Which statement is not true for the complex K_3[Fe(CN)_6] ?

The correct answer is Option (2) → It is weakly repelled by external magnetic field

A substance with unpaired electrons is paramagnetic and is attracted by a magnetic field; only a substance with all electrons paired is diamagnetic and is weakly repelled.

In \( \ce{K3[Fe(CN)6]} \), the charge balance 3(+1) + x + 6(−1) = 0 gives x = +3. Fe (Z = 26) is [Ar]3d⁶4s²; removing the two 4s electrons and one 3d electron gives \( \ce{Fe^{3+}} \): 3d⁵. \( \ce{CN^{-}} \) is a strong-field ligand and pairs these electrons into three 3d orbitals:

    3d: [↑↓][↑↓][↑][ ][ ]

One electron stays unpaired, so the complex is paramagnetic (μ = √3 ≈ 1.73 BM) and is attracted, not repelled, by a magnetic field. Statement (2) is the one that is not true.

• Option (1) It is an inner orbital or low spin complex — True for K₃[Fe(CN)₆]. The two empty 3d orbitals with 4s and 4p give d²sp³ hybridisation; inner (n−1)d orbitals are used and the spin is low.

• Option (2) It is weakly repelled by external magnetic field — Not true for K₃[Fe(CN)₆]. With one unpaired electron the complex is paramagnetic and is attracted by the field.

• Option (3) Geometry of the complex is octahedral — True for K₃[Fe(CN)₆]. Six \( \ce{CN^{-}} \) ligands on d²sp³ orbitals give an octahedron.

• Option (4) Its secondary valency is 6. — True for K₃[Fe(CN)₆]. The secondary valency is the coordination number: six \( \ce{CN^{-}} \) ligands are bonded to Fe.

NCERT (2026-27 edition):
Magnetic Properties of Coordination Compounds – “… The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing d⁶ ions. However, with species containing d⁴ and d⁵ ions there are complications. [Mn(CN)₆]^3– has magnetic moment of two unpaired electrons while [MnCl₆]^3– has a paramagnetic moment of four unpaired electrons. [Fe(CN)₆]^3– has magnetic moment of a single unpaired electron while [FeF₆]^3– has a paramagnetic moment of five unpaired electrons. … [Mn(CN)₆]^3–, [Fe(CN)₆]^3– and [Co(C₂O₄)₃]^3– are inner orbital complexes involving d²sp³ hybridisation, the former two complexes are paramagnetic and the latter diamagnetic.”

Magnetic Properties – “… Diamagnetic substances are repelled by the applied field while the paramagnetic substances are attracted.”