JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Coordination Compounds

The spin-only moment $\mu =\sqrt{n(n+2)}\mathrm{BM}$ rises with the number of unpaired electrons $n$. In $[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{2-}$ the $d^8$ nickel is square planar with strong-field cyanide, so $n=0$. In $[\mathrm{Cu}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ the $d^9$ copper has $n=1$. In $[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ the $d^8$ nickel is octahedral with weak-field water, so $n=2$. In $[\mathrm{MnB}{\mathrm{r}}_4{]}^{2-}$ the $d^5$ manganese is tetrahedral and high-spin, so $n=5$. The increasing order of magnetic moment is $[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{2-}<[\mathrm{Cu}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}<[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}<[\mathrm{MnB}{\mathrm{r}}_4{]}^{2-}$.