d–d Transition
The promotion of an electron between split d orbitals, responsible for the characteristic colours of transition metal complexes.
Formula / equation
ΔE = hν = Δ
Unit
nm
In depth
The complex absorbs the wavelength matching the splitting energy and transmits the complementary colour. Because such transitions are formally forbidden by the Laporte rule in centrosymmetric complexes, they are weak, which is why d–d colours are pastel rather than intense.
Examples in the real world
[Cu(H₂O)₆]²⁺ absorbing orange light around 800 nm and therefore appearing pale blue.