IB Chemistry HL

Revision Notes

13.1.6 Factors Affecting Colour

Factors Affecting Colour

  • 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 complexesChemistry 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

  • Ammonia has a greater charge density than water and so produces a larger split in the d-orbitals

Absorption spectra of two copper(II) complexes, downloadable IB Chemistry revision notes

Graph showing the replacement of the water molecules with four ammonia molecules causes a shift in maximum absorbance towards shorter wavelength

The nuclear charge

  • The strength of the attraction between the metal ion and lone pairs of electrons from the ligand can vary depending on the effective nuclear charge on the metal ion
  • For example, aqueous Mn(II) and Fe(III) have the same electronic configuration:

[Ar] 3d5

  • Mn(II) (Z=25) absorbs in the green region of the spectrum so appears pink
  • The higher effective nuclear charge on Fe(III) (Z= 26) causes a stronger interaction with the ligands, so it absorbs in the higher energy blue part of the spectrum and appears yellow/orange in colour

Oxidation state

  • When the same metal is in a higher oxidation state 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

Shape

  • 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

The Spectrochemical Series

  • The Japanese chemist, R. Tuschida, proposed a ranking of ligands base on their ability to separate the two sets of d-orbitals
  • This is known as the spectrochemical series
  • The higher the charge density of the ligand; the more strongly it influences the splitting of the d-orbitals so the greater the energy difference between them

Table showing the spectrochemical series for common ligands

Spectrochemical Series, downloadable IB Chemistry revision notes

  • You can see at either end of the series lies iodide ions, I, and carbon monoxide, CO
  • Iodide ions are large (think how many electron shells they have) so they have a relative low charge density and produce the weakest electric field so the separation energy of the d-orbitals is the smallest in the series
  • Chloride ions, Cl, on the other hand are smaller, have a higher charge density and consequently produce a large separation energy
  • However, size is not the only factor as carbon monoxide and cyanide ions produce a larger splitting due to complex interactions involving the pi bonds present in those molecules; those interactions occur when electrons in the d-orbitals of the transition metal interact with electrons in the p-orbitals of the ligands
  • You should be able to see why adding ammonia to aqueous copper(II) ions results in a darker blue complex
    • Ammonia is a stronger ligand than water so the separation energy is larger and the wavelength of absorbed light shorter
    • Shorter wavelength means moving towards to bluer higher energy end of the visible spectrum

Exam Tip

You do not need to learn the spectrochemical series as it is given in the data booklet in section 15. A list of polydentate ligands is also given in the data booklet in section 16.

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