JEE Main 2026 — Coordination Compounds Question with Solution

In $Ni(CO{)}_4$, Nickel is in $0$ oxidation state with configuration $[Ar]3d^{8}4s^2$. $CO$ is a strong field ligand, causing pairing of $4s$ electrons into $3d$ orbitals, resulting in $3d^{10}$ configuration. It is $s{p}^3$ hybridized and diamagnetic (zero unpaired electrons).
In $[Ni(CN{)}_4{]}^{2-}$, Nickel is in $+2$ oxidation state with configuration $[Ar]3d^8$. $C{N}^{-}$ is a strong field ligand, causing pairing of the two unpaired electrons in $3d$ orbitals. It is $ds{p}^2$ hybridized (square planar) and diamagnetic.
In $[NiC{l}_4{]}^{2-}$, Nickel is in $+2$ oxidation state with configuration $[Ar]3d^8$. $C{l}^{-}$ is a weak field ligand and cannot cause pairing of electrons. It has two unpaired electrons in $3d$ orbitals, making it $s{p}^3$ hybridized (tetrahedral) and paramagnetic.
Therefore, $Ni(CO{)}_4$ and $[Ni(CN{)}_4{]}^{2-}$ are diamagnetic, while $[NiC{l}_4{]}^{2-}$ is paramagnetic.