Selection Rules for Electronic Spectra of Transition Metal Complexes
- Page ID
- 1772
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.
Transition Types
- 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 forbidden, Laporte forbidden |
[Mn(H2O)6]2+ | 0.1 |
Spin allowed (octahedral complex), Laporte forbidden |
[Ti(H2O)6]3+ | 1 - 10 |
Spin allowed (tetrahedral complex), Laporte partially allowed by d-p mixing |
[CoCl4]2- | 50 - 150 |
Spin allowed, Laporte allowed e.g. charge transfer bands |
[TiCl6]2- or MnO4- | 1,000 - 106 |
Expected Values
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.