Laporte or parity selection rule

An electronic transition takes place through the interaction of the electric held of incident electromagnetic radiation with a component of the dipole moment of the absorbing atomic or molecular system. Such transitions usually involving light in the visible region of the spectrum can occur only between states that differ in parity that is, one state must have a symmetric (g) wave function and the other an antisymmetric (n) wave function. 

3 Energy level diagrams and crystal field spectra 

The probability of a transition, P which influences the intensity of an absorption band, depends on the square of the transition moment, Q, expressed as 

The transition moment, Q, is related to the oscillator strength,/, which is a measure of intensity of an absorption band (table 3.2), by 

The Laporte selection rule is weakened, or relaxed, by three factors first, by the absence of a centre of symmetry in the coordination polyhedron second, by mixing of d and p orbitals which possess opposite parities and third, by the interaction of electronic 3d orbital states with odd-parity vibrational modes. If the coordination environment about the cation lacks a centre of symmetry, which is the case when a cation occupies a tetrahedral site, some mixing of d 

---