vibronic coupling

If the Laporte rule strictly forbids d-d transitions in a centrosymmetric complex, how does a copper or nickel solution have any color at all? The escape hatch is that molecules are never frozen still — they are always wobbling, stretching and bending. Vibronic coupling is the marriage of these vibrations (vibr-) with the electronic transition (-onic), and it is how a forbidden band borrows just enough intensity to be seen.

Here is the mechanism in plain steps. A perfect octahedron has a center of symmetry, so its d-d band is parity-forbidden. But as the molecule vibrates, certain non-symmetric vibrations — ones that momentarily push the ligands out of their symmetric arrangement — destroy that center of inversion for an instant. During that fleeting asymmetric moment, the molecule no longer has the symmetry that forbade the transition, so the d orbitals mix a little with parity-odd orbitals (like the metal p orbitals), and a photon can be absorbed. The transition rides on the vibration: it is allowed only because the molecule is caught mid-wobble without its center of symmetry.

This is why Laporte-forbidden bands are faint rather than absent, and why they tend to be broad rather than sharp — each electronic jump is dressed in a spread of vibrational energies. It also explains a temperature signature: warm the sample and the molecule vibrates more vigorously, so the asymmetric distortions are larger and more frequent, and a vibronically allowed band actually grows more intense (cooling weakens it). Vibronic coupling is therefore the quiet reason behind the gentle, persistent colors of the great many octahedral transition-metal complexes that the selection rules would otherwise leave colorless.