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Selection Rules for Electronic Spectra of Transition Metal Complexes

The Selection Rules governing transitions between electronic energy levels of transition metal complexes are:

  1. ΔS = 0 The Spin Rule
  2. 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

  1. 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.
  2. d-d, these can occur in both the UV and visible region but since they are forbidden transitions have small intensities.
Expected intensities of electronic transitions
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.