A complex ion is an ion that contains a metal cation binded to one or more small molecules or ions (such as NH₃, CN^- or OH^-). The formation of a complex ion is a stepwise process and each step has its own characteristic equilibrium constant.

For the formation of Ag(NH₃)₂⁺, the reactions are:

Notice: when two equations are added together, the equilibrium constants multiply. To learn more about equilibrium constants, please visit General Equilibrium.

Complex ions are soluble in solutions because they are charged. The forces of attraction between the complex ions and water molecules are quite large. (ion-dipole intermolecular forces)

Some cations you may encounter include complex ions of ammonia and hydroxide.

Cations that form complex ions with excess NH₃ are:

1. 
2. 
3. 
4. 

Cations that form complex ions with excess OH^- are:

1. 
2. 
3. 

The formation of a complex ions is suspected when:

1. The addition of a reagent causes precipitate formation followed by dissolving of the precipitate on addition of excess reagent.

OR

2. The formation of a colored species in solution is observed.