TetraamminkobberII

Tetraamminkobber(II) is a coordination complex with the chemical formula [Cu(NH₃)₄]²⁺. It is a deep blue compound, often encountered as a sulfate salt, tetraamminkobber(II) sulfate, [Cu(NH₃)₄]SO₄. The central copper ion is in the +2 oxidation state and is coordinated to four ammonia ligands. This complex is planar square geometry.

The tetraamminkobber(II) ion is readily formed when copper(II) salts, such as copper(II) sulfate or copper(II) chloride, are dissolved in aqueous ammonia. The ammonia molecules displace water molecules from the hydration sphere of the copper(II) ion. The formation of this complex is a common demonstration in introductory chemistry labs due to the striking color change from the pale blue of hydrated copper(II) to the intense deep blue of the tetraamminkobber(II) complex.

This complex is soluble in water and also in aqueous ammonia. It can be decomposed by acidification, which protonates the ammonia ligands, releasing them as ammonium ions and reforming a hydrated copper(II) ion. The tetraamminkobber(II) ion is an important intermediate in various copper-related chemical reactions and is used in analytical chemistry for the detection of certain substances. It is also known for its use in Fehling's solution, a reagent used to test for reducing sugars.