Answer» - A large number of coordination compounds show wide range of colours due to d – d transition of electron and this can be explained by crystal field theory (CFT).
- The complex absorbs the light in one visible region (400 nm to 700 nm) and transmits the light in different visible region giving complementary colour.
- Consider an octahedral purple coloured complex of [Ti(H2O)6]3+ which absorbs green light and transmits purple colour. Similarly [Cu(H2O)6]2+ absorbs the light in the red region of radiation spectrum and transmits in the blue region, hence the complex appears blue.
- The absorption of light arises due to d-d transition of electron from lower energy level (t2g) to higher energy level (eg) in octahedral complex.
- The energy required for transition depends upon crystal field splitting energy Δ0. If Δ0 = ΔE, then the energy of an absorbed photon (hv) is ΔE = hv = \(\frac{hc}{\lambda}\) where λ, v and c are wavelength, frequency and velocity of the absorbed light.
- Higher the magnitude of Δ0 or ΔE, higher is the frequency or lower is the wavelength of the absorbed radiation.
- Since Δ0 depends upon nature of metal atom or ion, its oxidation state, nature of ligands and the geometry of the complex, different coordination compounds have different colours.
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