6.2.8 Coloured Complexes

• Most transition element complexes are coloured
• A transition element complex solution which is coloured, absorbs part of the electromagnetic spectrum in the visible light region
• The observed colour is the complementary colour which is made up of light with frequencies that are not absorbed
  • For example, copper(II) ions absorb light from the red end of the spectrum
  • The complementary colour observed is therefore pale blue (cyan)

The visible light region of the electromagnetic spectrum

Electron promotion

• In an isolated transition element ion (which is not bonded to any ligands), all of the 3d orbitals are degenerate
• However, when ligands are attached to the central metal ion through dative covalent bonds, these orbitals are split into two sets of non-degenerate orbitals
• The difference in energy between these two sets of orbitals is ΔE
• When light shines on a solution containing a transition element complex, an electron will absorb this exact amount of energy (ΔE)
• The amount of energy absorbed can be worked out by the equation:

ΔE = h x v

h = Planck's constant (6.626 x 10^-34 m² kg s^-1)

v = frequency (Hertz, Hz or s^-1)

• The electron uses the energy from the light to jump into a higher, non-degenerate energy level
  • This is also called electron promotion

• The other frequencies of light which are not absorbed combine to make the complementary colour
• The diagram below shows an example of electron promotion in an octahedral complex of a nickel(II) Ni²⁺ ion

Electron promotion in a Ni(II) complex when light shines on the solution

• Transition element complexes absorb the frequency of light which corresponds to the exact energy difference (ΔE) between their non-degenerate d orbitals
• The frequencies of light which are not absorbed combine to make the complementary colour of the complex
• It is the complementary colour which is seen
• However, the exact energy difference (ΔE) is affected by the different ligands which surround the transition element ion
• Different ligands will split the d orbital by a different amount of energy
• This depends on the repulsion that the d orbital experiences from these ligands
• Therefore, the size of ΔE and thus the frequency of light absorbed by the electrons will be slightly different
• 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 similar 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 ion having an oxidation state of +2 in both complexes
  • This is evidence that the ligands surrounding the complex ion affect the colour of the complex

• Different ligands may affect the complementary colour of a transition ion complex solution
• This is shown by ligand exchange reactions in copper(II) and cobalt(II) complexes, as this causes a change in colour of the complexes

Copper(II) & cobalt(II) ions

• The ligand exchange of [Cu(H₂O)₆]²⁺ and [Co(H₂O)₆]²⁺ by NH₃ ligands causes a change in the colour of the solutions
  • [Cu(H₂O)₆]²⁺ is light blue in colour whereas [Cu(NH₃)₄(H₂O)₂)]²⁺ is deep blue in colour
  • [Co(H₂O)₆]²⁺ is a pink solution whereas [Co(NH₃)₆]²⁺ is a brown solution

• The colour change results from the ammonia ligands, which cause the d orbitals to split by a different amount of energy (ΔE)
• Therefore, the size of ΔE and the frequency of light absorbed by the electrons will be slightly different
• As a result, a different colour of light is absorbed and thus a different complementary colour is observed

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

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

• Similarly, full ligand exchange by chloride ions in copper(II) and cobalt(II) complexes results in a change in complementary colour

Cu(OH)₂(H₂O)₄ (s)  +  4Cl^-   →   [CuCl₄]^2- (aq)  +  4H₂O(l)  +  2OH^- (aq)

                                                 pale blue precipitate                     yellow solution

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

Ligand exchange by chloride ligands causes a change in colour of the cobalt(II) complex solution

• As before, this suggests that different ligands will split the d orbitals differently