Selection Rules for Electronic Spectra of Transition Metal Complexes.

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 (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 centre 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 the Rules can occur through:

• a) Spin-Orbit coupling - this gives rise to weak spin forbidden bands
• b) Vibronic coupling - an octahedral complex may have allowed vibrations where the molecule is asymmetric.
  Absorption of light at that moment is then possible.
• c) π-acceptor and π-donor ligands can mix with the d-orbitals so transitions are no longer purely d-d.

Types of transition

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-	1000 - 106

Expected Values

The expected values should be compared to the following rough guide.
For M²⁺ complexes, expect Δ = 7500 - 12500 cm^-1 or λ = 800 - 1350 nm.
For M³⁺ complexes, expect Δ= 14000 - 25000 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 1000 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.

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