The species which has four unpaired electron is:

Options

A. `[Co(CN)_(6)]^(4-)`
B. `[Cr(H_(2)O)_(6)]^(3+)`
C. `[FeCl_(4)]^(2-)`
D. `[Fe(H_(2)O)_(6)]^(3+)`

Answer

To determine which species has four unpaired electrons, we will analyze the oxidation states and electron configurations of the given complexes step by step.

### Step 1: Identify the species
We need to check the electron configurations of the given complexes to find the one with four unpaired electrons. The complexes we will analyze are:
1. [Co(CN)6]^(4-)
2. [Fe(H2O)6]^(3+)
3. [FeCl4]^(2-)

### Step 2: Analyze [Co(CN)6]^(4-)
1. **Determine the oxidation state of cobalt (Co)**:
- Let the oxidation state of Co be \( x \).
- CN is a -1 ligand, and there are 6 CN ligands.
- The overall charge of the complex is -4.
- Therefore, \( x + 6(-1) = -4 \) → \( x - 6 = -4 \) → \( x = +2 \).

2. **Determine the electron configuration of Co**:
- Cobalt has an atomic number of 27, so the ground state configuration is [Ar] 4s² 3d⁷.
- For Co in the +2 oxidation state, we remove the 4s electrons: [Ar] 3d⁷.

3. **Determine the number of unpaired electrons**:
- In a strong field ligand environment (like CN), the 3d electrons will pair up.
- The configuration will be: ↑↓ ↑↓ ↑ ↑ ↑ (3 unpaired electrons).

### Step 3: Analyze [Fe(H2O)6]^(3+)
1. **Determine the oxidation state of iron (Fe)**:
- Let the oxidation state of Fe be \( y \).
- H2O is a neutral ligand, so its contribution is 0.
- The overall charge of the complex is +3.
- Therefore, \( y + 0 = +3 \) → \( y = +3 \).

2. **Determine the electron configuration of Fe**:
- Iron has an atomic number of 26, so the ground state configuration is [Ar] 4s² 3d⁶.
- For Fe in the +3 oxidation state, we remove two 4s electrons and one 3d electron: [Ar] 3d⁵.

3. **Determine the number of unpaired electrons**:
- In a weak field ligand environment (like H2O), the 3d electrons will remain unpaired.
- The configuration will be: ↑ ↑ ↑ ↑ ↑ (5 unpaired electrons).

### Step 4: Analyze [FeCl4]^(2-)
1. **Determine the oxidation state of iron (Fe)**:
- Let the oxidation state of Fe be \( z \).
- Cl is a -1 ligand, and there are 4 Cl ligands.
- The overall charge of the complex is -2.
- Therefore, \( z + 4(-1) = -2 \) → \( z - 4 = -2 \) → \( z = +2 \).

2. **Determine the electron configuration of Fe**:
- For Fe in the +2 oxidation state, we have: [Ar] 4s² 3d⁶.
- After ionization, we have: [Ar] 3d⁶.

3. **Determine the number of unpaired electrons**:
- In a weak field ligand environment (like Cl), the 3d electrons will remain unpaired.
- The configuration will be: ↑ ↑ ↑ ↑ ↑ (4 unpaired electrons).

### Conclusion
Among the analyzed complexes, [FeCl4]^(2-) has 4 unpaired electrons.

### Final Answer
The species which has four unpaired electrons is **[FeCl4]^(2-)**.