The correct answer is option 3. Ionisation Isomerism.
Ionization isomerism is a type of structural isomerism observed in coordination compounds. In this isomerism, the isomers arise from the interchange of ligands between the coordination sphere and the ionized part of a complex ion or between different ionized forms of the same complex. This isomerism is often associated with the dissociation or ionization of the complex in solution.
Let us break down the process and provide a more detailed explanation:
1. Definition:
Ionization isomerism occurs when the counterion in a complex can act as a ligand by displacing a ligand in the coordination sphere. This interchange can result in isomeric complexes.
2. Example:
Consider a general octahedral complex of the form \([MA_4B_2]X\), where \(M\) is the central metal ion, \(A\) and \(B\) are different ligands, and \(X\) is the counterion. In one isomer, \(A\) ligands are coordinated, and in the ionized form, \(B\) becomes the ligand in the coordination sphere.
3. Illustrative Reaction:
The interchange can be represented as follows:
\[ [MA_4B_2]X \rightleftharpoons [MB_4A_2]X' + [A_2B_4]X'' \]
Here, \(X\), \(X'\), and \(X''\) represent different counterions.
4. Explanation:
The interchange occurs in solution due to the dynamic nature of coordination compounds.
When the complex is dissolved in a solvent, it may undergo ionization, and the counterion may become a ligand, replacing one or more ligands in the coordination sphere.
5. Identification:
Ionization isomers can be identified by differences in their behavior in solution. Different ionization isomers may exhibit different conductivities, colors, or other properties.
6. Comparison with Other Isomerisms:
Ionization isomerism should not be confused with coordination isomerism. In coordination isomerism, the ligands are different, but the overall composition remains the same.
7. Importance:
Ionization isomerism is an important consideration in understanding the behavior of coordination compounds in solution. It has implications for the reactivity of coordination compounds and can affect the properties observed experimentally.
In summary, ionization isomerism arises when the counterion in a complex can act as a ligand, leading to the interchange of ligands between the coordination sphere and the ionized part of the complex. This phenomenon adds to the structural diversity of coordination compounds and is an important aspect of their behavior in solution. | 
