JEE Main 2021 · 25 July · morning · Single correct · +4 / −1
Which one of the following species responds to an external magnetic field?
- (a) [Fe(H₂O)₆]³⁺
- (b) [Ni(CN)₄]²⁻
- (c) [Co(CN)₆]³⁻
- (d) [Ni(CO)₄]
Answer and solution
Answer (a) [Fe(H₂O)₆]³⁺
Solution
Chemistry ma’am · 3 steps
Only unpaired electrons make a complex respond to a magnet, so count them on each metal ion.
- What to look for. A species is pulled into a magnetic field when it is paramagnetic, that is, when it has at least one unpaired electron. So find the metal's oxidation state, its d-electron count, and whether the ligand pairs those electrons up.
- Ligand strength. CN⁻ and CO are strong-field ligands. They force the d electrons to pair. H₂O is a weak-field ligand, so the electrons stay spread out (high spin).
Species Metal and d count Ligand field Unpaired electrons [Fe(H₂O)₆]³⁺ Fe³⁺, 3d⁵ weak, high spin t_(2g)³ e_g² 5, paramagnetic [Ni(CN)₄]²⁻ Ni²⁺, 3d⁸ strong, square planar (dsp²) 0 [Co(CN)₆]³⁻ Co³⁺, 3d⁶ strong, low spin t_(2g)⁶ 0 [Ni(CO)₄] Ni(0), 3d⁸ 4s² strong, electrons pushed into 3d¹⁰, sp³ 0 - So. Only [Fe(H₂O)₆]³⁺ has unpaired electrons (5 of them, μ ≈ 5.92 BM), so option (a) is the species that responds to the external magnetic field.
The idea to keep. Magnetic response means unpaired electrons. Find the d count, then ask: strong-field ligand like CN⁻ or CO pairs them up, weak-field ligand like H₂O leaves them unpaired.
Answer from the NTA final answer key. Solution by PrepWiser.