Advanced Higher Chemistry Unit 1 Ligands and colour of transition metal complexes

1. Advanced Higher Chemistry Unit 1 Ligands and colour of transition metal complexes

2. Transition Metal Ions : Colours • Transition metal ions and their complexes are often coloured. • They absorb light in the visible spectrum. • The colour seen is complementary to that absorbed i.e. it is a combination of the colours not absorbed. • White light is a combination of the primary colours red, blue and green.

3. Transition Metal Ions : Colours • If red light is absorbed green and blue are transmitted (green/blue or cyan) e.g.Cu2+ • If blue light is absorbed red and green are transmitted (yellow) e.g.CrO42- • If green light is absorbed red and blue are transmitted (magenta or purple) e.g.MnO4-

4. d Orbitals and ligands • When there are no ligands attached to a transition metal ion, the d orbitals are degenerate. • When ligands attach, the d orbitals split due to electrostatic repulsion.

5. The degree of splitting depends on the ligand.

6. The Spectrochemical Series The ability of ligands to cause the splitting () of the d orbitals is given by the Spectrochemical Series. For the most common ligands this is:

7. d-d transitions • When the complex absorbs light, electrons can become excited and can move from a lower energy d orbital to a d orbital of higher energy. • The energy is lost as heat not light. • Changing the ligand will change the split between d orbitals and, therefore, will change the colour.

8. The colours of many transition metal complexes can be explained in terms of d-d transitions but only if there are electrons in the d orbitals to begin with e.g. the intense purple colour of permanganate ions cannot be explained by d-d transitions because Mn has an oxidation state of +7 and so has no d electrons available. Its colour is due to charge-transfer transitions but this is out with the content of AH.