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 of d sub-levels 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. The nature of the transition metals involved significantly influences these effects.

IB Chemistry Tutor Summary: Crystal field theory shows how colour in transition metal complexes is due to light absorption leading to electronic transitions. When ligands surround a metal ion, they split its d orbital energies into two levels. The colour we see comes from the light not absorbed, which depends on the metal, ligands, and complex geometry. Strong-field ligands often result in blue or green colours, while weak-field ligands can lead to red or orange hues. For more detailed examples on how these processes result in various colours, refer to observed versus absorbed colours in transition metal complexes. Additionally, further insights into the influence of d-block elements on colour can be explored in the notes on the colour of d-block complexes.