Which of the following octahedral complex does not show geometrical isomerism (`A` and `B` are monodentate ligands) ?

Options

A. `[MA_(4)B_(2)]`
B. `[MA_(5)B]`
C. `[MA_(2)B_(4)]`
D. `[MA_(3)B_(3)]`

Explanation

To determine which octahedral complex does not show geometrical isomerism, we need to analyze the given options based on the types of ligands and their arrangements. Here’s a step-by-step solution:

### Step 1: Understand Geometrical Isomerism
Geometrical isomerism in coordination compounds occurs due to different spatial arrangements of ligands around the central metal atom. In octahedral complexes, this can manifest as:
- **Cis and Trans Isomers**: For complexes with two different types of ligands (e.g., MA4B2), where ligands can be adjacent (cis) or opposite (trans).
- **Facial and Meridional Isomers**: For complexes with three of one type of ligand and three of another (e.g., MA3B3), where ligands can be arranged to form a triangular face (facial) or a plane (meridional).

### Step 2: Analyze Each Option
1. **Option A: MA4B2**
- This complex can show geometrical isomerism because the two B ligands can be arranged in cis or trans positions relative to each other. Therefore, this option can exhibit isomerism.

2. **Option B: MA5B**
- In this complex, there are five A ligands and only one B ligand. Since there is only one B ligand, it cannot be arranged in a way to form cis or trans isomers. The absence of a second B ligand means there is no possibility for geometrical isomerism. Thus, this complex does not show geometrical isomerism.

3. **Option C: MA3B3**
- This complex can show both facial and meridional isomers due to the arrangement of three A ligands and three B ligands. Therefore, this option can exhibit isomerism.

### Step 3: Conclusion
Based on the analysis, **Option B (MA5B)** is the only complex that does not show geometrical isomerism due to the presence of only one B ligand.

### Final Answer
**Option B: MA5B does not show geometrical isomerism.**

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