A solution of potassium iodide is added to a solution of lead nitrate. A yellow precipitate of lead iodide forms, and as more iodide is added, the soluble adduct PbI42- is formed.
An aqueous solution of cobalt-II contains the pink adduct Co(OH2)62+. When hydrochloric acid is added to the solution, the blue CoCl42- adduct is formed. The reaction can be reversed by adding water to regenerate the Co(OH2)62+ adduct.