Formation of coloured ions

Outline why transition metals are coloured

1. Transition metal ions can absorb certain frequencies of visible light

2. This causes d-electrons to be promoted to higher energy level orbitals (excited state)

3. The remaining colours of light are transmitted or reflected

4. The combination of all these frequencies create a complementary colour that we observe

Why do complexes with a full or empty 3d shell appear colourless or white?

No electrons in the 3d shell can migrate to the higher energy level

What does the frequency of light absorbed by transition metal ions depend on?

∆E (in J)

What is the equation for ∆E?

∆E = h𝓥

• ∆E = Change in energy

• h = Planck’s constant* (6.63 × 10^-34 Js)

• 𝓥 = Frequency of light absorbed (hertz or s^-1)

*given in exam

What is the equation for 𝓥 (Frequency of light absorbed)?

𝓥 = c / λ

• 𝓥 = Frequency of light absorbed (hertz or s^-1)

• c = Speed of light (3.00 × 10⁸ ms^-1)

• λ = Wavelength of light (metres)

How to convert nanometres into metres?

Divide by 10⁹

How to convert metres into nanometres?

Multiply by 10⁹

What the the 3 factors affecting size of ∆E/Colour of transition metal complex?

• Change of ligand

• Change in co-ordination number

• Change in oxidation state of transition metal

Why does change in co-ordination number cause a change in size of ∆E/Colour of transition metal complex?

The splitting in the d-orbitals is different in octahedral complexes than in tetrahedral complexes

Why does change in ligand cause a change in size of ∆E/Colour of transition metal complex?

Different ligands have different effects on the energies of the d-orbitals and so when the ligand changes the size of ∆E changes and hence the colour changes

Why does change in oxidation state of transition metal ion cause a change in size of

As the oxidation state of the metal increases so does the size of the energy gap, ∆E and hence the colour changes

Spectrometry

The study of how electromagnetic radiation interacts with matter

What does a colorimeter measure the amount of? What does this give us a measure of?

• Amount of light absorbed by a solution

• Concentration

Outline the colorimetry method

1. The colorimeter must be set to 0. To do this measure the absorbance of a blank sample (Normally the solvent used to dissolve the transition metal ion)

2. White light is filtered into a narrow range of frequencies. Monochromatic light is produced

3. The monochromatic light passes through the sample and some light is absorbed. The sample is held in a vessel called a cuvette

4. Light not absorbed travels to the detector. The detector will measure the level of absorbance by comparing it to the absorbance of the blank sample

Why is the choice of monochromatic light colour important in colorimetry?

The colour produced from the filter must be absorbed by the metal ion solution

What are the x and y axis of the colorimetry graph?

x = concentration

y = absorbance