28.3 Colour of complexes
- Syllabus
- 9701–2028–2029
- Topic
- 28.3
- Level
- A2
Learning objectives
28.3.1Terms: degenerate and non-degenerate d orbitals• Define/use: degenerate and non-degenerate d orbitals28.3.2d-orbital splitting• Describe the splitting of degenerate d orbitals into two non-degenerate sets of d orbitals of higher energy, and use of Δ E in:: (a) octahedral complexes, two higher and three lower d orbitals; (b) tetrahedral complexes, three higher and two lower d orbitals28.3.3Why transition elements form coloured compounds• Explain why transition elements form coloured compounds in terms of the frequency of light absorbed as an electron is promoted between two non-degenerate d orbitals28.3.4Ligand effects on orbital splitting• Describe, in qualitative terms, the effects of different ligands on Δ E, frequency of light absorbed, and hence the complementary colour that is observed28.3.5Colours of Cu(II) and Co(II) complexes• Use the complexes of copper( II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions as examples of ligand exchange affecting the colour observed
Degenerate orbitals have equal energy; non-degenerate orbitals do not
| Term | Meaning | Application to d orbitals |
|---|---|---|
| degenerate | equal in energy | the five d orbitals of an isolated gaseous metal ion are degenerate |
| non-degenerate | not equal in energy | surrounding ligands split the five d orbitals into two sets with different energies |
Equal energy does not mean equal shape or orientation. The five d orbitals can have different spatial orientations while remaining degenerate before the ligand field is applied.
Ligand-field splitting changes energies within the same d subshell; it does not create a new shell or change the principal quantum number.
Octahedral and tetrahedral ligand fields split the five d orbitals oppositely
| Geometry | Higher-energy set | Lower-energy set | Gap |
|---|---|---|---|
| octahedral | 2: dz², dx²−y² | 3: dxy, dxz, dyz | ΔE between the two sets |
| tetrahedral | 3: dxy, dxz, dyz | 2: dz², dx²−y² | ΔE between the two sets |
Orbitals directed more toward the approaching ligand lone pairs experience greater repulsion and lie higher in energy. Octahedral ligands approach along the axes; tetrahedral ligands approach between the axes, so the 2:3 ordering reverses.
A valid text-first energy diagram has energy increasing upward, two horizontal levels separated by a labelled ΔE, and the correct 2/3 degeneracy on each level for the stated geometry.
ΔE is an energy separation, not an extra orbital. Do not copy the octahedral 2-higher/3-lower arrangement into a tetrahedral complex.
Selective visible-light absorption across ΔE produces the observed complementary colour
ΔE=hf=λhc
A photon is absorbed when its energy matches ΔE. A d electron is promoted from the lower non-degenerate set to the higher set. Removing that frequency from white light leaves the transmitted or reflected complementary colour that the observer sees.
| If ΔE changes… | Absorbed frequency f | Absorbed wavelength λ | Observed colour |
|---|---|---|---|
| larger | higher | shorter | complement of the newly absorbed colour |
| smaller | lower | longer | complement of the newly absorbed colour |
The complex does not simply emit its displayed colour in this syllabus model. It selectively absorbs light for a d–d promotion, and the colour seen is complementary to the principal colour absorbed.
Changing ligand changes ΔE, absorbed frequency and complementary colour
Different ligands interact differently with the metal's d orbitals. Therefore ligand identity can change ΔE; because ΔE = hf, this changes the absorbed frequency and wavelength, so the complementary colour observed changes.
| Ligand-field change | ΔE | Absorbed f | Absorbed λ | What must be concluded about observed colour? |
|---|---|---|---|---|
| stronger splitting | increases | increases | decreases | it changes to the complement of the higher-frequency absorbed light |
| weaker splitting | decreases | decreases | increases | it changes to the complement of the lower-frequency absorbed light |
For a fair ligand comparison, keep track of metal identity, oxidation state and geometry as well as ligand identity; each can affect the splitting. A ligand exchange can change colour while the metal remains Cu(II) or Co(II).
A ligand does not transfer its own colour to the complex. The colour follows from the new metal-centred energy gap and selective absorption.
Cu(II) and Co(II) ligand exchange gives diagnostic colour changes
| Metal(II) / ligand change | Starting species and colour | Product / observation |
|---|---|---|
| Cu²⁺, H₂O → OH⁻ | [Cu(H₂O)₆]²⁺, blue solution | [Cu(H₂O)₄(OH)₂], pale-blue precipitate |
| Cu²⁺, H₂O → excess NH₃ | blue hexaaqua solution; pale-blue precipitate first | [Cu(NH₃)₄(H₂O)₂]²⁺, deep-blue solution |
| Cu²⁺, H₂O ⇌ Cl⁻ | blue [Cu(H₂O)₆]²⁺ | yellow [CuCl₄]²⁻; an equilibrium mixture may look green |
| Co²⁺, H₂O → OH⁻ | [Co(H₂O)₆]²⁺, pink solution | [Co(H₂O)₄(OH)₂], blue precipitate |
| Co²⁺, H₂O → NH₃ | pink hexaaqua solution | ammine species; air oxidation can subsequently alter colour and oxidation state |
| Co²⁺, H₂O ⇌ Cl⁻ | pink [Co(H₂O)₆]²⁺ | blue [CoCl₄]²⁻ |
The incoming ligand changes the ligand field and sometimes coordination geometry, changing ΔE and the light absorbed. Adding water to concentrated chloride equilibria favours the hexaaqua complex and reverses the characteristic colour shift.
Identify the actual species and conditions before naming a colour. A precipitate is a distinct complex product, and any later oxidation of a Co(II) ammine is redox rather than ligand exchange alone.