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

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

Transition Metal Ions : Colours n If red light is absorbed green and blue are transmitted (green/blue or cyan) e.g.Cu 2+ n If blue light is absorbed red and green are transmitted (yellow) e.g.CrO 4 2- n If green light is absorbed red and blue are transmitted (magenta or purple) e.g.MnO 4 -

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

n The degree of splitting depends on the ligand.

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:

d-d transitions n 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. n The energy is lost as heat not light. n Changing the ligand will change the split between d orbitals and, therefore, will change the colour.

n 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.