Molecular autoionization (or self-ionization) is a reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing.:163 In most cases the oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic ( transfer), or non-protic.
Protic solvents often undergo some autoionization:
These solvents have all possess atoms with odd atomic numbers, either nitrogen or a halogen. Such atoms enable the formation of singly charged, nonradical ions (which must have at least one odd atomic number atom), which are the most favorable autoionization products. Protic solvents, mentioned previously, use hydrogen for this role. Autoionization would be much less favorable in a solvent such as sulfur dioxide or carbon dioxide which has only even atomic number atoms.
Autoionization is not restricted to neat liquids or solids. Solutions of metal complexes exhibit this property. For example, compounds of the type FeX2(terpyridine) are unstable with respect to autoionization [Fe(terpyridine)2]2+[FeX4]2-.