Chloropentamminecobalt chloride

Synthesis and reactions

The complex is prepared with a two-step process starting with oxidizing a solution of cobalt chloride and ammonia.

2 CoCl₂,6H₂O + 8 NH₃ + 2 NH₄Cl + H₂O₂ → 2 [Co(NH₃)₅(OH₂)]Cl₃ + 12 H₂O

This intermediate is then heated to induce coordination of one of the outer sphere chloride ligands:

[Co(NH₃)₅(OH₂)]Cl₃ → [Co(NH₃)₅Cl]Cl₂ + H₂O

The complex is a dication with idealized C_4v symmetry.

In an aqueous solution, chloropentaamminecobalt(III) chloride reforms aquopentammine complex. With concentrated sulfuric acid, chloropentaamminecobalt(III) chloride forms the hydrogen sulfate complex.

History

Cobalt complexes have been of long-standing interest in inorganic chemistry because they are numerous, easily prepared, and colorful. It was partly on the basis of his study of cobalt coordination chemistry that Alfred Werner was awarded the Nobel Prize in Chemistry. Prior to Werner, the models of amine complexes postulated chains of pentavalent nitrogen centers. This Jørgensen–Bloomstrand model was ovethrown by Werner who introduced the idea that coordination complexes feature metal atoms of octahedral and tetrahedral shapes, with ammonia and other ligands attached individually to the metal. Werner's model accounted for the inner sphere ligands being less reactive. In [Co(NH₃)₅Cl]Cl₂, two chloride ions are outer sphere (counter ions) and one is bound to the Co(III) center: reaction with excess silver nitrate would immediately precipitate the two chloride counter ions, but the bound chloride ion would not be precipitated.