Structural isomerism: ionization isomers

   Ionization isomers result from the interchange of an anionic ligand within the first coordination sphere with an anion outside the coordination sphere. Examples of ionization isomers are violet [Co(NH₃)₅Br][SO₄] (prepared by reaction scheme 1.1) and red [Co(NH₃)₅(SO₄)]Br (prepared by reaction sequence 1.2).  These isomers can be readily distinguished by appropriate qualitative tests for ionic sulfate or bromide, respectively.  The isomers are also easily distinguished by IR spectroscopy; free and coordinated sulfate ions give rise to one or three IR active S^_O stretching vibrations respectively.

                              (1.1)

                    (1.2)