Single correct · +4 / −1
The crystal field splitting energy (Δ₀) is largest for:
- (a) [Co(H₂O)₆]³⁺
- (b) [Co(H₂O)₆]²⁺
- (c) [Co(CN)₆]³⁺
- (d) [Co(F)₆]³⁺
Answer and solution
Answer (c) [Co(CN)₆]³⁺
Solution
Chemistry ma’am · 4 steps
Check the ligand first, then the metal's charge: the strongest-field ligand on Co³⁺ wins.
- What sets Δ₀. For the same metal, two things decide the size of Δₒ. The first is the ligand's place in the spectrochemical series. The second is the metal's oxidation state, because a higher charge pulls the ligands closer.
- Spectrochemical order. The relevant part of the series is F⁻ < H₂O < CN⁻. CN⁻ is a strong-field ligand: its π-acceptor orbitals take electron density back from the metal and widen the gap.
Complex Metal ion Ligand strength Δₒ [Co(H₂O)₆]³⁺ Co³⁺ weak to moderate medium [Co(H₂O)₆]²⁺ Co²⁺ weak to moderate smaller (lower charge) Co(III) with six CN⁻ Co³⁺ strong field largest Co(III) with six F⁻ Co³⁺ weak field small - A note on charge. Co(III) with six CN⁻ ions is really [Co(CN)₆]³⁻, so the 3+ printed in the option is a typo. The comparison does not change.
- So. The strongest-field ligand on the higher-charged metal ion gives the largest splitting. That is the cyanide complex, option (c).
The idea to keep. For the same metal, Δ₀ grows with ligand strength (I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO) and with the metal's oxidation state.
Answer from the NTA final answer key. Solution by PrepWiser.