The value of magnetic moment for [CoF_6]^3- is:

The correct answer is Option (4) → 4.9 BM

The magnetic moment of a first-row transition metal complex is given by the spin-only formula, where n is the number of unpaired electrons:

    \( \displaystyle \mu = \sqrt{n(n+2)}\ \text{BM} \)

In \( \ce{[CoF6]^3-} \), x + 6(−1) = −3, so cobalt is +3. Co (Z = 27) is [Ar] 3d⁷4s²; removing the two 4s electrons and one 3d electron gives \( \ce{Co^3+} \), [Ar] 3d⁶. \( \ce{F-} \) is a weak field ligand, so the d electrons do not pair up (high-spin, outer orbital sp³d² complex):

    3d: [↑↓] [↑] [↑] [↑] [↑], n = 4

    \( \displaystyle \mu = \sqrt{4(4+2)} = \sqrt{24} = 4.90\ \text{BM} \)

This is the same value as for any 3d⁶ ion with 4 unpaired electrons, such as \( \ce{Fe^2+} \) in the table below.

NCERT (2026-27 edition):
Magnetic Properties (Table 4.7: Calculated and Observed Magnetic Moments (BM))