Complex Ions

A complex ion is a charged species formed between a central metal ion and one or more surrounding ligands.
The central ion is usually a transition metal.
The ligands are molecules or ions that donate at least one pair of electrons to the metal ion to form a coordinate bond.
Some common ligands include: water (H2O), ammonia (NH3), chloride ions (Cl-) and cyanide ions (CN-).
Ligands can be monodentate, meaning they form one coordinate bond with the metal ion (like water and chloride ion), or multidentate, forming multiple coordinate bonds (like ethanedioate ion (C2O4^2-), which is bidentate, and EDTA4-, which is hexadentate).
Ligands cause a metal ion to become a complex ion.
The number of coordinate bonds a metal ion forms is called the coordination number.
The coordination number of a complex can indicate the geometry of the complex (for example, a coordination number of 4 often leads to tetrahedral geometry, while a coordination number of 6 often results in octahedral geometry).

Colour of Complex Ions

Complex ions of transition metals are often coloured due to the presence of d-d transitions.
In a d-d transition, an electron in a lower energy d orbital is excited to a higher energy d orbital within the same shell.
The energy required for this transition corresponds to the energy of light in the visible spectrum, resulting in the absorption of coloured light.
The colour observed is the complementary colour of the absorbed light.
The colour of a complex ion can be impacted by several factors: the identity of the metal ion, the oxidation state of the metal ion, the type and number of ligands, and the arrangement of ligands around the metal ion in the complex.

Complex ions are instrumental in many biological processes, notably in the transport of oxygen by the complex ion in haemoglobin.
They are involved in hard and soft water chemistry and can be used in water softening.
Cobalt is used in vitamin B12, and copper is utilised in photosynthesis by chlorophyll.
Complex ions also have significant roles around us in dyes and pigments, and the blue colour of many sapphires is due to the presence of the iron and titanium complex ions.
They play a role in many chemical reactions, including precipitation reactions, redox reactions, and acid-base reactions, and are therefore important in many areas of chemistry.