Colour in Aqueous ions (Edexcel International A Level (IAL) Chemistry): Revision Note

Exam code: YCH11

Stewart Hird

Last updated

Colour in Aqueous ions

Perception of colour

  • Most transition metal compounds appear coloured. This is because they absorb energy corresponding to certain parts of the visible electromagnetic spectrum

  • The colour that is seen is made up of the parts of the visible spectrum that aren’t absorbed

  • For example, a green compound will absorb all frequencies of the spectrum apart from green light, which is transmitted

  • The colours absorbed are complementary to the colour observed

The colour wheel, downloadable AS & A Level Biology revision notes

The colour wheel showing complementary colours in the visible light region of the electromagnetic spectrum

  • Complementary colours are any two colours which are directly opposite each other in the colour wheel

    • For example, the complementary colour of red is green and the complementary colours of red-violet are yellow-green

Splitting of 3d energy levels

  • In a transition metal atom, the five orbitals that make up the d-subshell all have the same energy.

  • Ions that have completely filled 3d energy levels (such as Zn2+) and ions that have no electrons in their 3d subshells (such as Sc3+) are not coloured

  • Transition metals have a partially filled 3d energy level

  • When ligands attach to the central metal ion the energy level splits into two levels with slightly different energies

    • If one of the electrons in the lower energy level absorbs energy from the visible spectrum it can move to the higher energy level

    • This process is known as promotion / excitation

  • The amount of energy absorbed depends on the difference between the energy levels

    • A larger energy difference means the electron absorbs more energy

  • The amount of energy gained by the electron is directly proportional to the frequency of the absorbed light and inversely proportional to the wavelength

orbital-splitting

Upon bonding to ligands, the d orbitals of the transition element ion split into sets of orbitals with different energies

Colour Changes in Aqueous ions

The size of the splitting energy ΔE in the d-orbitals is influenced by the following four factors:

  • The size and type of ligands

  • The nuclear charge and identity of the metal ion

  • The oxidation state of the metal

  • The shape of the complex

Chemistry of Transition Elements - Coloured Transition Metal Complexes, downloadable AS & A Level Chemistry revision notes

The large variety of coloured compounds is a defining characteristic of transition metals

Size and type of ligand

  • The nature of the ligand influences the strength of the interaction between ligand and central metal ion

  • Ligands vary in their charge density

  • The greater the charge density; the more strongly the ligand interacts with the metal ion causing greater splitting of the d-orbitals

  • The further it is then shifted towards the region of the spectrum where it absorbs higher energy

  • As a result, a different colour of light is absorbed by the complex solution and a different complementary colour is observed

  • This means that complexes with the same transition elements ions, but different ligands, can have different colours

    • For example, the [Cu(H2O)6]2+ complex has a light blue colour

    • Whereas the [Cu(NH3)4(H2O)2]2+ has a dark blue colour despite the copper(II) ion having an oxidation state of +2 in both complexes

Chemistry of Transition Elements - Copper(II) Change in Colour, downloadable AS & A Level Chemistry revision notes

Ligand exchange of the water ligands by ammonia ligands causes a change in colour of the copper(II) complex solution

Oxidation number

  • When the same metal has a higher oxidation number that will also create a stronger interaction with the ligands

  • If you compare iron(II) and iron (III):

    • [Fe(H2O)6]2+ absorbs in the red region and appears green

    • But, [Fe(H2O)6]3+ absorbs in blue region and appears orange

Coordination number

  • The change of colour in a complex is also partly due to the change in coordination number and geometry of the complex ion

  • The splitting energy, ΔE, of the d-orbitals is affected by the relative orientation of the ligand as well as the d-orbitals

  • Changing the coordination number generally involves changing the ligand as well, so it is a combination of these factors that alters the strength of the interactions

    • For example, the [Cu(H2O)6]2+ complex has a light blue colour

    • Whereas the [CuCl4]2– has a yellow colour due to the change in the ligand and the change in the coordination number from 6 to 4 

      • In practice, there would be a change from light blue to green (due to a mixture of both ions) to yellow

    • Since the reaction is reversible, it can also go from yellow, [CuCl4]2–,  to green to light blue, [Cu(H2O)6]2+ 

copper-chloride-yellow-complex

You've read 1 of your 5 free revision notes this week

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Download notes on Colour in Aqueous ions