The Selection Rules governing transitions between electronic energy levels of transition metal complexes are:
- ΔS = 0 The Spin Rule
- Δl = +/- 1 The Orbital Rule (or Laporte)
The first rule says that allowed transitions must involve the promotion of electrons without a change in their spin. The second rule says that if the molecule has a center of symmetry, transitions within a given set of p or d orbitals (i.e. those which only involve a redistribution of electrons within a given subshell) are forbidden.
Relaxation of these rules can occur through:
- Spin-Orbit coupling: this gives rise to weak spin forbidden bands
- Vibronic coupling: an octahedral complex may have allowed vibrations where the molecule is asymmetric.
Absorption of light at that moment is then possible.
- Mixing: π-acceptor and π-donor ligands can mix with the d-orbitals so transitions are no longer purely d-d.
- Charge transfer, either ligand to metal or metal to ligand. These are often extremely intense and are generally found in the UV but they may have a tail into the visible.
- d-d, these can occur in both the UV and visible region but since they are forbidden transitions have small intensities.
|Transition type||Example||Typical values of ε /m2mol-1|
|Spin allowed (octahedral complex),
|[Ti(H2O)6]3+||1 - 10|
|Spin allowed (tetrahedral complex),
Laporte partially allowed
by d-p mixing
|[CoCl4]2-||50 - 150|
e.g. charge transfer bands
|[TiCl6]2- or MnO4-||1,000 - 106|
The expected values should be compared to the following rough guide.
- For M2+ complexes, expect Δ = 7,500 - 12,500 cm-1 or λ = 800 - 1,350 nm.
- For M3+ complexes, expect Δ= 14,000 - 25,000 cm-1 or λ = 400 - 720 nm.
For a typical spin-allowed, but Laporte (orbitally) forbidden transition in an octahedral complex, expect ε < 10 m2mol-1. Extinction coefficients for tetrahedral complexes are expected to be around 50-100 times larger than for octrahedral complexes. B for first-row transition metal free ions is around 1,000 cm-1. Depending on the position of the ligand in the nephelauxetic series, this can be reduced to as low as 60% in the complex.