Compounds with the same molecular formula but with different structures and so different properties are called isomers and the phenomenon is called isomerism.
Different types of isomerism
1) Structural isomerism
If the difference in properties result from the difference in the arrangement of atoms within the molecule, it is called structural isomerism. The various types are:
a) Ionization Isomerism :- This type of isomerism arises when the compound gives different ions in solution. Eg : [CoBr(NH3)5]SO4 and [CoSO4(NH3)5]Br are ionization isomers. The former gives [CoSO4(NH3)5]2+ and (SO4)2- as ions and latter gives [CoSO4(NH3)5]+ and Br- as ions.
b) Co-ordination Isomerism :- Here the cation part and the anion part of the compound are complexed. But the ligands co-ordinated to the metal are different. Eg: [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4]
c) Linkage Isomerism :- If the ligand has two or more different atoms which can act as donor atoms, it can be linked to the metal differently. This type of ligands are called ambident groups. NO2 can bond to the metal either through N or through Oxygen. Eg: [CoNO2(NH3)5]Cl2 and [Co ONO(NH3)5]Cl2
d) Hydrate Isomerism :- This is similar to ionization isomerism. It is due to the difference in number of water molecules present as ligands and as molecules of hydration (in outside the sphere). Eg: [Cr(H2O)6]Cl3 - violet; [CrCl(H2O)5]Cl2.H2O - blue green; [CrCl2(H2O)4]Cl.2H2O - dark green
2) Stereo isomerism
Stereo isomers contain same atoms and atom-atom bonds but the special arrangements of atoms about the central atom are different. There are two types of stereo isomerism.
a) Geometrical isomerism :- This occurs in square planar and in octahedral complexes. When the ligands occupy the adjacent position it is called the 'cis' isomer and if it is in the opposite side, it is the 'trans' isomer.
b) Optical isomerism :- Substance which can rotate the plane of plane polarised light is called optically active substance. Optical isomers differ in their action towards polarized light. This type of isomerism exists when a molecule and its mirror image are not superimpossible (chiral). The isomer which rotate the plane of polarized light to the right is called dextro rotatory isomer and the one that rotates the plane of polarised light to the left is called laevo rotatory isomer.

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