JEE Main · 2021 · 25 Jul 2021 · Shift 1 · Coordination Compounds

A species is drawn into an external magnetic field only when it is paramagnetic, that is only when it has unpaired electrons. Cyanide is a strong field ligand: in $[\mathrm{Co}(\mathrm{CN}{)}_6{]}^{3-}$ the $d^6$ cobalt(III) centre pairs all six electrons in the $t_{2g}$ set, and in square planar $[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{2-}$ the $d^8$ nickel(II) centre pairs its eight electrons below an empty $d_{x^2-y^2}$ orbital. In $[\mathrm{Ni}(\mathrm{CO}{)}_4]$ the metal is nickel(0) with a filled $d^{10}$ shell. Water, however, is a weak field ligand, so the $d^5$ iron(III) centre in $[\mathrm{Fe}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{3+}$ remains high spin with five unpaired electrons. The species that responds to an external magnetic field is $[\mathrm{Fe}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{3+}$.