Which of the following is flexidentate?

Options

A. CO
B. en
C. EDTA
D. `CI^(-)`

Answer

To determine which of the given ligands is a flexidentate ligand, we need to analyze each option based on the definition of flexidentate ligands. A flexidentate ligand is one that can exhibit variable denticity, meaning it can bind to a metal ion through different numbers of coordination sites.

### Step-by-Step Solution:

1. **Understanding Flexidentate Ligands**:
- A flexidentate ligand is defined as a ligand that can bind to a metal ion through varying numbers of donor atoms (coordination sites). It can be monodentate, bidentate, or even hexadentate depending on the metal and the conditions.

2. **Analyzing the Options**:
- **Option 1: CO (Carbon Monoxide)**:
- CO is a monodentate ligand as it can only donate one lone pair of electrons from the carbon atom to the metal. Therefore, it does not qualify as a flexidentate ligand.

- **Option 2: EN (Ethylenediamine)**:
- Ethylenediamine is a bidentate ligand as it has two nitrogen atoms that can each donate a lone pair of electrons. It does not exhibit variable denticity, so it is not flexidentate.

- **Option 3: EDTA (Ethylenediaminetetraacetic acid)**:
- EDTA has multiple donor sites (six in total) and can bind to a metal ion in different ways. It can act as a hexadentate ligand (binding through all six sites) or a pentadentate ligand (binding through five sites) depending on the metal ion and the conditions. This variability in denticity qualifies EDTA as a flexidentate ligand.

- **Option 4: Chloride (Cl-)**:
- Chloride is a monodentate ligand as it can only donate one lone pair of electrons. Therefore, it is not flexidentate.

3. **Conclusion**:
- Among the options provided, the only ligand that qualifies as a flexidentate ligand is **EDTA**.

### Final Answer:
**EDTA (Ethylenediaminetetraacetic acid) is the flexidentate ligand.**

---