In its coordination
isomer [Cr(NH3)6][Co(CN)6], the NH3 ligands are bound to Cr
3+ and the
CN
– ligands to Co
3+ This form of isomerism arises when the counter ion in a complex salt
is itself a potential ligand and can displace a ligand which can then
become the counter ion An example is provided by the ionisation
isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4 5