Notes On Isomerism - CBSE Class 12 Chemistry
Two (or) more compounds with the same chemical formula but different structural arrangements are called isomers. The phenomenon is known as isomerism. Isomers differ in one (or) more physical or chemical properties because of the difference in the arrangement of the atoms. Isomerism in coordination compounds may be broadly divided into two types - structural isomerism and stereo isomerism. Structural isomerism arises due to the presence of different kinds of bonds between the metal and the ligands. This further divided into several types: Ionisation isomerism, solvate isomerism, ligand isomerism, linkage isomerism, polymerisation isomerism, coordination isomerism and coordination position isomerism. Ionisation isomerism:  Compounds that have the same stoichiometric composition, but yield different ions in a solution are called ionisation isomers and phenomenon is called ionisation isomerism. This type of isomerism is due to the exchange of groups between the complex ion and the ions outside it. Ex: [Co(NH3)5Br]SO4        +   BaCl2               →      [Co(NH3)5Br]Cl2               +   BaSO4 Pentaamminebromido       Barium                      Pentaamminebromido            Barium  cobalt(III) sulphate              chloride                       cobalt(III) chloride                sulphate Solvate isomerism:  This is also known as hydrate isomerism if water is involved as a solvent. Compounds that have the same composition, but differ by whether (or) not solvent molecules are present as ligands in the coordination sphere (or) are merely present as free solvent molecules outside the coordination sphere are known as solvate isomers. Ex: CrCl3.6H2O Linkage isomerism:  Linkage isomerism arises when a ligand can bind to a metal ion through any one of two (or) more different donor atoms. This type of isomerism is possible in the coordination compounds that have ambidentate ligands. Ex: In ambidentate nitrite ion, either nitrogen (or) oxygen atoms could act as the electron pair donors. Coordination isomerism:  This type of isomerism arises when both the positive and negative ions are complex ions. It may be caused by an interchange of ligands between the anion and the cation. Ex: In hexa ammine cobalt (III) and hexa cyano chromate(III), the ammonia ligands are bound to the cobalt(+3) ion, while the cyanide ligands are bound to the chromium (+3) ion. Stereo isomerism: Compounds with the same chemical formula and chemical bonds, but different arrangement of atoms (or) groups in space are called stereo isomers. Stereo isomerism can be subdivided into geometrical isomerism and optical isomerism. Geometrical isomerism is also known as cis-trans isomerism. The isomer in which the two identical groups are adjacent to each other is known as a cis-isomer while the isomer in which the two identical groups are diagonally opposite to each other is known as a trans isomer. Optical isomerism: When the solutions of certain coordination compounds are placed in the path of plane polarised light they rotate its plane through a certain angle that may be either to the left (or) to the right. This property of a complex, of rotating the plane of polarised light is called its optical activity. A complex that possesses this property is said to be optically active. Complexes that rotate the plane of polarised light towards right are said to be dextro rotatory (or) d-form while complexes that rotate it towards the left are called leavo rotatory (or) l-form. The d- and l- forms are mirror images and cannot be superimposed on each other. Such molecules or ions that cannot be superimposed are called chiral. Since these forms are related to each other as mirror images they are commonly called enantiomers.

#### Summary

Two (or) more compounds with the same chemical formula but different structural arrangements are called isomers. The phenomenon is known as isomerism. Isomers differ in one (or) more physical or chemical properties because of the difference in the arrangement of the atoms. Isomerism in coordination compounds may be broadly divided into two types - structural isomerism and stereo isomerism. Structural isomerism arises due to the presence of different kinds of bonds between the metal and the ligands. This further divided into several types: Ionisation isomerism, solvate isomerism, ligand isomerism, linkage isomerism, polymerisation isomerism, coordination isomerism and coordination position isomerism. Ionisation isomerism:  Compounds that have the same stoichiometric composition, but yield different ions in a solution are called ionisation isomers and phenomenon is called ionisation isomerism. This type of isomerism is due to the exchange of groups between the complex ion and the ions outside it. Ex: [Co(NH3)5Br]SO4        +   BaCl2               →      [Co(NH3)5Br]Cl2               +   BaSO4 Pentaamminebromido       Barium                      Pentaamminebromido            Barium  cobalt(III) sulphate              chloride                       cobalt(III) chloride                sulphate Solvate isomerism:  This is also known as hydrate isomerism if water is involved as a solvent. Compounds that have the same composition, but differ by whether (or) not solvent molecules are present as ligands in the coordination sphere (or) are merely present as free solvent molecules outside the coordination sphere are known as solvate isomers. Ex: CrCl3.6H2O Linkage isomerism:  Linkage isomerism arises when a ligand can bind to a metal ion through any one of two (or) more different donor atoms. This type of isomerism is possible in the coordination compounds that have ambidentate ligands. Ex: In ambidentate nitrite ion, either nitrogen (or) oxygen atoms could act as the electron pair donors. Coordination isomerism:  This type of isomerism arises when both the positive and negative ions are complex ions. It may be caused by an interchange of ligands between the anion and the cation. Ex: In hexa ammine cobalt (III) and hexa cyano chromate(III), the ammonia ligands are bound to the cobalt(+3) ion, while the cyanide ligands are bound to the chromium (+3) ion. Stereo isomerism: Compounds with the same chemical formula and chemical bonds, but different arrangement of atoms (or) groups in space are called stereo isomers. Stereo isomerism can be subdivided into geometrical isomerism and optical isomerism. Geometrical isomerism is also known as cis-trans isomerism. The isomer in which the two identical groups are adjacent to each other is known as a cis-isomer while the isomer in which the two identical groups are diagonally opposite to each other is known as a trans isomer. Optical isomerism: When the solutions of certain coordination compounds are placed in the path of plane polarised light they rotate its plane through a certain angle that may be either to the left (or) to the right. This property of a complex, of rotating the plane of polarised light is called its optical activity. A complex that possesses this property is said to be optically active. Complexes that rotate the plane of polarised light towards right are said to be dextro rotatory (or) d-form while complexes that rotate it towards the left are called leavo rotatory (or) l-form. The d- and l- forms are mirror images and cannot be superimposed on each other. Such molecules or ions that cannot be superimposed are called chiral. Since these forms are related to each other as mirror images they are commonly called enantiomers.