Which of the following complex ions is not expected to absorb visible light?
- [Ni(CN)4]2-
- [Cr(NH3)6]3+
- [Fe(H2O)6]2+
- [Ni(H2O)6]2+
Answer
(1) [Ni(CN)4]2-
Solution
[Cr(NH3)6]3+ (3d3), [Fe(H2O)6]2+ (3d6, high spin) and [Ni(H2O)6]2+ (3d8) have unpaired electrons and weak / moderate crystal field splitting d–d transitions absorb visible light (coloured).
[Ni(CN)4]2-: Ni2+ (3d8) with the strong field ligand CN- dsp2, square planar, all electrons paired (diamagnetic).
The splitting by CN- is very large, the energy needed for promotion lies outside the visible region.
[Ni(CN)4]2- is not expected to absorb visible light.
Answer: (1)