Be forms `[BeF_(4)]^(2-)`, but `Al` forms `[AiF_(6)]^(3-)`.
Reason (R ): Be does not have `d`-orbitals in the valence shell but `Al` has.

Options

A. If both `(A)` and `(R )` are correct and `(R )` is the correct explanation of `(A)`.
B. If both `(A)` and `(R )` are correct, but `(R )` is not the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R )` is incorrect.
D. If `(A)` is incrrect, but `(R )` is correct.

Answer

To analyze the question regarding the formation of `[BeF4]^(2-)` and `[AlF6]^(3-)`, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding the Elements:**
- Beryllium (Be) has an atomic number of 4, with the electronic configuration of 1s² 2s². This means that its valence shell consists of only the 2s orbital.
- Aluminium (Al) has an atomic number of 13, with the electronic configuration of 1s² 2s² 2p⁶ 3s² 3p¹. In addition to the 3s and 3p orbitals, Al has access to the 3d orbitals, which are empty in its ground state.

2. **Coordination Numbers:**
- Beryllium can form a coordination number of 4, which corresponds to the formation of the tetrahedral complex `[BeF4]^(2-)`. This is due to the fact that it can only utilize its 2s orbital for bonding.
- Aluminium, on the other hand, can form a coordination number of 6, leading to the formation of the octahedral complex `[AlF6]^(3-)`. This is possible because Al can utilize its 3s, 3p, and empty 3d orbitals to accommodate more bonds.

3. **Role of d-Orbitals:**
- The absence of d-orbitals in beryllium limits its ability to expand its coordination number beyond 4. Therefore, it cannot form complexes with more than four ligands.
- Aluminium, having vacant d-orbitals, can expand its valence shell and accommodate more ligands, allowing it to form complexes with a coordination number of 6.

4. **Conclusion:**
- The assertion that Be forms `[BeF4]^(2-)` and Al forms `[AlF6]^(3-)` is correct.
- The reason provided, that Be does not have d-orbitals in the valence shell while Al does, is also correct and serves as a valid explanation for the difference in coordination numbers.

### Final Answer:
Both the assertion and reason are correct, and the reason is a correct explanation for the assertion.