A reaction of cobalt (III) chloride and ethylenediamine is a `1 : 2` mole ratio generates two isomeric products A (violet coloured) and B (green coloured). A can show optical activity, but, B is optically inactive. What type of isomers does A and B represent ?

Options

A. Coordination isomers
B. Linkage isomers
C. Ionisation isomers
D. Geometrical isomers

Explanation

To solve the problem, we need to analyze the reaction between cobalt (III) chloride and ethylenediamine, which leads to the formation of two isomeric products A and B.

### Step-by-Step Solution:

1. **Identify the Reactants and Products:**
- The reactants are cobalt (III) chloride (CoCl3) and ethylenediamine (C2H8N2).
- The reaction occurs in a 1:2 mole ratio, meaning one mole of CoCl3 reacts with two moles of ethylenediamine.

2. **Determine the Coordination Number:**
- Cobalt (III) has a coordination number of 6 when it forms complexes.
- Ethylenediamine is a bidentate ligand, meaning it can form two bonds with the cobalt ion.

3. **Construct the Complex:**
- The cobalt (III) ion will coordinate with two ethylenediamine molecules and two chloride ions to form the complex.
- The general formula for the complex can be represented as [Co(en)2Cl2], where "en" represents ethylenediamine.

4. **Identify the Isomers:**
- The complex [Co(en)2Cl2] can exhibit geometrical isomerism due to the different arrangements of the ligands around the cobalt ion.
- Isomer A (violet) is likely to be the cis isomer, where the two chloride ions are adjacent to each other.
- Isomer B (green) is the trans isomer, where the two chloride ions are opposite each other.

5. **Optical Activity:**
- Isomer A (cis) does not have a plane of symmetry, making it chiral and optically active.
- Isomer B (trans) has a plane of symmetry, making it optically inactive.

6. **Conclusion:**
- The two isomers A and B represent geometrical isomers (cis and trans isomers) of the complex [Co(en)2Cl2].

### Final Answer:
A and B represent geometrical isomers (cis and trans isomers).