Choose the correct statement regarding complex `CuF_(6)^(-4)`

Options

A. Chelation always increase stability
B. CFSE have strong effect on stability
C. `Fe("dipy")_(3)^(+3)` is unstable
D. All of these

Answer

To solve the question regarding the complex \( \text{CuF}_6^{4-} \), we need to analyze each of the provided statements and determine their validity based on the properties of coordination compounds.

### Step-by-Step Solution:

1. **Understanding Chelation and Stability**:
- Chelation refers to the formation of a stable ring structure when a polydentate ligand binds to a metal ion. This process generally increases the stability of the complex.
- In the case of \( \text{CuF}_6^{4-} \), the ligand is fluoride (\( \text{F}^- \)), which is a monodentate ligand. Therefore, \( \text{CuF}_6^{4-} \) does not exhibit chelation.

**Hint**: Remember that only polydentate ligands can form chelate complexes.

2. **Analyzing the First Statement**:
- The first statement claims that "chelation always increases stability." While this is true for polydentate ligands, it does not apply to \( \text{CuF}_6^{4-} \) because it does not form chelate structures. Hence, this statement is not correct in the context of \( \text{CuF}_6^{4-} \).

**Hint**: Distinguish between polydentate and monodentate ligands when considering chelation.

3. **Crystal Field Splitting Energy (CFSE)**:
- The second statement discusses the effect of crystal field splitting energy (CFSE) on stability. In \( \text{CuF}_6^{4-} \), fluoride is a weak field ligand, leading to a high-spin complex with less pairing energy. Therefore, the statement regarding CFSE having a strong effect on stability is correct.

**Hint**: Recall that weak field ligands typically lead to high-spin complexes.

4. **Evaluating the Third Statement**:
- The third statement mentions that "Fe-Di-Py (diphenyl) is unstable." However, diphenyl is a bidentate ligand and can form stable chelate complexes. Thus, this statement is incorrect.

**Hint**: Bidentate ligands usually enhance stability through chelation.

5. **Conclusion on the Options**:
- Since the first statement is incorrect for \( \text{CuF}_6^{4-} \), the second statement is correct, and the third statement is incorrect, we can conclude that the only correct statement among the options provided is related to CFSE.

### Final Answer:
The correct answer is option **B**: "CFSE has a strong effect on stability."