JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Coordination Compounds

Each complex is read for its unpaired electrons. With ammonia and with oxalate, cobalt(III) ($d^6$) is low spin and fully paired, $t_{2g}^6$, so both are diamagnetic. $[\mathrm{MnC}{\mathrm{l}}_6{]}^{3-}$ has manganese(III) ($d^4$) in the weak chloride field, $t_{2g}^3{e}_g^1$, four unpaired. $[\mathrm{Mn}(\mathrm{CN}{)}_6{]}^{3-}$ has the same ion in the strong cyanide field, $t_{2g}^4$, two unpaired. $[\mathrm{Co}{\mathrm{F}}_6{]}^{3-}$ is high-spin cobalt(III), $t_{2g}^4{e}_g^2$, four unpaired. $[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{3-}$ is low-spin iron(III) ($d^5$), $t_{2g}^5$, one unpaired, while $[\mathrm{Fe}{\mathrm{F}}_6{]}^{3-}$ is high-spin iron(III), $t_{2g}^3{e}_g^2$, five unpaired. Among the paramagnetic complexes the value four is shared by $[\mathrm{MnC}{\mathrm{l}}_6{]}^{3-}$ and $[\mathrm{Co}{\mathrm{F}}_6{]}^{3-}$, so the number of species carrying the same count of unpaired electrons is two.