Transition metal complexes are formed when transition metals are bonded to one or more neutral or negatively charged non-metal species, referred to as ‘ligands’. Without these bonds, all the d orbitals are equal in energy – however, once they are present, some d orbitals move to a higher energy than they were at before, whilst some move to a lower energy, creating an energy gap. This is due to the fact that, due to their differing shapes, some d orbital are nearer to the ligands than others. Electrons can move from the lower energy d orbitals to the higher energy d orbitals by absorbing a photon of light; the wavelength of the absorbed light depends on the size of the energy gap. Any unabsorbed wavelengths of light pass through unabsorbed, and this causes the coloured appearance of the compounds.