Ionisation isomerism occurs when the counter ion in a complex salt becomes a ligand. In [Co(NH₃)₅(SO₄)]Br, SO₄ displaces a ligand, while in [Co(NH₃)₅Br]SO₄, Br⁻ becomes the ligand. This is similar to ‘hydrate isomerism’ when water is involved as a solvent.

Optical isomers are mirror images that cannot be superimposed, known as enantiomers. Enantiomers are chiral and rotate the plane of polarized light either to the right (dextro – d) or to the left (laevo – l) in a polarimeter.

In octahedral coordination entities like [Co(NH₃)₃(NO₂)₃], if ligands occupy adjacent positions at the corners of an octahedral face, it is the facial (fac) isomer. When they are around the meridian, it is the meridional (mer) isomer.

Geometrical isomerism occurs in complexes of the formula [MX₂(L–L)₂] when didentate ligands like ethane-1,2-diamine (en) are present.

In [MX₂L₂], square planar complexes, the ligands X can be arranged adjacent (cis) or opposite (trans). Such isomerism is not possible in tetrahedral geometry. Octahedral complexes [MX₂L₄] also exhibit similar cis-trans isomerism.