tetrammiinkobaltIII

Tetramminecobalt(III) is a coordination complex with the chemical formula [Co(NH₃)₄(H₂O)₂]³⁺ or [Co(NH₃)₄Cl₂]⁺. It is a salt containing the tetramminecobalt(III) cation. The cobalt ion is in the +3 oxidation state and is coordinated to four ammonia ligands and two other ligands, which can be water molecules or chloride ions depending on the specific salt. These complexes are generally diamagnetic and exhibit octahedral geometry.

The synthesis of tetramminecobalt(III) complexes typically involves reacting cobalt(II) salts with ammonia and an oxidizing agent. For example, the reaction of cobalt(II) chloride with concentrated ammonia and hydrogen peroxide can yield tetramminecobalt(III) chloride. The color of these complexes varies; for instance, trans-dichlorotetramminecobalt(III) chloride is a deep green solid, while cis-dichlorotetramminecobalt(III) chloride is a violet solid. Aquated forms are typically pink or red.

Tetramminecobalt(III) complexes have been important in the historical development of coordination chemistry. Alfred Werner, the father of modern coordination theory, extensively studied these compounds and used them to demonstrate his theories about the structure of inorganic complexes, including the concept of isomers. The stability of the cobalt(III) oxidation state in these ammine complexes made them ideal for elucidating principles like isomerism and coordination numbers. These complexes are relatively stable and can be isolated as solid salts.