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

As $SCN^{-}$ and $CN^{-}$ are weak and strong ligands respectively so during complex formation their behaviours are different.
$SCN^{-}$ being a weak ligand can not pair inner 3d electrons so
$Fe^{3 +}$ i.e. $\left[A r\right]_{18}3d^{5}$
For weak ligand, $t_{2 g}^{3}eg^{2},$ i.e. n = 5
As $CN^{-}$ being a strong ligand can pair inner 3d electrons so
For strong ligand, $t_{2 g}^{5}eg^{0}$ , i.e. n = 1
Magnetic moment $=\sqrt{n \left(\right. n + 2 \left.\right)}B.M.$
$=\sqrt{5 \left(\right. 5 + 2 \left.\right)}=\sqrt{35}B.M.$ for weak ligand.
Magnetic moment for strong ligand $=\sqrt{1 \left(\right. 1 + 2 \left.\right)}=\sqrt{3}B.M.$
Difference in magnetic moment $=\sqrt{35}-\sqrt{3}=4$