JEE Main 2018 — d and f Block Elements Question with Solution

The complex having higher number of unpaired electrons will have higher value of spin only magnetic moment. In all these complexes, the central metal ion is in $+2$ oxidation state. ${\mathrm{Zn}}^{2+}$ has $3d^{10}$ outer electronic configuration with 0 unpaired electron. ${\mathrm{Ni}}^{2+}$ has $3d^8$ outer electronic configuration with 2 unpaired electrons. ${\mathrm{Co}}^{2+}$ has $3d^7$ outer electronic configuration with 3 unpaired electrons. ${\mathrm{Mn}}^{2+}$ has $3d^5$ outer electronic configuration with 5 unpaired electrons Hence the correct order of spin-only magnetic moments is $${\left[{\mathrm{MnCl}}_4\right]}^{2-}>{\left[{\mathrm{CoCl}}_4\right]}^{2-}>{\left[{\mathrm{NiCl}}_4\right]}^{2-}>{\left[{\mathrm{ZnCl}}_4\right]}^{2-}\text{. }$$