• 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

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