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Which of the following complex ions is expected to absorb visible light? (At. no. Zn = 30, Sc = 21, Ti = 22, Cr = 24)
A
$[Zn(NH_3)_6]^{2+}$
B
$[Sc(H_2O)_3(NH_3)_3]^{3+}$
C
$[Ti(en)_2(NH_3)_2]^{4+}$
D
$[Cr(NH_3)_6]^{3+}$
Detailed Solution
A complex absorbs visible light when the metal ion has a partly filled d subshell, so that d–d transitions are possible.
$Zn^{2+}$: $3d^{10}$, completely filled; no d–d transition, colourless.
$Sc^{3+}$: $3d^0$; no d electrons, colourless.
$Ti^{4+}$: $3d^0$; no d electrons, colourless.
$Cr^{3+}$: $3d^3$, with three unpaired electrons in $t_{2g}$; $t_{2g} \rightarrow e_g$ transitions absorb visible light.
Hence $[Cr(NH_3)_6]^{3+}$ is expected to absorb visible light.
$Zn^{2+}$: $3d^{10}$, completely filled; no d–d transition, colourless.
$Sc^{3+}$: $3d^0$; no d electrons, colourless.
$Ti^{4+}$: $3d^0$; no d electrons, colourless.
$Cr^{3+}$: $3d^3$, with three unpaired electrons in $t_{2g}$; $t_{2g} \rightarrow e_g$ transitions absorb visible light.
Hence $[Cr(NH_3)_6]^{3+}$ is expected to absorb visible light.