CHEM2210 Chapter Notes - Chapter 4: Laporte Rule
Definitions and Theory/Past Exam Questions:
An orbital is the region of space an electron occupies (high probability) e.g. d-
orbital. It is labelled in terms of the angular momentum of the electron.
A configuration is assignment of electrons to a set of orbitals e.g. dxy1 dyz0
dxz0
A term is an energy level of the (multi)electron systems – it describes one or
more (degenerate) electron configurations e.g. 2 T2g
Why do d-d transitions display low intensities?
• Laporte forbidden
- i.e. they involve transition between orbitals of the same type
(same orbital angular momentum
- They only gain intensity by mixing with other orbitals (p etc),
relaxing the laporte rule (extent of mixing is typically small – the d-
orbitals are core like, not strongly overlapping with the 4s and 4p
orbitals (and the corresponding ligand orbitals)
By what mechanisms do d-d transitions gain intensity?
• Gain intensity by symmetry breaking.
• Transitions between d-orbitals gain intensity by mixing with other
orbitals
• Mixing between p and d orbitals however cannot occur in
centrosymmetric complexes - mechanisms which remove the centre of
inversion lead to d-d transitions gaining intensity.
- These can be either static (associated with the ligand or packing
effects in solid state) or dynamic (vibrations of the complex).
Typically, do d-d transitions from octahedral or tetrahedral complexes display
higher intensities? Why?
• Orbitals mix when they display the same symmetry.
E.g. Tetrahedral complex - the set of p orbitals and one set of d-orbitals (dxy,
dxz, dyz) do have the same symmetry (t2) and thus mix, relaxing the laporte
selection rule.
- The same is not true for octahedral complexes. Because they
are centrosymmetric, the set of p-orbitals and two sets of d-
orbitals do not fall into the same group i.e. the p orbitals are in
the t1u group whereas the d-orbitals (dxy, dxz, dyz) are in the t2g
group.
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Document Summary
An orbital is the region of space an electron occupies (high probability) e. g. d- orbital. It is labelled in terms of the angular momentum of the electron. A configuration is assignment of electrons to a set of orbitals e. g. dxy1 dyz0 dxz0. A term is an energy level of the (multi)electron systems it describes one or more (degenerate) electron configurations e. g. 2 t2g. Why do d-d transitions display low intensities: laporte forbidden i. e. they involve transition between orbitals of the same type (same orbital angular momentum. These can be either static (associated with the ligand or packing effects in solid state) or dynamic (vibrations of the complex). Why: orbitals mix when they display the same symmetry. Tetrahedral complex - the set of p orbitals and one set of d-orbitals (dxy, dxz, dyz) do have the same symmetry (t2) and thus mix, relaxing the laporte selection rule. The same is not true for octahedral complexes.
