How does crystal field theory apply to transition metal complexes?

Crystal field theory explains the colour, magnetic properties, and structures of transition metal complexes.

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 theory is particularly useful in explaining the behaviour of transition metal complexes. Transition metals are unique because they have incompletely filled d orbitals, which can accept electrons from ligands to form coordination complexes.

In the context of transition metal complexes, the surrounding charge distribution is provided by the ligands - ions or molecules that donate electron pairs to the metal. The ligands approach the metal ion, and the degenerate (same-energy) d orbitals of the metal ion are affected by the electric field of the ligands. This causes the energy levels of the d orbitals to split into two sets of different energies. The extent of this splitting depends on the nature of the ligands and the geometry of the complex.

The splitting of the d orbitals explains many properties of transition metal complexes. For instance, the colour of these complexes is due to the absorption of certain wavelengths of light, which corresponds to the energy difference between the split d orbitals. When light is absorbed, an electron is excited from a lower energy d orbital to a higher energy one. The remaining wavelengths of light are then transmitted or reflected, and this is the colour we observe.

Furthermore, the magnetic properties of transition metal complexes can also be explained by CFT. If there are unpaired electrons in the d orbitals, the complex will be paramagnetic (attracted to a magnetic field). If all the electrons are paired, the complex will be diamagnetic (not attracted to a magnetic field).

Lastly, the structure of the complex (whether it is tetrahedral, square planar, or octahedral, for example) can be predicted by CFT. The geometry that results in the lowest overall energy for the complex is the one that is adopted. This is determined by the arrangement of the ligands around the metal ion that causes the least repulsion between the electrons in the d orbitals and the negative charge on the ligands.