The text says:

This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace ligand which can then become the counter ion.

For other structural isomers that I studied in coordination compounds I simply thought that those compounds (structural isomers) separately exist. In fact I think of structural isomers in coordination chemistry this way only that these are independently existing with different properties but same molecular formula so we study them.

But the text above is confusing it seems that they convert like resonating structures or conformation isomers.

What is true? If they do interchange then isn't this a contradiction to Werner's theory that postulates that secondary valencies are non ionisable.

note:the below problems is related to one of the comments:

Please Consider this problem-"Give evidences that [Co(NH3)5Cl]So4 and [Co(NH3)5 So4]Cl are ionisation isomers".

Here we test by dissloving them in water and reacting them separately with silver and barium ions to test for counter ions.One gives AgCl and other BaSo4 as ppt. .Now if there is equlibrium like keto- enol tautomerism then only one of the two isomers should be stable in water then why such kinds of evidences are given?

  • $\begingroup$ the counter will become ligand by donating its pair of electrons $\endgroup$
    – JM97
    Aug 29, 2016 at 15:28
  • $\begingroup$ @JM97 But since coordination number is fixed the existing ligand need be ionised for counter ion to become ligand $\endgroup$ Aug 30, 2016 at 4:13
  • 1
    $\begingroup$ ligand exchange will not need ionisation of existing ligand first. Counter ion will attack the metal atom and simultaneously existing ligand will leave the cordination sphere. Similar to what happens in SN2 reaction. They would be equilibrium like in keto-enol tautomerism $\endgroup$
    – JM97
    Aug 30, 2016 at 5:34
  • 1
    $\begingroup$ chem.libretexts.org/Core/Inorganic_Chemistry/… $\endgroup$
    – JM97
    Aug 30, 2016 at 5:38
  • $\begingroup$ @JM97 Please Consider this problem-"Give evidences that [Co(NH3)5Cl]So4 and [Co(NH3)5 So4]So4 are ionisation isomers" $\endgroup$ Sep 2, 2016 at 10:21

1 Answer 1


Actually, in the ionisation isomerism the ligands do not interchange. They get displaced. Thus one ion becomes counter ion for the other. When one has look at the molecular formula it seems the ligands in the coordination sphere are interchanged with the ligands outside the sphere. So, this does not cross out Werner’s theory as there is displacement of ion and not just interchange.

  • $\begingroup$ Please fix that caps lock and use punctuation marks so your answer is intelligible. Recommending deletion since this is incomprehensible. $\endgroup$
    – M.A.R.
    Aug 29, 2016 at 18:55

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