(i) Geometrical isomerism : This type of isomerism arises inheteroleptic complexes due to different possible geometric arrangementsof the ligands. Important examples of this behaviour are found withcoordination numbers 4 and 6. In a square planarcomplex of formula [MX2L2](X and L are unidentate), the two identical ligands may be arranged adjacentto each other in a cis-isomer, or opposite to each other in a trans isomer.
Another type of geometrical isomerism occurs in octahedral coordinationentitiesof the type [Ma3b3] like [Co(NH3)3(NO2)3]. If three donor atoms ofthe same ligands occupy adjacent positions at the corners of an octahedralface, we have the facial (fac) isomer. When the positions are around themeridian of the octahedron,we get the meridional (mer) isomer.(ii) Optical isomerism : Optical isomerism is common in octahedralcomplexes involving bidentate ligands. In a coordination entity of the[PtCl2(en)2]2+, only the cis-isomer shows optical activity.(iii) Linkage isomerism : Linkage isomerism arises in a coordinationcompound containing ambidentate ligand. A simple example is providedby complexes containing the thiocyanate ligand, NCS‌–, which may bindthrough the nitrogen to give M-NCS or through sulphur to give M-SCN.This behaviour was seen in the complex [Co(NH3)5(NO2)]Cl2, which isobtained as the red form, in which the nitrite ligand is bound throughoxygen (–ONO), and as the yellow form, in which the nitrite ligand isbound through nitrogen (–NO2).
(iv) Coordination isomerism : This type of isomerism arises from theinterchange of ligands between cationic and anionic entities of differentmetal ions present in a complex. An example is provided by [Co(NH3)6][Cr(CN)6], in which the NH3 ligands are bound to Co‌3+ and the CN‌–ligands to Cr‌3+. In its coordination isomer [Cr(NH3)6][Co(CN)6], the NH3ligands are bound to Cr‌3+ and the CN‌–ligands to Co‌3+.
(v) Ionisation isomerism : This form of isomerism arises when the counterion 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 theionisation isomers [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4.
(vi) Solvate isomerism : This form of isomerism is known as ‘hydrateisomerism’ in case where water is involved as a solvent. This is similarto ionisation isomerism. Solvate isomers differ by whether or not asolvent molecule is directly bonded to the metal ion or merelypresent as free solvent molecules in the crystal lattice. An example isprovided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvateisomer [Cr(H2O)5Cl]Cl2⋅H2O (grey-green).