Colour of Coordination Compounds

Most transition-metal complexes are coloured because they absorb light of a particular wavelength from the visible region and transmit the rest; the colour we perceive is the complementary colour of the light that was absorbed (for example, hydrated Cu²⁺ absorbs orange light and appears blue, blue being orange's complementary colour on the colour wheel). Crystal Field Theory explains this: the ligand field splits the metal's d orbitals into the t2g and eg sets, and when white light falls on the complex, the metal ion absorbs the specific wavelength of visible light whose energy exactly matches the crystal field splitting energy, promoting an electron from the lower t2g level to the higher eg level -- a process called a d-d transition -- while transmitting the rest of the light, which is what we see as the complex's colour. Using [Ti(H₂O)₆]³⁺ as a worked example: Ti³⁺ has a d¹ configuration, with its single electron occupying one of the t2g orbitals; absorbing light at 20000 cm⁻¹ (corresponding to Δ₀ = 239.7 kJ/mol) promotes that electron to the eg level, and the light transmitted after this absorption is purple, so the complex appears purple. Octahedral titanium(III) complexes of other ligands (bromide, fluoride) show different colours because those ligands produce a different magnitude of crystal field splitting. Complexes of met …