tegenanion

A counteranion, also referred to as a counter-anion, is the anion paired with a given cation in a salt or ionic compound. It serves to balance the overall electrical charge of the solid or solution and defines the specific salt form of the compound. In a salt, the cation and anion are not covalently bonded but are held together by electrostatic forces.

A cation can form salts with many different anions, and the choice of counteranion strongly influences the properties of the material. For example, sodium chloride consists of Na+ with the Cl− counteranion, while a salt such as [Et4N]+ BF4− pairs a different cation with the BF4− counteranion. In ionic liquids and pharmaceutical salts, bulky or specialized counteranions (such as PF6−, BF4−, or NTf2−) are chosen to tailor solubility, conductivity, melting point, and stability.

The counteranion affects a range of properties beyond simple charge balance. Solubility in water and organic solvents, lattice energy, hygroscopicity, and thermal stability can all depend on which anion accompanies a given cation. In synthesis and catalysis, selecting an appropriate counteranion can influence reaction rates, crystallization behavior, and the performance of the salt in an application.

In naming, the salt is typically described by the cation first and the counteranion second (for example, sodium chloride or tetraethylammonium tetrafluoroborate). The term emphasizes the role of the anion as the counterpart to the specific cation in the compound.