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Which of the following complex ions is not expected to absorb visible light?
A
$[Ni(H_2O)_6]^{2+}$
B
$[Ni(CN)_4]^{2-}$
C
$[Cr(NH_3)_6]^{3+}$
D
$[Fe(H_2O)_6]^{2+}$
Detailed Solution
A complex absorbs visible light (and so appears coloured) when it has unpaired d electrons that can undergo d–d transitions of suitable energy.
$[Ni(CN)_4]^{2-}$: $Ni^{2+}$ is $3d^8$; the strong field ligand $CN^-$ pairs all the electrons ($dsp^2$, square planar), leaving no unpaired electron. With the large splitting it does not absorb in the visible region.
$[Ni(H_2O)_6]^{2+}$: $Ni^{2+}$ ($3d^8$) with a weak field ligand has 2 unpaired electrons; it is green.
$[Cr(NH_3)_6]^{3+}$: $Cr^{3+}$ ($3d^3$) has 3 unpaired electrons; it is coloured.
$[Fe(H_2O)_6]^{2+}$: $Fe^{2+}$ ($3d^6$, high spin) has 4 unpaired electrons; it is pale green.
Hence $[Ni(CN)_4]^{2-}$ is not expected to absorb visible light.
$[Ni(CN)_4]^{2-}$: $Ni^{2+}$ is $3d^8$; the strong field ligand $CN^-$ pairs all the electrons ($dsp^2$, square planar), leaving no unpaired electron. With the large splitting it does not absorb in the visible region.
$[Ni(H_2O)_6]^{2+}$: $Ni^{2+}$ ($3d^8$) with a weak field ligand has 2 unpaired electrons; it is green.
$[Cr(NH_3)_6]^{3+}$: $Cr^{3+}$ ($3d^3$) has 3 unpaired electrons; it is coloured.
$[Fe(H_2O)_6]^{2+}$: $Fe^{2+}$ ($3d^6$, high spin) has 4 unpaired electrons; it is pale green.
Hence $[Ni(CN)_4]^{2-}$ is not expected to absorb visible light.