Laporte rule
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The Laporte rule in coordination chemistry states that when the transition is
- g → g
or
- u → u
in the ligand field splitting diagram, the transition is forbidden.
The rule originates from a quantum mechanical selection rule, which states that during an electron transition, parity should be inverted.
g = gerade u = ungerade
A designation of g means there is symmetry with respect to an inversion center. That is, if all the atoms (or orbitals) are inverted across the metal center, the resulting compound would look exactly how it did before having inversion applied to it (this includes same orientation in space). A designation of u means otherwise, namely that there is no inversion center.
Moreover, it's very important to note that these rules can be relaxed. This rule basically says that having inversion symmetry "forbids" transitions. Spectroscopy indicates that this isn't the case (citation would probably be HKK Inorganic textbook that I don't have at hand now). Due to asymmetric vibrations, complexes are not perfectly symmetric all the time. This "relaxes" the Laporte rule and allows for transitions that would theoretically be forbidden.
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