Which complex of `Co^(2+)` will have the weakest crystal field splitting

Options

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

Explanation

To determine which complex of Co²⁺ will have the weakest crystal field splitting, we need to analyze the ligands associated with the cobalt complex and their respective ligand field strengths.

### Step-by-Step Solution:

1. **Identify the Oxidation State**:
The question specifies Co²⁺, which indicates cobalt is in a +2 oxidation state.

2. **Understand Crystal Field Splitting**:
Crystal field splitting refers to the energy difference between the split d-orbitals in a transition metal complex. The extent of this splitting depends on the nature of the ligands surrounding the metal ion.

3. **Rank the Ligands by Strength**:
Ligands can be classified based on their field strength, which affects the crystal field splitting. The general order of ligand strength (from weak to strong) is:
- Iodide (I⁻) < Bromide (Br⁻) < Chloride (Cl⁻) < Fluoride (F⁻) < Hydroxide (OH⁻) < Water (H₂O) < Ammonia (NH₃) < Ethylenediamine (en) < Cyanide (CN⁻)

4. **Identify the Ligands in the Complexes**:
Common ligands for cobalt complexes include:
- CN⁻ (cyanide)
- en (ethylenediamine)
- Cl⁻ (chloride)
- H₂O (water)

5. **Determine the Weakest Ligand**:
Among the ligands listed, chloride (Cl⁻) is a weaker field ligand compared to cyanide (CN⁻), ethylenediamine (en), and water (H₂O).

6. **Conclusion**:
Therefore, the complex of Co²⁺ with chloride ligands (e.g., [CoCl₄]²⁻) will have the weakest crystal field splitting due to the weak field strength of chloride ligands.

### Final Answer:
The complex of Co²⁺ that will have the weakest crystal field splitting is [CoCl₄]²⁻.