Why are transition metal compounds different colours

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Offline seansutherland

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i have been working on this in my head for a few days now. i understand all about the energy levels and how things get colour by absorbing a photon and then reflecting the other light. which gives it colour. but what effect does for example the oxygen in copper (II) oxide  have to make it black and the carbonate in Copper (II) carbonate have to make it cyan. im confused about this. as im asking myself so the energy  differences between d orbitals. do they change?  i feel like the energy must change to give new wavelengths of colour being accepted.
i would appreciate any discussion about this. as i need the answer fairly quickly. thanks sean.


Offline Bored chemist

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Why are transition metal compounds different colours
« Reply #1 on: 03/04/2010 10:28:03 »
Most of the colours of the transition metals are due to electrons hopping between different d orbitals. However, there's another type of transition that gives rise to some of the very intense colours. It's called a charge transfer colour. Copper(II) is quite a strong oxidant, it would like to get one of its electrons back. It can't quite get this from the oxide ion next to it because the oxide ion is a rather poor reducing agent.  But if a photon of the right energy turns up it can promote an electron from the oxide ion to the copper ion. This temporarily gives a copper (I) oxide and a rather odd looking O- ion. The electron falls back to the oxide and this energy is released as heat.

There's yet another complicating factor. The d orbitals in metal ions are fairly well defined- they have definite energies but, once they get crowded tightly together in a big crystal like CuO, they all perturb one another so you get lots of different energy levels. Because of this just about any visible photon  has enough energy to kick an electron from the oxygen onto one of the copper ions, so the CuO absorbs light of any wavelength. That's why it's  black.

The intense purple colour of the MnO4- ion is also a charge transfer colour.
Officially , according to quantum mechanics, the transitions from one d orbital to another are forbidden. In practice they do take place but not as easily as the allowed transitions from one atom to another. That's why the MnO4- ion is much more strongly coloured than, for example, Cu++.

Another thing that makes a difference to the energy levels is what is round them, that's why a solution of Cu++ ions in water changes colour when you add ammonia.
« Last Edit: 03/04/2010 10:30:39 by Bored chemist »
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