Q.Draw enantiomers of [Cr(ox)₃]³⊖

The NO$_2^-$ ligand can bind through N (nitrito-N, $-NO_2$) or through O (nitrito-O, $-ONO$); complexes differing only in this attachment are linkage isomers.

$[Co(NH_3)_5(NO_2)]^{2+}$ has the nitrite ligand bonded to cobalt through its nitrogen atom (nitro form), while $[Co(NH_3)_5(ONO)]^{2+}$ has the same ligand bonded through one of its oxygen atoms (nitrito form).

Both complexes have the identical molecular formula and the same set of ligands, differing only in which donor atom of the ambidentate NO$_2^-$ ligand is attached to the metal. This type of isomerism, arising from an ambidentate ligand binding through two different donor atoms, is called linkage isomerism.

When two ligands (here Cl- and SO4^2-) exchange places between 'inside the coordination sphere' and 'outside as counter ion', the compounds are ionisation isomers.

In [Co(NH3)5Cl]SO4, Cl- is a ligand bound to Co, and SO4^2- is the free counter ion — this compound gives a precipitate with BaCl2 (free SO4^2-) but not readily with AgNO3.

In [Co(NH3)5SO4]Cl, SO4^2- is bound inside the coordination sphere and Cl- is the free counter ion — this compound gives an immediate precipitate with AgNO3 (free Cl-) but not readily with BaCl2.

Both compounds have the same overall formula but ionise differently in water, producing different ions — this is the defining feature of ionisation isomerism (distinct from hydrate isomerism, which involves water molecules, or coordination isomerism, which involves swapping ligands between two different metal centres).

Swapping which ligands sit on the cation vs. the anion between two complex-salt formulas is coordination isomerism.

In $[Cu(NH_3)_4][PtCl_4]$ and $[Pt(NH_3)_4][CuCl_4]$, both compounds have the same overall composition (2 metal atoms, 4 $NH_3$, 4 $Cl^-$), but the distribution of ligands between the two complex ions is exchanged: in the first, $NH_3$ ligands are bound to Cu (cation) and $Cl^-$ to Pt (anion); in the second, this is reversed. This interchange of ligands between the cationic and anionic coordination entities of a compound made of two different complex ions is called coordination isomerism.