Question

For the octahedral complexes of $F{e}^{3+}$ in $SC{N}^{-}$ (thiocyanato -S) and in $C{N}^{-}$ 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

Answer: 4

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