Which is a diamagnetic complex?

Options

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

Answer

To determine which complex is diamagnetic, we need to analyze the electron configurations of the given complexes and identify the presence of unpaired electrons. A complex is considered diamagnetic if all its electrons are paired.

### Step-by-Step Solution:

1. **Understanding Magnetic Properties**:
- A complex is **diamagnetic** if it has all paired electrons.
- A complex is **paramagnetic** if it has one or more unpaired electrons.

2. **Identify the Complexes**:
- The complexes to analyze are:
1. [Fe(H2O)6]³⁺
2. [Fe(H2O)6]²⁺
3. [Fe(CN)6]³⁻
4. [Fe(CN)6]⁴⁻

3. **Determine the Oxidation States**:
- For [Fe(H2O)6]³⁺: Iron is in the +3 oxidation state.
- For [Fe(H2O)6]²⁺: Iron is in the +2 oxidation state.
- For [Fe(CN)6]³⁻: Iron is in the +3 oxidation state.
- For [Fe(CN)6]⁴⁻: Iron is in the +2 oxidation state.

4. **Electron Configuration of Iron**:
- The atomic number of Iron (Fe) is 26.
- The electron configuration is [Ar] 4s² 3d⁶.

5. **Analyze Each Complex**:
- **[Fe(H2O)6]³⁺**:
- Iron in +3 state: 3d⁵ configuration.
- Water is a weak field ligand, leading to unpaired electrons.
- Result: **5 unpaired electrons** (paramagnetic).

- **[Fe(H2O)6]²⁺**:
- Iron in +2 state: 3d⁶ configuration.
- Water is a weak field ligand, leading to unpaired electrons.
- Result: **4 unpaired electrons** (paramagnetic).

- **[Fe(CN)6]³⁻**:
- Iron in +3 state: 3d⁵ configuration.
- Cyanide is a strong field ligand, leading to pairing of electrons.
- Result: **1 unpaired electron** (paramagnetic).

- **[Fe(CN)6]⁴⁻**:
- Iron in +2 state: 3d⁶ configuration.
- Cyanide is a strong field ligand, leading to pairing of electrons.
- Result: **0 unpaired electrons** (diamagnetic).

6. **Conclusion**:
- The complex that is diamagnetic is **[Fe(CN)6]⁴⁻** because it has all paired electrons.