Assertion The number of unpaired electrons in `[Ni(CO)_(4)]` is zero
Reason In this compounds 4s-electrons of Ni atom enter the inner d-orbitals to facilitate the `sp^(3)` hybridisation in Ni atom .

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 and `(R )` is the correct explanation of `(A ) ` .
C. If`(A)` is correct, but `( R )` is incorrect .
D. Both `(A)` and `(R ) ` are incorrect .

Answer

To solve the question, we need to analyze both the assertion and the reason provided regarding the coordination compound `[Ni(CO)_(4)]`.

### Step-by-Step Solution:

1. **Determine the Oxidation State of Nickel in `[Ni(CO)_(4)]`:**
- The carbonyl (CO) ligand is neutral, so it does not contribute to the oxidation state.
- Let the oxidation state of Ni be \( x \).
- The equation for the oxidation state is:
\[ x + 0 \cdot 4 = 0 \implies x = 0 \]
- Thus, the oxidation state of nickel in this complex is 0.

**Hint:** Remember that neutral ligands do not affect the oxidation state of the central metal atom.

2. **Write the Electronic Configuration of Nickel:**
- The atomic number of Ni is 28. The ground state electronic configuration is:
\[ [Ar] \, 3d^8 \, 4s^2 \]

**Hint:** Use the periodic table to find the atomic number and derive the electronic configuration.

3. **Identify the Hybridization:**
- In `[Ni(CO)_(4)]`, the strong field ligand CO causes pairing of electrons.
- The 4s electrons can be promoted to the inner d orbitals to allow for hybridization.
- The hybridization of Ni in this complex is \( sp^3 \) because it forms four bonds with the four CO ligands.

**Hint:** Strong field ligands like CO lead to low-spin complexes, which promote electron pairing.

4. **Count the Number of Unpaired Electrons:**
- After hybridization, the 3d orbitals will have all electrons paired due to the strong field nature of CO.
- The final configuration in the hybridized state will have no unpaired electrons.

**Hint:** Check the filled orbitals after hybridization to determine the number of unpaired electrons.

5. **Evaluate the Assertion and Reason:**
- The assertion states that the number of unpaired electrons in `[Ni(CO)_(4)]` is zero, which is correct.
- The reason states that the 4s electrons of Ni enter the inner d orbitals to facilitate the \( sp^3 \) hybridization, which is also correct.
- Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

**Hint:** When evaluating assertions and reasons, ensure both statements are true and that the reason logically supports the assertion.

### Final Conclusion:
Both the assertion and the reason are correct, and the reason is the correct explanation of the assertion.