Metal Complexes and Colour

Metal Complexes

Metal complexes are formed when metal ions bond to ligands. These can be negatively charged ions such as Cl- or neutral molecules like water.
The number of ligands that can bond to a metal ion is called its coordination number.
Most metal ions form octahedral complexes, where six ligands are arranged around the metal ion. However, they can also form other shapes such as tetrahedral or square planar.
The type of bond that holds the ligand and metal ion together is known as a dative covalent bond, also known as coordinate bonding.
Metal ions in complexes can exhibit variable oxidation states, meaning they can lose different numbers of electrons.
There are different types of ligands such as monodentate (one point of attachment to the central ion), bidentate (two points of attachment) and multidentate (multiple points of attachment).

The colour of a metal complex depends on the wavelength of light absorbed by the metal ion, which in turn depends on the difference in energy between the d-orbitals.
When visible light is shone through a solution containing a metal complex, certain wavelengths (colours) will be absorbed. The transmitted or reflected light will be the complementary colour to the absorbed one.
The specific colours observed with different metal ions and ligands are due to this d-orbital splitting.
In transition metal complexes, the absorption of light promotes an electron from a lower-energy d-orbital to a higher-energy d-orbital. The energy difference between these orbitals corresponds to the energy of the light absorbed.
The energy differences and thus the colours depend on the type of ligand and the oxidation state of the metal ion. Therefore, different metal ions and different ligands result in different colours.
Crystal field theory can be used to explain the colour of metal complexes due to the splitting of d-orbitals in a magnetic field.

Common observations include: Copper(II) complexes are often blue or green, iron(III) complexes are yellow/brown, and nickel(II) complexes are green.
Some compounds like [CuCl4]2- are yellow because they have a different number of ligands and a different shape to the more common blue Cu2+ ion, leading to a different d-orbital energy splitting.
A spectrophotometer can be used to detect and measure the amount of light absorbed by a metal complex at different wavelengths, providing a way to investigate these colour changes quantitatively.
The fact that different metal-ligand combinations absorb light of different colours can be harnessed in applications ranging from chemical analysis to colour photography.