Which one of the following is paramagnetic in nature ?

Options

A. `Ni(CO)_(4)`
B. `[Ni(CN)_(4)]^(2-)`
C. `K_(4)[Fe(CN)_(6)]`
D. `[FeF_(6)]^(4-)`

Answer

To determine which of the given coordination compounds is paramagnetic in nature, we need to analyze the electronic configurations and the presence of unpaired electrons in each complex.

### Step-by-step Solution:

1. **Understanding Paramagnetism and Diamagnetism**:
- Paramagnetic substances have unpaired electrons, which contribute to a net magnetic moment.
- Diamagnetic substances have all electrons paired, resulting in no net magnetic moment.

2. **Analyzing Each Complex**:
- We will examine the oxidation state of the metal in each complex and its electronic configuration to identify unpaired electrons.

3. **Complex 1: Ni(CO)4**:
- Oxidation state of Ni = 0.
- Electronic configuration of Ni = [Ar] 3d^8 4s^2.
- CO is a strong field ligand, causing pairing of electrons.
- Result: All electrons are paired → **Diamagnetic**.

4. **Complex 2: Ni(CN)4^2-**:
- Oxidation state of Ni = +2.
- Electronic configuration of Ni^2+ = [Ar] 3d^8.
- CN is a strong field ligand, causing pairing of electrons.
- Result: All electrons are paired → **Diamagnetic**.

5. **Complex 3: K4[Fe(CN)6]**:
- Oxidation state of Fe = +2.
- Electronic configuration of Fe^2+ = [Ar] 3d^6.
- CN is a strong field ligand, causing pairing of electrons.
- Result: All electrons are paired → **Diamagnetic**.

6. **Complex 4: FeF6^2-**:
- Oxidation state of Fe = +2.
- Electronic configuration of Fe^2+ = [Ar] 3d^6.
- F is a weak field ligand, which does not cause pairing of electrons.
- Result: There are 4 unpaired electrons → **Paramagnetic**.

7. **Conclusion**:
- The only complex that is paramagnetic is **FeF6^2-** (Option 4).

### Final Answer:
The paramagnetic complex is **FeF6^2-** (Option 4).