Tetraamine copper(II) complex

Left: Copper(II) chloride solution, Center: Precipitate of copper oxychloride after the addition of ammonia, Right: More ammonia was added, dissolving the precipitate and forming the tetraamine copper(II) complex.

The tetraammine copper(II) complex, also called the cuprammonium complex, is an ionic complex of copper with the formula [Cu(NH₃)₄(H₂O)₂]²⁺. It has an intense ultramarine blue coloration to the naked eye but when portayed digitally is somewhat muted in color. It is formed by the addition of ammonia to solutions of copper(II) ions, or by dissolving basic copper compounds in aqueous ammonia. The ion is the main constituent of the compounds Schweizer's reagent and tetraammine copper(II) sulfate, among others.