JEE Advanced · 2021 · 03 Oct 2021 · Paper 1 · d and f Block Elements

For ${\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$: Cr³⁺ has electronic configuration [Ar]3d³. NH₃ is a strong field ligand causing pairing, but with only 3 electrons, all remain unpaired in t₂g orbitals. Number of unpaired electrons n = 3.
$\mu =\sqrt{n(n+2)}=\sqrt{3(5)}=\sqrt{15}\approx 3.87$ BM

For ${\left[{\mathrm{CuF}}_6\right]}^{3-}$: Cu³⁺ has configuration [Ar]3d⁸. F⁻ is a weak field ligand, so no pairing occurs beyond what's already present. In octahedral field: t₂g⁶ e_g². Number of unpaired electrons n = 2.
$\mu =\sqrt{2(4)}=\sqrt{8}\approx 2.84$ BM