coloured complex, whereas [Cr(H2O)5Cl]Cl2.H2O is grey-green coloured complex.
Class II. Stereo Isomerism:
These compounds have a similar number of atoms and similar bonding of ligands with central metal atoms. But the differences in the spatial arrangement of atoms in 3D space; hence this type of isomerism is called stereoisomerism. These are of two types-
Cis and trans isomerism
In cis isomerism, the ligands are attached to each central metal atom on the same side so that the effective distance between two ligands is minimum.
In trans isomerism, the ligands are attached to each central metal atom on the diagonally opposite sides so that the effective distance between two ligands is maximum.
Cis and trans types of isomerism can be exhibited by square planar and octahedral complexes. In cis isomerism, ligands are placed at 90o to each other, and in trans isomerism, they are placed at 180o to each other. Tetrahedral complexes do not show this type of isomerism.
Example- Square planar complex type MA2B2
cis-[Pt(NH3)2Cl2], known as cisplatin, has its therapeutic value for the treatment of cancer while trans-[Pt(NH3)2Cl2] has no therapeutic value. Instead, it is toxic.
Optical isomerism
The coordination complexes which are chiral exhibit optical isomerism. The tendency of the solution of coordination compounds to rotate the polarised plane light either clockwise or anti-clockwise is called dextrorotatory and laevorotatory, respectively. These are the two optically active isomers that are mirror images of each other.
Octahedral complexes of the type [M(xx)3]n± , [M(xx)2 AB]n± and [M(xx)2 B2 ]n± show optical isomerism.
Conclusion:
Coordination compounds consist of anions or neutral molecules linked together by covalent bonds. Coordination compounds and coordination complexes are both terms that describe the same thing. The ligands are the molecules or ions attached to the central atom (also known as complexing agents).The development of coordination chemistry required the presence of coordination molecules with the same formula but a different arrangement of ligands. The different isomers of coordination compounds show different colours due to differences in their bonding patterns, arrangement or their rotation.
