Formation of Complex Ions

Metal ions are always hydrated in aqueous solutions. The water molecules act as Lewis bases, sharing their lone pair of electrons with the metal ions, which behave as Lewis acids.

When a Lewis base stronger than water is added, it displaces the water molecules and surrounds the central metal ion, forming a complex ion. The molecule or ion acting as the Lewis base is called a ligand.

In hexammine cobalt(III) chloride, hexammine cobalt is a complex ion, where the 6 ammonia molecules are the ligands that octahedrally surround the central cobalt ion.

Because transition metal ions have a high charge density and empty d orbitals to accommodate shared electrons, they are particularly prone to forming complex ions.

The equilibrium constant for the reaction between the metal ion and the ligand is called the formation constant, K_f. The larger the value of K_f, the more stable the complex ion.

Forming such stable complex ions often increases the solubility of sparingly soluble metal salts.

Consider silver sulfide, which exists in solution in an equilibrium of aqueous ions and undissolved solid.

If silver sulfide is added to sodium cyanide solution, the silver ions combine with cyanide to form