Chemguide: Support for CIE A level Chemistry

Learning outcomes 12.3(a), 12.3(b) and 12.3(c)

These statements are about the colours of transition metal complexes.


Basic material




Additional material for CIE

Problems with statement 12.3(c)



The syllabus statement mentions these complexes with copper(II) ions. The example which everyone concentrates on is the major colour difference between the pale blue (cyan) aqua complex, and the very dark blue complex with ammonia - and this is discussed on that page.


Most of the ligands here, of course, are still water, and there isn't much difference between the colour of the precipitate and that of the original hexaaquacopper(II) ion solution. I'm not sure exactly what CIE want here. Up to November 2013, nothing specific had been set on it.

For teachers:

There is also a major problem with a statement in the teacher support material. This says:

"Students should be made aware that the more nucleophilic the ligand (i.e. more basic, as NH3 is compared to H2O), the larger the energy gap between the two sets of d orbitals is, and hence the higher is the frequency of absorption."

OH- ions are much more powerful nucleophiles and are much more strongly basic than either water or ammonia. If this statement was true, then hydroxide ions would cause more splitting than either of the other two. That isn't the case. So that sentence in the support material must be wrong! The energy gap produced by a ligand isn't directly related to its abilities as a base or a nucleophile.

CIE's questions about colour

This is another good example of CIE's questions not following what it asks in either the syllabus or the teacher support material.

The teacher support material follows the general line of my Chemguide page, and talks about the importance of the fact that the colour observed is the complement of the one absorbed. That means that you would need to learn the colour wheel shown on the Chemguide page.

However, the questions asked so far on the whole couldn't be answered by working out the complementary colour, because you were given examples where the light absorbed covered a wide range of frequencies. These were given as an absorption spectrum - something that neither the syllabus nor the teacher support material mentions.




To help you, they included the following table:

energy of photoncolour of photon

Look at the graph labelled C.












The relationships between wavelengths, frequency and energy






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© Jim Clark 2011 (last modified May 2014)