The Laporte rule is a rule that explains the intensities of absorption spectra for chemical species. It is a

selection rule In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...

that rigorously applies to atoms, and to molecules that are

centrosymmetric In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements. In such a point group, for every point (x, y, z) in the unit cell there is an indistinguishable point (-x, -y, -z). Such point grou ...

, i.e. with an inversion centre. It states that electronic transitions that conserve parity are forbidden. Thus transitions between two states that are each symmetric with respect to an inversion centre will not be observed. Transitions between states that are antisymmetric with respect to inversion are forbidden as well. In the language of symmetry, ''g'' (gerade = even (German)) → ''g'' and ''u'' (ungerade = odd) → ''u'' transitions are forbidden. Allowed transitions must involve a change in parity, either ''g'' → ''u'' or ''u'' → ''g''. For atoms s and d orbitals are gerade, and p and f orbitals are ungerade. The Laporte rule implies that s to s, p to p, d to d, etc. transitions should not be observed in atoms or centrosymmetric molecules. Practically speaking, only d-d transitions occur in the visible region of the spectrum. The Laporte rule is most commonly discussed in the context of the electronic spectroscopy of

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

complexes. However, low-intensity f-f transitions in the actinide elements can be observed in the

near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...

region. The rule is named after Otto Laporte who published it in 1925 with William Frederick Meggers.

Optical properties of transition metal complexes

Octahedral complexes have a center of symmetry and thus should show no d-d bands. In fact, such bands are observed, but are weak, having intensities orders of magnitude weaker than "allowed" bands. The extinction coefficients for d-d bands are in the range 5–200 L mol^-1 cm^-1 . The allowedness of d-d bands arises because the centre of symmetry for these chromophores is disrupted for various reasons. Complexes are not perfectly symmetric all the time. Transitions that occur as a result of an asymmetrical vibration of a molecule are called vibronic transitions, such as those caused by vibronic coupling. Through such asymmetric vibrations, transitions are weakly allowed. The Laporte rule is powerful because it applies to complexes that deviate from idealized O_h symmetry. For example, the d-d transitions for r(NH₃)₅Clsup>2+ are weak (ε < 100) even though the complex is only of C_4v symmetry. This is because the Jahn-Teller distortion removes O_h symmetry, but does not affect centrosymmetry; the complex still has a centre of inversion. The Laporte rule helps explain the intense colors often observed for the tetrahedral complexes. The tetrahedral point group lacks the inversion operation, so the Laporte rule does not apply. Illustrative of this effect are the disparate extinction coefficients for octahedral vs tetrahedral complexes of Co(II). For o(H₂O)₆sup>2+, which is pink, ε ≈ 10. For oCl₄sup>2-, which is deep blue, ε ≈ 600.

Note on spin-selection rule

Complementing the Laporte rule, is the spin-selection rule, which forbids transitions that involve changes in spin state. Violations of ''both'' the Laporte and spin-selection rules results in particularly low extinction coefficients. Illustrative of this combined effect is the faintness of even concentrated solutions of octahedral Mn(II) and Fe(III) complexes.

References

{{reflist Absorption spectroscopy Eponymous chemical rules

Optical properties of transition metal complexes

Note on spin-selection rule

See also

References