Isomerism in co-ordination compounds.
Compounds having the same molecular formula but different structures or spatial arrangements are called isomers and the phenomenon is referred as isomerism.
(1) Structural isomerism: Here the isomers have different arrangement of ligands around the central metal atom. It is of the following types :
(i) 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: [Co Br(NH3)5] SO4 [Co SO4(NH3)5] Br
Pentaaminebromo cobalt (III) Sulphate
Pentaaminesulphato cobalt (III) bromide
SO42– present in ionisation sphere Br– present in ionization sphere Gives white precipitate with BaCl2 Gives light yellow precipitate with AgNO3
(ii) 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(NH3)6] [Cr(CN)6] [Cr (NH3)6] [Co(CN)6] hexaamine cobalt (III) hexacyano chromate (III) hexaamine chromium (III) hexacyanocobalt (III) complex cation contains → NH3 ligand (with cobalt) complex anion contains → NH3 ligand (with chromium) complex anion contains →CN– ligand (with chromium) complex anion contains → CN– ligand (with cobalt)
(iii) 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 : [CoONO (NH3)5] Cl2 [Co NO2 (NH3)5]Cl2
Pentaamminenitritocobalt (III) Pentaaminenitrocobalt (III) chloride
:O–NO– oxygen atom donates lone pair of electrons (nitrito) NO2– nitrogen atom donates lone pair of electrons (nitro)
(iv) 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 : (i) [CrH2O)6]Cl3 hexaaquachromium (III) chloride (violet)
(ii) [Cr(H2O)5Cl]Cl2.H2O pentaaquachlorochromium (III) chloride monohydrate (blue green)
(iii) [Cr(H2O)4Cl]Cl2.2H2O tetraaquadichlorochromium (III) chloride dihydrate (green)
(2) Stereo isomerism or space isomerism : Here the isomers differ only in the spatial arrangement of atoms of groups about the central metal atom. It is of two types :
(i) 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.
Complexes of co-ordination number 4
Tetrahedral geometry : In this case all the four ligands are symmetrically arranged with respect to one another as such geometrical isomerism is not possible.
Square planar geometry : The four ligands occupy position at the four corners and the metal atom or ion is at the center and lie in the same plane. 
Type : I [Ma2b2],
M = Pt, a = Cl, b = NH3
Example :
[PtCl(NH3)(Py)2]
Note : Square planar complexes of types Ma4, Ma3b, Mab3 do not exhibit geometrical isomerism as all the spatial arrangements of the ligands relative to each other are equivalent.
Complexes of co-ordination number 6
Octahedral geometry : Here the metal atom or ion lies at the center and 1 to 6 position are occupied by the ligands. 
Cis–Positions : 1–2, 2–3, 3–4, 4–5
Trans – Position : 1–4, 2–5, 3–6
Type –I Ma4b2, M=Co, a=NH3, and b=Cl 
Example :
[CoCl2(NH3)4]+ ion
Type –II Ma3b3, M = Rh, a = Cl, and b = Py
Example : [Rh Cl3(Py)3
Type –III [M(aa)2(en)2]++, 
(bidentate), (monodentate)
(ii) 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)
Example : (a) [Ma2b2c2]n+1 ; [Ma2b2c2]n+1 ; [Pt(Py)2(NH3)2Cl2]2+
(b) [Ma b c e f] ; Pt(py) NH3NO2Cl Br]
(c) [M(AA)3]n+ ;[M(AA)3]n+ ; [CO(en)3]3+
(d) [M(AA)2 a2]n+ ; [Co(en)2 Cl2]+
(e) [M(AA)2ab]n ; [Co(en)2NH3Cl]2+
