How does crystal field theory explain colour in transition metal complexes?

Crystal field theory explains colour in transition metal complexes through the absorption of light causing electronic transitions.

In more detail, crystal field theory (CFT) is a model that describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to a static electric field produced by a surrounding charge distribution. This surrounding charge distribution is created by ligands - ions or molecules that bind to the central metal ion in a complex. The ligands create an electric field that affects the energy levels of the d orbitals of the transition metal ion.

In an isolated atom, the five d orbitals are degenerate - they have the same energy. However, when ligands approach the metal ion, they cause these five d orbitals to split into two different energy levels. This splitting is what gives rise to colour in transition metal complexes.

When white light, which contains all colours, shines on a transition metal complex, certain wavelengths of light are absorbed. This absorbed light corresponds to the energy difference between the two sets of d orbitals. The remaining light, which is not absorbed, reaches our eyes and gives the complex its colour. For example, if a complex absorbs light in the red region of the spectrum, it will appear green, the complementary colour to red.

The extent of the splitting, and hence the colour observed, depends on several factors. These include the nature of the metal ion, the type of ligands attached to it, and the geometry of the complex. For instance, strong-field ligands like cyanide (CN-) cause a large splitting of the d orbitals and complexes with these ligands often appear blue or green. On the other hand, complexes with weak-field ligands like iodide (I-) usually appear red or orange because they cause a smaller splitting.

In summary, the colour of transition metal complexes is a direct result of the d orbital splitting caused by the surrounding ligands, as explained by crystal field theory. This splitting leads to the absorption of certain wavelengths of light, with the remaining light giving the complex its observed colour.