13.2 Coloured Complexes

Crystal Field Theory

You should know that there are 5 d-orbitals each usually at the same energy – we say that they are degenerate.

Crystal field theory models ligands as “point-charges” - occupying no space.

Many complex ions are octahedral – ligands form covalent bonds along the x,y and z axes.

Some d-orbitals lie on the axes (dz² and d x²-y²) - their energy is raised.

The other three orbitals lie between the axes and their energy is lowered.

The difference in energy is the Crystal Field Splitting Energy, ∆_o.

It is affected by the oxidation number of the metal ion, the ligand, the geometry of the complex and the metal at its centre.

Different metals cause different amounts of splitting – ie, different ∆o.

Larger metals tend towards larger ∆o – providing that their oxidation states aren’t different.

Look at the relative sizes of metals in the d-block.

• Why would you expect 2+ ions of Period 3 transition elements to have similar ∆o?

• Why would you expect 2+ ions of V and Ta to have quite different ∆o?

Different oxidation states also cause different amounts of splitting – ie, different ∆o.

Larger oxidation states produce larger ∆o for the same metal.

We’re not expected to know exactly why a tetrahedral complex ion will have a different splitting than a square planar complex.

Nor are we expected to know which would be larger.

Crystal Field Theory divides ligands into weak-field ligands and strong field ligands.

Strong-field ligands cause bigger splitting because their charge density is higher.

Below, different complexes of the same ion have different Splitting energies.

The tetrahedral split is smaller – the energy gap is less.

Energy is proportional to frequency, and inversely proportional to wavelength

E ∝ f E ∝ 1/λ

So the frequency of light needed to promote electrons is low – and the wavelength long.

Long wavelengths are at the red end

A solution will appear blue-green – the opposite side of a colour wheel.

After this there are two alternatives:

i) The next electron could singly occupy a higher orbital

ii) The next electron could spin pair in a lower energy orbital

Both are higher energy options than for the first 3 electrons but pairing is the highest energy option.

The extra energy needed to spin pair is the Pairing Energy, P.

We really aren’t expected to know anymore.