JEE Main 2021 — Coordination Compounds Question with Solution

Spin only magnetic moment, $$\mu =\sqrt{n(n+2)}BM$$

where, n = number of unpaired electrons and $$\mu$$ $$\propto$$ n.

(i) $$[Fe{F}_6{]}^{3-}\Rightarrow F{e}^{3+}=(3d^5),F^{-}$$ (weak field ligand).

Thus, pairing of electron does not take place.

(ii) $$[Co(N{H}_3{)}_6{]}^{3+}\Rightarrow C{o}^{3+}=(3d^6),N{H}_3$$ (strong field ligand).

Thus, pairing of electron takes place.

(iii) $$[NiC{l}_4{]}^{2-}\Rightarrow N{i}^{2+}=(3d^8),C{l}^{-}$$ (weak field ligand).

Thus, pairing of electron takes place.

(iv) $$[Cu(N{H}_3{)}_4{]}^{2+}\Rightarrow C{u}^{2+}(3d^9),N{H}_3$$ (strong field ligand).

Thus, pairing of electron takes place.

So, the decreasing order of $$\mu$$ is

$$\underset{(n=5)}{{\mu }_i}>\underset{(n=2)}{{\mu }_{iii}}>\underset{(n=1)}{{\mu }_{iv}}>\underset{(n=0)}{{\mu }_{ii}}$$