28.3.3—Why transition elements form coloured compounds

A transition-metal complex can absorb visible light when a d electron is promoted from a lower split d level to a higher one. The absorbed photon has energy ΔE = hf, so its frequency is set by the size of the splitting.

The colour seen is not the colour absorbed: it is the complementary mixture of wavelengths that pass through or are reflected. A transition is possible only when the metal has an appropriate partially filled d set.

If a complex absorbs mainly orange-red light, the transmitted or observed colour is toward the blue-green complement. Changing ligands can change ΔE and therefore shift the observed colour.

“The d orbitals emit the colour” is the wrong model for the syllabus explanation; the key event is selective absorption during promotion between non-degenerate levels.