JEE Main 2026 Jan 28 Shift 1, Chemistry Q20: Colour, Magnetism And Stability Of Co-ordination Compounds

Determine the electron configuration and geometry of each Ni complex.

Ni(CO)${}_4$: Ni is in oxidation state 0, configuration 3d${}^{10}$ after pairing under strong-field CO ligands. Geometry sp${}^3$, all electrons paired $\Rightarrow$ diamagnetic.

$[{\text{Ni(CN)}}_4{]}^{2-}$: Ni${}^{2+}$ is 3d${}^8$. CN${}^{-}$ is a strong-field ligand causing pairing; geometry dsp${}^2$ (square planar), all paired $\Rightarrow$ diamagnetic.

$[{\text{NiCl}}_4{]}^{2-}$: Ni${}^{2+}$ is 3d${}^8$. Cl${}^{-}$ is weak-field; geometry sp${}^3$ (tetrahedral) with two unpaired electrons $\Rightarrow$ paramagnetic.

$\therefore$ Ni(CO)${}_4$ and $[{\text{Ni(CN)}}_4{]}^{2-}$ are diamagnetic; $[{\text{NiCl}}_4{]}^{2-}$ is paramagnetic. Option (3).