Assertion. `SiF_(6)^(2-)` is known but `SiCl_(6)^(2-)` is not.
Reason. Size of F is small and its lone pair of electrons interact with d-orbitals of Si strongly.

Options

A. If both assertion and reason are true, and reason is the correct explanation of the assertion.
B. If both assertion and reason are ture, but reason is not the correct explanantion of the assertion.
C. If assertion is true, but reason is false.
D. If both assertion and reason are false

Answer

To solve the assertion and reason provided in the question, we will analyze both the assertion and the reason step by step.

### Step 1: Understanding the Assertion
The assertion states that \( SiF_6^{2-} \) is known while \( SiCl_6^{2-} \) is not.

**Explanation:**
- Silicon can form various hexafluorides and hexachlorides. The formation of \( SiF_6^{2-} \) indicates that silicon can effectively bond with fluorine to form a stable anion.
- In contrast, \( SiCl_6^{2-} \) does not exist as a stable species.

### Step 2: Analyzing the Reason
The reason provided is that the size of fluorine is small and its lone pair of electrons interacts strongly with the d-orbitals of silicon.

**Explanation:**
- Fluorine is a small atom, which allows its lone pairs to be close enough to the silicon atom. This proximity enables effective overlap and interaction with the vacant d-orbitals of silicon.
- This interaction stabilizes the \( SiF_6^{2-} \) complex.
- On the other hand, chlorine is larger than fluorine, which results in less effective overlap between its lone pairs and the d-orbitals of silicon. This leads to weaker interactions and ultimately makes \( SiCl_6^{2-} \) unstable or non-existent.

### Step 3: Conclusion
Based on the analysis:
- The assertion is true: \( SiF_6^{2-} \) is known, while \( SiCl_6^{2-} \) is not.
- The reason is also true: The small size of fluorine allows for strong interaction with silicon's d-orbitals, which is not the case with chlorine.

Thus, both the assertion and the reason are correct, and the reason correctly explains the assertion.

### Final Answer
- **Assertion:** True
- **Reason:** True
- **Explanation:** The small size of fluorine allows for effective interaction with silicon's d-orbitals, stabilizing \( SiF_6^{2-} \), while the larger size of chlorine prevents such stabilization, leading to the non-existence of \( SiCl_6^{2-} \).

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