Question

For the octahedral complexes of $Fe^{3+}$in $SCN$ (thiocyanato-S) and in $CN^{-}$ ligand environments, the difference between the spin-only magnetic moments in Bohr magnetons (when approximated to the nearest integer) is
[Atomic number of $Fe = 26$]

Answer 1

$Fe\left(26\right)\to\left[Ar\right]_{18} 3d^{6} 4s^{2}$
$Fe^{3+}\to\left[Ar\right]_{18} 3d^{5} 4s^{0}$
$SCN^{-}$ is weak field ligand hence pairing will not occur.
$\therefore Fe^{3+}\to$
Unpaired electrons. $n = 5$
$\therefore \mu_{1} = \sqrt{n \left(n +2\right)}B.M =\sqrt{35}B.M$
For strong field ligand, $CN^{-}$,pairing will occur.
$\therefore Fe^{3+}\to$
Hence, unpaired, $n = 1$
$\therefore \mu_{2} = \sqrt{n \left(n +2\right)} = \sqrt{3}B.M$
$\therefore $ Difference in magnetic moment
$\left(\mu_{1} - \mu_{2}\right) = \sqrt{35} - \sqrt{3} = 4$