L3 - Coloured Complexes

To begin with this lesson we first must continue to develop our understanding of complex ions before we relate it to Newton's Colour wheel.

Bonding in Complex Ions

• A brief look at [Fe(H₂O)₆]⁺³ can help us to see how this bonding is represented:

Why not more ligands?

• Due to steric Hindrance - you simply cannot fit any more waters around the ion due to it's small size - larger ions can fit less and this goes some way to explain the coordination numbers of 4 and 2.

What does it look like?

Complex Ions and Colours

• As their is a different number of electrons, charge density, ligand on each complex ion the complexes can absorb visible light of different colours for different complexes.
• This is a key learning point in IB Chemistry
• The identity of the metal ion and the ligand give rise to the colour of the complex due to the extent of d-d orbital splitting.
• We only need to know how this splits for Octahedral complexes in the IB.

d-d orbital splitting

So what happens to the 3d shell if it is not involved in bonding with the ligands? It is Split.

In the below diagram you can see that the presence of the ligands causes the d orbitals to SPLIT into 2 levels - a lower energy level with 3 orbitals and a higher energy level with 2 orbitals.

As you can see from the right hand diagram - when the electrons are excited they can jump from the lower energy level to the higher energy level.

When they do this they absorb Energy in the visible region of Light - this absorbance leads to a transmission of intense colour!

Why do different Complexes have different colours?

• The degree of d-d splitting depends on the central ion - which has different numbers of electrons - and the ligands.
• We can use the Spectrochemical Series and even our knowledge of colour science to work out which ligand causes the most d-d splitting.
• In this series (found below and on page 14 of the data booklet) Iodide ions cause the LEAST d-d splitting and CN- ions cause the most.
• When you increase d-d splitting you Decrease the wavelength of light absorbed.
• if E = hf
• then we know that if E increases so does f (frequency)
• We should also know that f is inversely proportional to wavelength

• Take the above diagram showing two different ligands on the same Metal Ion
• You can see that the difference in Magnitude of the d-d splitting is greater for the CN- rather than the Water
• This is represented by the difference between the energy levels in each one - in the Water the energy levels are closer together.
• Hence we can see that Stronger ligands (found to the right of the spectrochemical series) split the d-d orbitals more than weaker ligands.

Predicting the Magnitude of d-d Splitting

1. Using the Spectrochemical Series

• This is a list starting with the smallest difference in splitting (I-) to the largest splitting difference (CN-)
• If a ligand is exchanged (read - replaced) with another, stronger, ligand then that would mean the colour transmitted would change - i.e. if I- was replaced with CN- the colour of the complex would completely change.

2. Using the colour transmitted

• You need to know that E = hf and f ∝ ƛ
• Understanding these relationships means that:
  • as d-d splitting increases - E absorbed increases
  • This means that f (frequency) increases (as h is planck's constant)
  • This means that ƛ (Wavelength) gets smaller

• This means that if you have the colours of 4 different complexes you can tell that the complex that transmits the highest wavelength colour must absorb at the lowest wavelength
• This means that the d-d splitting must be highest for this complex

• In the above table examples we will be asked to pick the complex which has the largest d-d splitting
• Using the Colour Wheel (again found on page 14 of the data booklet) we can see that the largest wavelength transmitted comes from the orange
• We can also use the colour wheel to find the colour absorbed - in this example we use complimentary colours to explain this. A complimentary colour is the opposite colour on the colour wheel. When one colour is absorbed you see the opposite colour transmitted. In our example blue light is absorbed.
• This means that the shortest wavelength is absorbed in this complex is blue light.
• As this has the shortest wavelength it has the highest Energy
• This means that the Orange Complex has the largest d-d splitting