Answer the question on the basis of passage given below:
Transition metals form a large number of complexes or coordination compounds in which the metal atoms are bound to a number of anions or neutral molecules. The valence bond theory explains the formation, magnetic behaviour and geometrical shapes while the crystal field theory explains the effect of different crystal fields on the degeneracy of d-orbital energies of the central metal atom/ion. This provides for the quantitative estimation of orbital separation energies, magnetic moments and spectral and stability parameters.
Amongst the following ions which should have the highest magnetic moment value?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \([CoF_6]^{3-}\)
The spin-only magnetic moment rises with the number of unpaired electrons n:
\( \mu = \sqrt{n(n+2)}\ \text{BM} \)
So the ion with the most unpaired electrons has the highest moment. Strong-field ligands (\( \ce{CN^{-}} \)) pair electrons; weak-field ligands (\( \ce{F^{-}} \), \( \ce{Cl^{-}} \)) do not.
\( \ce{[NiCl4]^{2-}} \): Ni (Z = 28) [Ar]3d84s2 → \( \ce{Ni^{2+}} \) 3d8; weak field, tetrahedral:
3d: [↑↓] [↑↓] [↑↓] [↑ ] [↑ ] → n = 2, \( \mu = \sqrt{8} = 2.83\ \text{BM} \)
\( \ce{[Mn(CN)6]^{4-}} \): charge balance x + 6(−1) = −4, x = +2. Mn (Z = 25) [Ar]3d54s2 → \( \ce{Mn^{2+}} \) 3d5; strong-field \( \ce{CN^{-}} \) pairs the electrons into three orbitals (low spin):
3d: [↑↓] [↑↓] [↑ ] [ ] [ ] → n = 1, \( \mu = \sqrt{3} = 1.73\ \text{BM} \)
\( \ce{[Cr(NH3)6]^{3+}} \): Cr (Z = 24) [Ar]3d54s1 → \( \ce{Cr^{3+}} \) (4s electron and two 3d electrons removed) 3d3:
3d: [↑ ] [↑ ] [↑ ] [ ] [ ] → n = 3, \( \mu = \sqrt{15} = 3.87\ \text{BM} \)
\( \ce{[CoF6]^{3-}} \): charge balance x + 6(−1) = −3, x = +3. Co (Z = 27) [Ar]3d74s2 → \( \ce{Co^{3+}} \) 3d6; weak-field \( \ce{F^{-}} \), high spin:
3d: [↑↓] [↑ ] [↑ ] [↑ ] [↑ ] → n = 4, \( \mu = \sqrt{24} = 4.90\ \text{BM} \)
[CoF6]3−, with four unpaired electrons, has the highest magnetic moment.
Shortcut: compare only n (2, 1, 3, 4); there is no need to work out μ.
NCERT (2026-27 edition):
Magnetic Properties – …
