JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Coordination Compounds

All three ions hold nickel in the $+2$ state and their colour comes from $d$–$d$ transitions, which are Laporte forbidden in any centrosymmetric field. $[\mathrm{NiC}{\mathrm{l}}_4{]}^{2-}$ is tetrahedral and has no centre of symmetry, so $d$ and $p$ orbitals mix and the restriction is lifted, making its absorption the most intense. $[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ is octahedral and centrosymmetric, so its transitions become allowed only through unsymmetrical vibrations and its green colour is correspondingly faint. $[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{2-}$ is square planar and diamagnetic, the strong field of cyanide having paired all the $d$ electrons and pushed the splitting into the ultraviolet, so it is almost colourless. The order of intensity is $[\mathrm{NiC}{\mathrm{l}}_4{]}^{2-}>[\mathrm{Ni}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}>[\mathrm{Ni}(\mathrm{CN}{)}_4{]}^{2-}$.