JEE Main · 2021 · 20 Jul 2021 · Shift 2 · Coordination Compounds

The spin-only moment $\mu =\sqrt{n(n+2)}$ BM equals 1.73 BM only when $n=1$. In $[\mathrm{Cu}(\mathrm{N}{\mathrm{H}}_3{)}_4]\mathrm{C}{\mathrm{l}}_2$ the metal is $\mathrm{C}{\mathrm{u}}^{2+}$, a $3d^9$ ion carrying one unpaired electron. In ${\mathrm{O}}_2^{-}$ the antibonding ${\pi }^{\ast }$ set holds three electrons and in ${\mathrm{O}}_2^{+}$ it holds one, so each of these ions also has a single unpaired electron. In CuI the metal is $\mathrm{C}{\mathrm{u}}^{+}$, which is $3d^{10}$ with every $d$ orbital doubly filled, so $n=0$ and the moment is zero. CuI is the species whose spin-only moment is not 1.73 BM.