JEE Main · 2025 · 02 Apr 2025 · Shift 2 · d and f Block Elements

First the diamagnetic complexes are counted. $[\mathrm{Zn}(\mathrm{H}{}_2\mathrm{O}){}_6]\mathrm{Cl}{}_2$ ($\mathrm{Zn}{}^{2+}$, $d^{10}$) and $[\mathrm{Cu}(\mathrm{PPh}{}_3){}_3\mathrm{I}]$ ($\mathrm{Cu}{}^{+}$, $d^{10}$) are diamagnetic, whereas $\mathrm{K}{}_2[\mathrm{NiCl}{}_4]$ (tetrahedral $\mathrm{Ni}{}^{2+}$ with two unpaired electrons) and $\mathrm{K}{}_3[\mathrm{Mn}(\mathrm{CN}){}_6]$ (low-spin $\mathrm{Mn}{}^{3+}$ with unpaired electrons) are paramagnetic, so the number of diamagnetic complexes is $n=2$. Among Ni, Zn, Mn and Cu the least enthalpy of atomisation belongs to zinc, whose fully filled $3d^{10}4s^2$ configuration gives weak metallic bonding. The ion formed is $\mathrm{Zn}{}^{2+}$, which is $3d^{10}$ with no unpaired electrons, so its spin-only magnetic moment $\mu =\sqrt{0(0+2)}$ is 0 BM.