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transition metal complex question

"Suggest why the compound [Co(NH3)6]Cl3 has a different colour from that of [Co(NH3)4Cl2]Cl."
At first I thought it would be due to the oxidations states of cobalt but they're the same for both compounds (3+). I'm guessing it has something to do with the fact that chlorine is covalently bonded in one of the molecules but I am not sure.
Please could someone help.
Thanks!
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Avatar for Kim J
Kim J
13
Basically different ligands cause different splitting of the degenerate d orbital. In the case of your question, both are octahedral complexes, so the splitting pattern will be same. However, there are different ligands attached , so the energy used for the splitting is different, and the light fequency absorbed will differ. This leads to a different light frquency being reflected as the complementary color.

This explanation (below) is from savemyexams CIE notes 2022. I found it helpful so maybe it can help answer your question in a more detailed way than I did.

Transition element complexes absorb the frequency of light which corresponds to the exact energy difference (ΔE) between their non-degenerate d orbitals/
The frequencies of light which are not absorbed combine to make the complementary colour of the complex
Different ligands will split the d orbital by a different amount of energy. This depends on the repulsion that the d orbital experiences from these ligands. Therefore, the size of ΔE and thus the frequency of light absorbed by the electrons will be slightly different
As a result, a different colour of light is absorbed by the complex solution and a different complementary colour is observed.
This means that complexes with similar transition elements ions, but different ligands, can have different colours.

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