JEE Main 2023 · 29 January · evening · Single correct · +4 / −1
Correct order of spin only magnetic moment of the following complex ions is :
(Given At.no. Fe : 26, Co : 27)
- (a) [CoF₆]³⁻ > [FeF₆]³⁻ > [Co(C₂O₄)₃]³⁻
- (b) [FeF₆]³⁻ > [Co(C₂O₄)₃]³⁻ > [CoF₆]³⁻
- (c) [FeF₆]³⁻ > [CoF₆]³⁻ > [Co(C₂O₄)₃]³⁻
- (d) [Co(C₂O₄)₃]³⁻ > [CoF₆]³⁻ > [FeF₆]³⁻
Answer and solution
Answer (c) [FeF₆]³⁻ > [CoF₆]³⁻ > [Co(C₂O₄)₃]³⁻
Solution
Chemistry ma’am · 3 steps
Find each metal's d count, decide high spin or low spin from the ligand, then count unpaired electrons.
- Oxidation state. All three ions carry 3 negative charges with 6 donor sites, and each F⁻ or C₂O₄²⁻ is a negative ligand. So iron and cobalt are both +3: Fe³⁺ is 3d⁵ and Co³⁺ is 3d⁶.
- Ligand strength. F⁻ is a weak-field ligand, so Δₒ is small and electrons spread out (high spin). With Co³⁺, oxalate gives a Δₒ large enough to pair all six electrons in t_(2g) (low spin), so [Co(C₂O₄)₃]³⁻ is diamagnetic.
Complex Configuration Unpaired n μ = √n(n+2) [FeF₆]³⁻ d⁵, high spin, t_(2g)³ e_g² 5 √35 = 5.92 BM [CoF₆]³⁻ d⁶, high spin, t_(2g)⁴ e_g² 4 √24 = 4.90 BM [Co(C₂O₄)₃]³⁻ d⁶, low spin, t_(2g)⁶ e_g⁰ 0 0 BM - So. The order is [FeF₆]³⁻ > [CoF₆]³⁻ > [Co(C₂O₄)₃]³⁻, which is option (c).
The idea to keep. Magnetic moment depends only on unpaired electrons. A high-spin d⁵ ion has the most unpaired electrons possible (5), and a low-spin d⁶ ion has none.
Answer from the NTA final answer key. Solution by PrepWiser.