JEE Main 2023 · 30 January · morning · Single correct · +4 / −1
Which of the following is correct order of ligand field strength?
- (a) S²⁻<C₂ O₄²⁻<NH₃ < en <CO
- (b) CO<en<NH₃<C₂ O₄²<S²⁻
- (c) S²⁻<NH₃< en <CO<C₂ O₄²
- (d) NH₃<en<CO<S²⁻<C₂ O₄²
Answer and solution
Answer (a) S²⁻<C₂ O₄²⁻<NH₃ < en <CO
Solution
Chemistry ma’am · 3 steps
Sort them by donor type: a large π-donor sulfur at the weak end, the π-acceptor CO at the strong end.
- Recall the series. The spectrochemical series ranks ligands by how much they split the d orbitals. Its middle stretch runs I⁻ < Br⁻ < S²⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO.
Ligand Donor atom and bonding Field strength S²⁻ large S, π-donor weakest of the five C₂O₄²⁻ O donor, chelating weak NH₃ N donor, pure σ-donor moderate en two N donors, chelating a little stronger than NH₃ CO C donor, σ-donor and π-acceptor strongest of the five - Why this order. π-donors such as S²⁻ push electron density into the t_(2g) set and shrink Δₒ. N donors split more strongly than O donors. CO accepts electron density back from the metal, and this back bonding widens Δₒ the most.
- So. The increasing order is S²⁻ < C₂O₄²⁻ < NH₃ < en < CO, which is option (a).
The idea to keep. For field strength, think donor atom first: halides and S are weak, O is next, N is stronger, and C donors that accept π back bonding (CN⁻, CO) are the strongest.
Answer from the NTA final answer key. Solution by PrepWiser.