Here the isomers have different arrangement of ligands around the central metal atom. It is of the following types :

(i) `text(Ionisation isomerism)` : The co-ordination compound having the same composition or molecular formula but gives different ions in solution are called ionization isomers. There is exchange of anions between the co-ordination sphere and ionization sphere.

Example : `[CoBr(NH_3)_5 ]SO_4` - `text(pentaamminebromocobalt) (III) text(sulphate)`; `SO_4^(2-)` present in ionisation sphere gives white precipitate with `BaCl_2`

`[Co SO_4 (NH_3 )_5 ] Br` - `text(pentaamminesulphatocobalt) (III) text(bromide)`; `Br^-` present in ionization sphere gives light yellow precipitate with `AgNO_3`

(ii) `text(Co-ordination isomerism)` : In this case compound is made up of cation and anion and the isomerism arises due to interchange of ligands between complex cation and complex anion.

Example : `[Co(NH_3)_6 ] [Cr(CN)_6]` - `text(hexaamminecobalt) (III) text(hexacyanochromate) (III)`

complex cation contains `->` `NH_3` ligand (with cobalt)
complex anion contains `->` `CN^-` ligand (with chromium)

`[Cr (NH_3)_6][Co(CN)_6]` - `text(hexaamminechromium)(III) text(hexacyanocobalt)(III)`

complex cation contains `->` `NH_3` ligand (with chromium)
complex anion contains `->` `CN^-` ligand (with cobalt)

(iii) `text(Linkage isomerism)` : In this case isomers differ in the mode of attachment of ligand to central metal ion and the phenomenon is called linkage isomerism.

Example : `[Co ONO (NH_3)_5]Cl_2` - `text(pentaamminenitritocobalt) (III) text(chloride)`; `:O-NO^-` oxygen atom donates lone pair of electrons (nitrito)

`[CoNO_2(NH_3)_5]Cl_2` - `text(pentaamminenitrocobalt) (III) text(chloride)`; `NO_2^-` nitrogen atom donates lone pair of electrons (nitro)

(iv) `text(Hydrate isomerism)` : Hydrate isomers have the same composition but differ in the number of water molecules present as ligands and the phenomenon is called hydrate isomerism.

Examples : (a) `[Cr(H_2O)_6]Cl_3` - `text(hexaaquachromium) (III) text[chloride (violet)]`

(b) `[Cr(H_2O)_5 Cl] Cl_2.H_2O` - `text(pentaaquachlorochromium) (III) text[chloride monohydrate (blue green)]`

(c) `[Cr(H_2O)_4 Cl_2] Cl.2H_2O` - `text(tetraaquadichlorochromium) (III) text[chloride dihydrate (green)]`

Here the isomers differ only in the spatial arrangement of atoms of groups about the central metal atom. It is of two types :

(i) `text(Geometrical or Cis-trans isomerism)` : This isomerism arises due to the difference in geometrical arrangement of the ligands around the central atom. When identical ligands occupy positions opposite to each other called cis-isomer.When identical ligands occupy positions opposite to each other called trans-isomer. It is very common in disubstituted complexes with co-ordination number of `4` and `6`.

(ii) `text(Optical isomerism)` : (a) Optical isomers are mirror images of each other and have chiral centers.

(b) Mirror images are not super imposable and do and have the plane of symmetry.

(c) Optical isomers have similar physical and chemical properties but differ in rotating the plane of plane polarized light.

(d) Isomer which rotates the plane polarized light to the right is called dextro rotatory (`d`-form) and the isomer which rotates the plane polarized light to the left is called laevorotatory (`l`-form).

(a) The ligands occupy different positions around the central metal ion.

(b) When two identical ligands are coordinated to the metal ion from the same side then it is cis isomer. (Latin, cis means same).

(c) If the two identical ligands are coordinated to the metal ion from opposite side then it is Trans isomer (Latin, Trans means across)

(d) These geometrical isomers differ in physical as well as in chemical properties.

(e) Geometrical isomerism is most important in compounds with coordination numbers `4` and `6`.

(f) `4`-coordinated complexes with tetrahedral geometry do not exhibit Cis-Trans isomerism.

(g) It is exhibited by `4`-coordinated complexes with square planar geometry.

(a) Optically active complexes are those which are nonsuperimposable over the mirror image structure.

(b) An optically active complex is one which is asymmetric in nature i.e., not divisible into two identical halves.

(c) The complex which rotates plane polarised light to left hand side is laevo rotatory i.e., `l` or `'-'` and if the complex rotates the plane polarised light to right hand side then it is dextrorotatory `d` or `'+'`.

(d) Thus complexes which have same physical and chemical properties but differ in their action towards plane polarised light are called as optical isomers.

(e) The `d` and `l` isomers of a compound are called as Enantiomers or Enantiomorphs.

(f) Only those `6`-coordinated complexes in which there are chelating agents i.e. bidentate ligands, exhibit optical isomerism. This is due to the absence of elements of symmetry in the complex.

(g) Optical isomerism is not found in square planar complexes on account of the presence of axis of symmetry.