Which among the following is a polydentate ligand?

Options

A. Oxalate
B. Ethane-1, 2-diamine
C. `EDTA^(4-)`
D. `SCN^-`

Answer

To determine which among the given options is a polydentate ligand, we need to analyze each option based on the number of donor sites they possess. A polydentate ligand is defined as a ligand that can attach to a central metal atom at multiple points (more than two donor sites).

### Step-by-Step Solution:

1. **Identify the Options**:
- Option 1: Oxalate
- Option 2: Ethane-1,2-diamine
- Option 3: EDTA4-
- Option 4: SCN^-

2. **Analyze Option 1: Oxalate**:
- The structure of oxalate ion (C2O4^2-) contains two carboxylate groups (C(=O)O^-).
- Each carboxylate group has two oxygen atoms that can donate lone pairs.
- Therefore, oxalate can form two coordination bonds with a metal ion.
- **Conclusion**: Oxalate is a bidentate ligand (2 donor sites).

3. **Analyze Option 2: Ethane-1,2-diamine**:
- The structure of ethane-1,2-diamine (NH2-CH2-CH2-NH2) contains two amine groups.
- Each nitrogen atom has a lone pair that can be donated to form a bond with a metal ion.
- Therefore, ethane-1,2-diamine can also form two coordination bonds.
- **Conclusion**: Ethane-1,2-diamine is a bidentate ligand (2 donor sites).

4. **Analyze Option 3: EDTA4-**:
- EDTA4- (ethylenediaminetetraacetate) has a complex structure with two nitrogen atoms and four oxygen atoms from the acetate groups.
- The two nitrogen atoms can donate lone pairs, and each of the four oxygen atoms can also donate lone pairs.
- This gives a total of six donor sites (2 from nitrogen and 4 from oxygen).
- **Conclusion**: EDTA4- is a hexadentate ligand (6 donor sites), making it a polydentate ligand.

5. **Analyze Option 4: SCN^-**:
- The thiocyanate ion (SCN^-) can bind through either the sulfur (S) or the nitrogen (N) atom.
- However, it can only form a bond through one of these atoms at a time, making it ambidentate.
- **Conclusion**: SCN^- is not a polydentate ligand as it has only one donor site at a time.

### Final Conclusion:
The only option that qualifies as a polydentate ligand is **Option 3: EDTA4-**.