Colours of Ions

Chemistry: Cambridge International A Level, topic 2.4.6

d Sub-Shell Splitting

The colour of transition metals ions depends on their ligands and their geometries.

Without ligands, all of the d orbitals have the same energy.
In the presence of ligands, the orbitals will split. Some of them gain energy, and some of them lose energy.
This is shown in the following diagram.
The difference in energy of the upper level and the lower level is given the symbol ΔE.

Electrons will occupy the lower energy orbitals first. This is called the ground electronic state of the ion.
If an electron absorbs energy equal to the energy gap, it can move to occupy the higher energy orbitals. This is called an excited electronic state.
Electrons will absorb frequencies of light that contain enough energy to jump the energy gap.
This is shown pictorially on the next slide.

You can calculate the energy gap from the wavelength of absorbed light by using the following formula:
$\Delta E = h\nu = \frac{hc}{\lambda}$
ν is the frequency of light in hertz, h is planck’s constant (which will be given if you need it), c is the speed of light in ms^-1 and λ is the wavelength of the light in metres.
The energy gap depends on the metal ion, it's oxidation state, it's ligands, and also it's coordination number.

The colour of a transition metal ion depends on the colour of the light it absorbs.

When a transition metal ion is in light, it will absorb the frequencies which correspond to the d sub-shell energy gap.
The rest of the frequencies will be reflected.
You only see the reflected light.

Suppose you have a metal complex which absorbs red light.
The red light is removed from the light you can see.
You see the rest of the colours in the spectrum.
So the complex appears blue.
Metal ions that absorb red light do NOT appear red, because there’s no red light for you to see.

Every transition metal ion will be a different colour with different ligands.
But we can identify all the hexaaqua ions (the ones with six water ligands), and we also know the colours of some other specific ones.
The ones you need to know are on the next slide.

On the previous slide, we gave you the colours of certain ions. While these colours are true, you might not always observe them.
Iron(III) in solution usually appears yellow or orange if its concentrated.
This is because hexaaqua iron(III) is quite acidic, and will lose protons to become Fe(H₂O)₅(OH^-) (and this is yellow).
If you’re asked what colour hexaaqua iron(III) is, you should say purple. Just be aware that if it's in solution, you’ll have a yellow solution.
We’ll talk about the acidic properties in a later module.

You can use the colour of ions to find their concentrations. This uses a technique called spectroscopy.

You can shine white light through a coloured filter to remove everything but that colour of light.
You can then let this light fall on a sample of a transition metal solution.
The more light it absorbs, the higher the concentration of the solution.
We can compare the amount of light absorbed to a calibration curve (this is explained on the next slide).

When doing spectroscopy, we can’t calculate the concentration of a sample without comparing it to known concentrations.
We first measure the absorbances of solutions whose concentrations we know. Then we plot these on a graph.
This graph is called a calibration curve. An example is on the next slide.
We then use the absorbance of the unknown sample to work out from the graph what its concentration is.