Which complex does not show geometrical isomerism but show optical isomerism ?

Options

A. `[M(A A)_(2)a_(2)]`
B. `[M(A A)a_(4)]`
C. `[Mabcd]` tetrahedral
D. `[Mabcd]` square palanar

Explanation

To determine which complex does not show geometrical isomerism but shows optical isomerism, we can analyze the given complexes step by step.

### Step 1: Understanding Geometrical and Optical Isomerism
- **Geometrical Isomerism** occurs in coordination compounds when ligands can be arranged differently in space, leading to distinct isomers (e.g., cis and trans forms).
- **Optical Isomerism** occurs when a compound can exist in two non-superimposable mirror images (enantiomers), typically due to the presence of chiral centers.

### Step 2: Analyzing the Given Complexes
1. **Complex 1: MAA₂**
- Here, A is a bidentate ligand and A is a monodentate ligand.
- The geometry is octahedral.
- This complex can show geometrical isomerism because the arrangement of the ligands can lead to different spatial configurations (cis/trans).
- **Conclusion**: This complex shows geometrical isomerism.

2. **Complex 2: MA₄**
- This complex has four monodentate ligands.
- In an octahedral geometry, the arrangement of the ligands can lead to geometrical isomers.
- **Conclusion**: This complex shows geometrical isomerism.

3. **Complex 3: MABCD (Tetrahedral)**
- This complex has four different ligands (A, B, C, D).
- Tetrahedral complexes with all different ligands do not show geometrical isomerism because there are no equivalent positions to swap.
- However, it does not have a plane of symmetry or a center of symmetry, making it chiral.
- **Conclusion**: This complex does not show geometrical isomerism but shows optical isomerism.

4. **Complex 4: MABCD (Square Planar)**
- This complex has four different ligands arranged in a square planar geometry.
- Square planar complexes can exhibit geometrical isomerism (e.g., cis/trans).
- Additionally, it has a plane of symmetry, which means it cannot be optically active.
- **Conclusion**: This complex shows both geometrical isomerism and is optically inactive.

### Final Answer
The complex that does not show geometrical isomerism but shows optical isomerism is **Complex 3: MABCD (Tetrahedral)**.