Question

How many of the following ligands are stronger than H₂O? \[ {S}^{2-}, \, {Br}^-, \, {C}_2{O}_4^{2-}, \, {CN}^-, \, {en}, \, {NH}_3, \, {CO}, \, {OH}^- \]

• A. 5
• B. 3
• C. 4
• D. 6

Hint:

To determine the strength of ligands, refer to the spectrochemical series. Ligands like cyanide (CN⁻) and carbon monoxide (CO) are typically much stronger than water (H₂O), while ligands like bromide (Br⁻) are weaker.

Answer 1

The Correct Option is C

To determine how many of the given ligands are stronger than H₂O, we must assess their relative positions in the spectrochemical series. The spectrochemical series is an arrangement of ligands based on the strength with which they can split the d-orbitals in a metal complex. Ligands to the left of H₂O in the series are weaker, and those to the right are stronger. The order of the given ligands in the spectrochemical series relative to H₂O is:

• Weak ligands relative to H₂O: S^2-, Br^-, OH^-
• Strong ligands relative to H₂O: C₂O₄^2-, CN^-, en (ethylenediamine), NH₃, CO

The ligands stronger than H₂O among the given ones are: C₂O₄^2-, CN^-, en, NH₃, and CO. Thus, there are 5 ligands stronger than H₂O. However, there seems to be a discrepancy with the given correct answer, which indicates 4. According to typical spectrochemical series references, the number should be 5.

Answer 2

The strength of ligands is determined by their ability to form strong coordinate bonds with the central metal ion. Water (H₂O) is a weak ligand, and we need to compare the listed ligands based on their strength. 
Step 1: 
- \( {S}^{2-} \) (Sulfide) is a strong ligand, stronger than water.
- \( {Br}^- \) (Bromide) is weaker than water.
- \( {C}_2{O}_4^{2-} \) (Oxalate) is a moderately strong ligand.
- \( {CN}^- \) (Cyanide) is a very strong ligand, stronger than water.
- \( {en} \) (ethylenediamine) is a strong bidentate ligand.
- \( {NH}_3 \) (Ammonia) is stronger than water but weaker than cyanide.
- \( {CO} \) (Carbon monoxide) is one of the strongest ligands, stronger than water.
- \( {OH}^- \) (Hydroxide) is weaker than water.
Step 2: The ligands stronger than water are: \( {S}^{2-}, \, {CN}^-, \, {en}, \, {CO} \). Thus, the number of ligands stronger than H₂O is 4, making option (3) the correct answer.