Module – 24.3
Two or more substances having the same molecular formula but different
structural or spatial arrangement are called isomers.
They are of two types.
a. Structural isomerism
b. Stereo isomerism
a. Structural isomerism:
Two or more complexes have same molecular composition, but different
properties are known as isomerism. Those are called as isomers. The isomers
differ in the arrangement of ligands within the complest are called structural
isomers and the phenomenon is called structural isomerism.
1. Ionization isomerism:
This is due to difference in ionisable groups.
Example: i) [Co(NH3)5Cl]SO4 and [Co(NH3)5SO4]Cl
ii) [Pt(NH3)4Cl2]Br2 and [Pt(NH3)4Br2]Cl2
2. Polymerization isomerism:
Compounds having the same emperial formula, but different molecular weights.
Example: [Pt(NH3)2Cl] and [Pt(NH3)4][PtCl4]
3. Hydrate isomerism:
Isomerism differing in the number of water molecules attached the metal ion as
ligands in coordination sphere
Example: [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2H2O
4. Linkage isomerism:
Isomers of this type have different kinds of linkages of a ligand to the central
Example: [Co(NH3)5(NO2)]Cl2 and [Co(NH3)5CoNO]Cl2
5. Coordination isomerism:
This isomerism caudes by the interchange of ligands between the two complex
Example: [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]
6. Ligand isomerism:
Some ligands capable of existing isomerism can give different isomeric complexes.
Stereo isomerism in complex compounds:
Two or more complexes having same molecular composition but different in
arrangement of atoms (or) groups in space is called as stereo isomerism.
It is of two types.
a. Geometrical isomerism
b. Optical isomerism
Stereo isomerism in coordina
coordination number 6 compounds:
The arrangement of six ligands in a complex around central metal ion in two
different ways. Those are regular hexagon and regular octahedron.
If the complex Ma4b2 has hexagonal structure it can give three isomers
corresponding to (1, 2), (1, 3) and (1, 4) positions of b, while octahedral
arrangement can give only two isomers. X – ray analysis, confirmed that
coordination number 6 complexes exhibit octahedral arrangement by giving two
isomers of Ma4b2 type. Hence coordination number 6 complexes have octahedral
1. No isomerism is possible in [Ma6] or Ma5b of complexes
Example: [Co(NH3)6]3+; [Co(NH3)5Cl]2+
2. In [Ma4b2] and [Ma4bc] type complexes can give two isomers. Those are Cis
and trans isomers.
Example: [Co(NH3)4Cl2]+ gives Cis and trans isomers.
3. In [Ma3b3] type of complexes can exhibit geometrical isomerism.
Example: [Co(NH3)3Cl3] gives Cis – trans isomers.
4. In [M(aa)2b2] type of complexes having two bidentate ligands, can exhibit
Example: [Co(en)2Cl2]+ gives Cis and trans isomers.
1. The Cis isomer of [M(aa)2b2] type complexes has no plane of symmetry. So it
can give two isomers (d and l)
2. Complexes of [M(aa)3] type having three bidetate ligands are also
unsymmetric and gives optical isomerism
Example: [Pt(en)3]4+ gives optical isomers.
Stereo isomerism in coordination numbers “4” compounds:
Complex compounds of coordination number “4” type gives either tetrahedral or
square planar structure.
1. No isomers are possible [Ma4], [Ma3b] types complexes
Example: Ni(Co)4, [Cu(NH3)4]2+
2. [Ma2b2] and [Ma2bc] complexes of some metals like Pt (II), Pd (II), Ni (II), Cu (II)
can give square plannnar structure. These gives dsp2 hybridisation
Example: [Pt(NH3)2(CN)2] gives Cis trans isomers
The geometrical isomers of [Pt(NH3)2Cl2] are
Some complex compounds of Cu, Zn, Ni can give tetrahedral structure. These
gives sp3 hybridisation
[Mabcd] type of complex gives optical isomerism, if thay have tetrahedral.
Three isomers are possible, if they have square planner 5 structure.
Example: [Pt(NH3)(Py)ClBr] gives three isomers.
Applications of complex compounds:
I. Applications in equilibrium analysis:
a. Separation of AgCl from Hg2Cl2:
In the Ist group silver ions precipitated as white ppt of AgCl. It is soluble in
ammonia due to the formation of [Ag(NH3)2]Cl
On the other hand, white p of Hg2Cl2 turns black on treatment with ammonia.
b. Separation of Cu and Cd:
Candmium is separated from a mixture of copper and cadmium ion by adding
KCN. Copper ion forms a stable complex with KCN in the solution.
Where as cadmium complex being unstable, decomposes to give Cd+2 ions. These
ions gives yellow precipitate with H2S
c. Test for Fe+2 (ferrous ions) and Fe+3 (ferric ion):
Ferrous salt solution gives deep blue colour precipitate with potassium ferry
cyanide. These tests indicate the presence of ferrous ion.
Ferric salt precipitate with potassium ferro cyanide. This indicates the presence of
d. Test for K+ and ion:
Potassium salt solution gives yellow precipitate with sodium cobalt nitrate. Thie
test indicates the presence of potassium ion.
Nessler’s reagent is used for identification of ammonium ion. It gives reddish
e. Test for nickel ions:
Nickel salts react with dimethyl glyoxime in presence of ammonium hydroxide to
give red precipitate of nickel dimethyl glyoxime
f. The complex “Ferroin” is used as indicator in some redox titrations.
II. Applications in quantitative analysis:
a. Estimation of several cations like Mg+2, Zn+2, Cu+2, Ni2+ using EDTA as titrant in
presence of suitable indicator
Example: Erio chrome black – T
b. Dimethyl glyoxime is used for estimation of nickel.
Example: 8 – hydroxyl quinolene is used for estimation of zinc
c. Hardness of water can be calculated by EDTA
1. Give any three applications of complex compounds in quatitative analysis
2. List various types of structural isomerism possible for coordination
compounds, giving one example each
3. Draw the structures of optical isomers of
1. [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br exhibit
a. Hydrate isomerism
b. Ionization isomerism
c. Ligand isomerism
d. Co – ordination isomerism
2. Blue colour/precipitate will be obtained when K4[Fe(CN)6] reacts with
a. Fe (II) ions
b. Cu (II) ions
c. Fe (III) ions
d. Cu (I) ions
3. Which one of the following square planner complexes will show Cis – trans
4. The number of isomers possible for square plannar complex. K2[PdClBr2(SN)]
1. Silver chloride is soluble in ammonia due to the formation of
2. Geometrical isomerism would be expected form?
3. [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2H2O exhibit
a. Hydrate isomerism
b. Ionization isomerism
c. Ligand isomerism
d. Coordination isomerism
4. Which of the following octahedral complexes does not show geometrical
isomerism (A and B are monodentate ligands)?
5. Name the type of isomerism exhibited by the following isomers
a. [Cr(NH3)6] [Co(CN)6] and [Co(CN)6][Cr(NH3)6]
b. [Co(Py)2(H2O)2Cl2]Cl and [Co(Py)2(H2O)Cl3]H2O
c. [Pt(NH3)4Br2]Cl2 and [Pt(NH3)4Br2]Cl2
d. [Co(NH3)5NO2]Cl2 and [Co(NH3)5Ono)Cl2
a. Coordination isomerism
b. Hydrate isomerism
c. Ionization isomerism
d. Linkage isomerism
6. Draw structures of geometrical isomers [Fe(NH3)2(CN)4]
1. Why is geometrical isomerism not possible in tetrahedral complexes having
two different types of unidentate ligands coordinated with the central metal
2. Give evidence the [Co(NH3)5Cl)SO4 and [Co(NH3)5SO4]Cl are ionization isomers?
3. Indicate the types of isomerism exhibited by the following complexes and
draw the structure for these isomers
4. Platinum (II) forms square planar complexes and platinum (IV) gives octahedral
complexes. How many geometrical isomers are possible for each of the
following complexes? Describe their structures?
Solutions to exercise questions:
1. Tetrahedral complexes do not show geometrical isomerism because the
relative positions of the unidentate ligands attached to the central metal atom
are the same w.r.t each other.
2. When ionization isomers are dissolved in water, they ionize to give different
ions. These ions then react differently with different reagents to give different
a. A pair of optical isomers
It can also show linkage isomerism
[Co(NH3)5(NO2)](NO3)2 and [Co(NH3)5(ONO)](NO3)2
It can also show ionization isomerism
[Co(NH3)5(NO2)](NO3)2 and [Co(NH3)5](NO3)(NO2)
b. Geometrical (Cis-, trans isomers of [Pt(NH3)(H2O)Cl2] can exist
a. No isomers are possible for a square planar complex of th type Ma3
b. No isomers are possible for an octahedral complex of the type Mab5
c. Cis and trans isomers are possible for a square planar complex of the type
d. Cis and trans isomers are possible for an octahedral complex of the type