Assertion (A): Both reactions are Lewis acid-base recations?
i. `NH_(3) + BF_(3) rarr H_(3)N: BF_(3)`
ii. `Mg +S rarr Mg^(2+) +S^(2-)`
Reason (R) : Lewis acid-base reaction involve the donation of lone pair electrons from base to acid. this donation results in a corrdinate bond.

Options

A. If both (A) and (R) are correc, 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 incorrect, but (R) is correct.

Explanation

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

### Step 1: Analyze the First Reaction
The first reaction given is:
\[ NH_3 + BF_3 \rightarrow H_3N:BF_3 \]

- In this reaction, ammonia (NH3) acts as a Lewis base because it has a lone pair of electrons that it can donate.
- Boron trifluoride (BF3) acts as a Lewis acid because it has an incomplete octet and can accept electrons.
- When NH3 donates its lone pair to BF3, a coordinate bond is formed, resulting in the complex \( H_3N:BF_3 \).

### Step 2: Analyze the Second Reaction
The second reaction given is:
\[ Mg + S \rightarrow Mg^{2+} + S^{2-} \]

- In this reaction, magnesium (Mg) loses two electrons to form \( Mg^{2+} \), and sulfur (S) gains two electrons to form \( S^{2-} \).
- This process involves the transfer of electrons but does not involve the formation of a coordinate bond.
- Instead, it results in the formation of ionic bonds, which is different from Lewis acid-base reactions.

### Step 3: Conclusion on Assertion and Reason
- The assertion states that both reactions are Lewis acid-base reactions. This is **false** because only the first reaction qualifies as a Lewis acid-base reaction.
- The reason states that Lewis acid-base reactions involve the donation of lone pair electrons from the base to the acid, resulting in a coordinate bond. This is **true**.

### Final Answer
- The assertion is false, and the reason is true. Therefore, the correct answer is D: Assertion is false, Reason is true.

---