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Crystal Field Theory says that because of d-orbital splitting caused by different ligands we have 2 different energy levels for d-orbitals.

It also says that transition metals are coloured because photons excite electrons, and when they have the crystal field splitting energy through a specific wavelength of light, the electrons to go to the higher energy d-orbital.

When electrons come back down they emit the same wavelength of light and that's the colour we see.

But then why is $\ce{[Ni(H2O)6]^2+}$ green and $\ce{[Cu(H2O)_6]^2+} $ blue? They have the same type and number of ligands.

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The Cu2+ and Ni2+ complexes, while having the same ligands, have different amounts of d-electrons (8 for Ni and 9 for Cu), meaning their ∆o value is going to be different.

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For both complexes the t2g is completely filled meaning the ∆o only increases based on the electrons in the eg. This is why Cu2+ will absorb higher energy light (orange light) in order to excite an electron to the eg orbital where as Ni will absorb lower energy light (red light) for its promotion. I believe this change in ∆o is rather minor though in comparison to ligand effects and charge and this may be one of the few cases where the change really is due to the number of d electrons. Also keep in mind the ∆o increases as you go down any group of the periodic table which is why you often see more low spin complexes.

Someone please correct me if I'm wrong here, but this is at least my understanding of it.

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When electrons come back down they emit the same wavelength of light and that's the colour we see.

I think this is a major misconception held by students. The color of these transition metal complexes is due to the absorption of light in the visible region. Very crudely, imagine you have six colors in the rainbow, if you remove a certain color portion from that rainbow, the transmitted light is no longer white to our eyes. For example, copper (II) absorbs red, that is why it appears blue to us. Yes, the electron has to relax back to its ground state, it does so non-radiatively.

The spacing (∆o) in the energy level determines the wavelength which will be absorbed.

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  • $\begingroup$ but if 2 complexes absorb the same wavelength of light wouldn't they show the same colour? $\endgroup$ – muhammad haider Jun 19 at 13:00
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    $\begingroup$ @muhammadhaider yes, but the colour depends on the exact energy difference which will, in general, not be the same even for different metal complexes with a similar structure. $\endgroup$ – matt_black Jun 19 at 13:12

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