Which of the following is diamagnetic complex

Options

A. (a) `[Co(OX)_3]^(3-)`, `[Fe(CN)_6]^(3-)`
B. (b) `[Co(Ox)_3]^(3-)`,`[FeF_6]^(3-)`
C. (c) `[Fe(OX)_3]^(3)`, `[FeF_6]^(3-)`
D. (d) `[Fe(CN)_6]^(3), [CoF_6]^(3-)`

Explanation

To determine which of the given complexes is diamagnetic, we need to analyze the electronic configurations of the metal ions in each complex and the nature of the ligands involved. A complex is diamagnetic if it has no unpaired electrons.

### Step-by-Step Solution:

1. **Identify the complexes**: We need to analyze the following complexes:
- Co(ox)₃³⁻
- Fe(CN)₆³⁻
- CoF₆³⁻
- FeF₆³⁻

2. **Determine the oxidation states**:
- For Co(ox)₃³⁻: Oxalate (ox) has a charge of -2, so the oxidation state of Co is +3.
- For Fe(CN)₆³⁻: CN is a strong field ligand and has a charge of -1, so the oxidation state of Fe is +3.
- For CoF₆³⁻: Fluoride (F) has a charge of -1, so the oxidation state of Co is +3.
- For FeF₆³⁻: The oxidation state of Fe is also +3.

3. **Determine the electronic configurations**:
- Co³⁺: [Ar] 3d⁶
- Fe³⁺: [Ar] 3d⁵

4. **Analyze the ligand field strength**:
- Oxalate (ox) is a strong field ligand, which means it will cause pairing of electrons in the d-orbitals.
- CN⁻ is also a strong field ligand, leading to pairing of electrons.
- F⁻ is a weak field ligand, which does not favor pairing.

5. **Fill the d-orbitals**:
- For Co(ox)₃³⁻ (Co³⁺): The 3d⁶ configuration will fill as follows under strong field conditions:
- T₂g: ↑↓ ↑↓ ↑↓ (all paired, no unpaired electrons)
- For Fe(CN)₆³⁻ (Fe³⁺): The 3d⁵ configuration will fill as follows under strong field conditions:
- T₂g: ↑↓ ↑↓ ↑ (one unpaired electron)
- For CoF₆³⁻ (Co³⁺): The 3d⁶ configuration will fill as follows under weak field conditions:
- T₂g: ↑ ↑ ↑ ↑ ↑ (all unpaired, 4 unpaired electrons)
- For FeF₆³⁻ (Fe³⁺): The 3d⁵ configuration will fill as follows under weak field conditions:
- T₂g: ↑ ↑ ↑ ↑ ↑ (all unpaired, 5 unpaired electrons)

6. **Determine the magnetic properties**:
- Co(ox)₃³⁻: Diamagnetic (no unpaired electrons)
- Fe(CN)₆³⁻: Paramagnetic (one unpaired electron)
- CoF₆³⁻: Paramagnetic (four unpaired electrons)
- FeF₆³⁻: Paramagnetic (five unpaired electrons)

### Conclusion:
The diamagnetic complex among the given options is **Co(ox)₃³⁻**.