Although d-d transitions are Laporte forbidden transitions, they do occur in transition metal complexes, why?
According to Laporte rule, g → g or u → u are forbidden transitions. Since all d-orbitals are gerade (g), hence d - d transitions in transition metal complexes are forbidden transitions. These should not cause change in dipole moment i.e. |∫Ψ M Ψ* d𝜏|² is zero. However they do become allowed due to following reasons-- Vibration of compounds may distort symmetry (octahedral or tetrahedral) of compounds and compounds thus lose centre of symmetry. Thus, due to coupling of electronic and vibrational wave functions (vibronic coupling) d-d transitions become allowed.
- There may be some mixing of d-orbitals with p or f orbitals and thus we may have dp → dp transition rather than pure d-d-transition.
- Metal d-orbitals may overlap with ligand orbitals and thus pure d-orbital character is lost.