3.1.10 (HL)—Transition element colors

Light can be absorbed to promote an electron between split d-orbitals. The observed colour is complementary to the colour absorbed.

Use the colour wheel to map absorbed colour to observed complementary colour, and explain the absorption through d-orbital splitting and promotion.

Worked calculation: for absorbed light of wavelength $600\,\mathrm{nm}=600\times10^{-9}\,\mathrm{m}$, $f=c/\lambda=(3.00\times10^8\,\mathrm{m\,s^{-1}})/(600\times10^{-9}\,\mathrm{m})=5.00\times10^{14}\,\mathrm{s^{-1}}$. Use the colour wheel separately: absorption in the orange region means the observed colour is the complementary blue region. Frequency and wavelength describe the absorbed radiation, not the colour label by themselves.

If a complex absorbs orange light, use the colour wheel to predict the observed complementary blue. The absorbed photon promotes a d electron between ligand-split levels; the observed colour is transmitted or reflected light, not the colour of the absorbed radiation.

The simple d–d model requires an electron in a lower split d level and an available higher d level. A d⁰ or d¹⁰ ion therefore has no d–d transition in this model. The energy gap—and hence absorbed wavelength—also depends on the metal ion, oxidation state, ligand and geometry, so an exact shade cannot be predicted without the coordination environment.