Coordination Compounds : Daily Practice Problems (DPP) - 3 - Class 12

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1
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  9217610408,8699438881,8699438882
     DPP : 03 / Co-ordination compounds
ISOMERISM IN COORDINATION COMPOUNDS
Two or more molecular formula but different properties are called isomers and the phenomenon is called isomerism.
Isomerism can be broadly classified into two major categories
(A) STRUCTURAL ISOMERISM
The isomers have same molecular formula but different structural arrangement of atoms or groups of atoms around the
central metal ion are called structural isomers. Different types of structural isomerism are discussed here
I. Ionization Isomerism. Compounds which have same molecular formula but give different ions in solutions are
called ionization isomers. This type of isomerism occurs when the counter ion in a coordination compound is
itself a potential ligand. Some examples are given below
()() 2
2
3 2 4 3 3 4
5 5
Co NH NO SO Co NH NO SO
+
- ? ? ? ? ? +
? ? ? ?
()() 3 4 3 3 4 3
5 5
Co NH SO NO Co NH SO NO
+
- ? ? ? ? ? +
? ? ? ?
II. Hydrate Isomerism. The compounds which have the same molecular formula but differ in the number of water
molecules present as ligands or as molecules of hydration are called hydrate isomers. This type of isomerism is
similar to ionization isomerism and may occur inside and outside the coordination sphere as a coordinated group
or a water of hydration.
For example, there are three isomers for CrCl
3
.6H
2
O.
[Cr(H
2
O) 6
]Cl
3
; [Cr(H
2
O) 5
Cl]Cl
2
.H
2
O ; [Cr(H
2
O) 4
Cl
2
]Cl.2H
2
O
III. Coordination isomerism. This type of isomerism occurs in compounds containing both cationic and anionic
coordination entities and the isomers differ in the distribution of ligands in the coordination entity of cationic and
anionic parts, e.g.,
[Co(NH
3
) 6
] [Cr(CN) 6
] ; [Cr(NH
3
) 6
] [Co(CN) 6
]
IV. Coordination position isomerism. This type of isomerism occurs in polynuclear complexes in which coordination
groups may be present in the same number but may arrange themselves differently with respect to the different
metal ions present, i.e., it is a result of the exchange of ligands between the different metal ions present in the
same coordination sphere. An example of this is
(NH
3
) 4
Co
NH
2
Unsymmetric form
Co(NH
3
) 2
Cl
2
Cl
2
O
2
Cl(NH
3
) 3
Co
NH
2
Symmetric form
Co(NH
3
) 3
Cl Cl
2
O
2
COORDINATION COMPOUNDS
DAILY  PRACTICE  PROBLEMS - 3
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1
SCF-07, 1
st
 floor, Sector -15, Panchkula
  9217610408,8699438881,8699438882
     DPP : 03 / Co-ordination compounds
ISOMERISM IN COORDINATION COMPOUNDS
Two or more molecular formula but different properties are called isomers and the phenomenon is called isomerism.
Isomerism can be broadly classified into two major categories
(A) STRUCTURAL ISOMERISM
The isomers have same molecular formula but different structural arrangement of atoms or groups of atoms around the
central metal ion are called structural isomers. Different types of structural isomerism are discussed here
I. Ionization Isomerism. Compounds which have same molecular formula but give different ions in solutions are
called ionization isomers. This type of isomerism occurs when the counter ion in a coordination compound is
itself a potential ligand. Some examples are given below
()() 2
2
3 2 4 3 3 4
5 5
Co NH NO SO Co NH NO SO
+
- ? ? ? ? ? +
? ? ? ?
()() 3 4 3 3 4 3
5 5
Co NH SO NO Co NH SO NO
+
- ? ? ? ? ? +
? ? ? ?
II. Hydrate Isomerism. The compounds which have the same molecular formula but differ in the number of water
molecules present as ligands or as molecules of hydration are called hydrate isomers. This type of isomerism is
similar to ionization isomerism and may occur inside and outside the coordination sphere as a coordinated group
or a water of hydration.
For example, there are three isomers for CrCl
3
.6H
2
O.
[Cr(H
2
O) 6
]Cl
3
; [Cr(H
2
O) 5
Cl]Cl
2
.H
2
O ; [Cr(H
2
O) 4
Cl
2
]Cl.2H
2
O
III. Coordination isomerism. This type of isomerism occurs in compounds containing both cationic and anionic
coordination entities and the isomers differ in the distribution of ligands in the coordination entity of cationic and
anionic parts, e.g.,
[Co(NH
3
) 6
] [Cr(CN) 6
] ; [Cr(NH
3
) 6
] [Co(CN) 6
]
IV. Coordination position isomerism. This type of isomerism occurs in polynuclear complexes in which coordination
groups may be present in the same number but may arrange themselves differently with respect to the different
metal ions present, i.e., it is a result of the exchange of ligands between the different metal ions present in the
same coordination sphere. An example of this is
(NH
3
) 4
Co
NH
2
Unsymmetric form
Co(NH
3
) 2
Cl
2
Cl
2
O
2
Cl(NH
3
) 3
Co
NH
2
Symmetric form
Co(NH
3
) 3
Cl Cl
2
O
2
COORDINATION COMPOUNDS
DAILY  PRACTICE  PROBLEMS - 3
2
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st
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V. Linkage Isomerism. The compounds which have the same molecular formula but differ in the mode of
attachment of ligand to the metal atom or ion are called linkage isomers. For example, in
2
NO
- ion, the nitrogen
atom as well as the oxygen atom can donate their lone pairs as shown in figure.
Co
NH
3
H
3
N
H
3
N
NH
3
NH
3
NO
2
Co
NH
3
H
3
N
H
3
N
NH
3
[Co(NH
3
) 5
NO
2
]Cl
2
ONO
2+ 2+
NH
3
(Red)(Yellow brown) Linkage isomers of pentamminenitrocobalt(III) chloride
VI. Polymerization isomerism. This term is used to describe compounds which have the same stoichiometric
composition but whose molecular composition are multiples of the simplest stoichiometric arrangements, e.g.,
I. [Pt(NH
3
) 2
Cl
2
] II. [Pt(NH
3
) 4
] [PtCl
4
] III. [Pt(NH
3
) 3
Cl]
2
 [PtCl
4
]
VII. Ligand Isomerism. Many ligands are themselves capable of existing in isomeric states. When such ligands are
associated to complexes, the complexes are isomers of each other. Such isomers are known as ligand isomers
and the phenomenon is known as ligand isomerism.
NH
2
Cl
NH
2
Cl
NH
2
Cl
o-Chloroaniline m-Chloroaniline p-Chloroaniline
(B) STEREO ISOMERISM
When the same molecular formula represents two or more compounds which differ in the spatial arrangement of atoms
or groups, then such compounds are called stereo isomers. The phenomenon is known as stereo  isomerism (stereo-
occupying space). It is further classified into two types
I. Geometrical Isomerism or Cis-Trans Isomerism
i. In cis isomers the two identical ligands occupy the positions adjacent to each other.
ii. In trans isomers, the two identical ligands occupy the positions diagonal to each other.
Geometrical isomers have different physical and chemical properties.
(a) Geometrical isomerism in Complexes of CN 4
The complexes having coordination number 4 acquire tetrahedral or square planar geometry. The geometrical
isomerism is not possible in tetrahedral geometry because any two positions are adjacent to each other.
However, square planar complexes show geometrical isomerism.
In square planar complexes the positions 1, 2 ; 2, 3 ; 3, 4 and 1, 4 are cis with respect to each other while the
positions 1, 3 and 2, 4 are trans to each other.
Square planar complexes of the type MA
2
X
2
, MA
2
XY or MABX
2
 can exist in cis trans isomers. Here A and B are
neutral ligands like H
2
O, NH
3
, Py, etc., while X and Y are anionic ligands like SCN
–
, Cl
–
, Br
–
, I, etc.
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1
SCF-07, 1
st
 floor, Sector -15, Panchkula
  9217610408,8699438881,8699438882
     DPP : 03 / Co-ordination compounds
ISOMERISM IN COORDINATION COMPOUNDS
Two or more molecular formula but different properties are called isomers and the phenomenon is called isomerism.
Isomerism can be broadly classified into two major categories
(A) STRUCTURAL ISOMERISM
The isomers have same molecular formula but different structural arrangement of atoms or groups of atoms around the
central metal ion are called structural isomers. Different types of structural isomerism are discussed here
I. Ionization Isomerism. Compounds which have same molecular formula but give different ions in solutions are
called ionization isomers. This type of isomerism occurs when the counter ion in a coordination compound is
itself a potential ligand. Some examples are given below
()() 2
2
3 2 4 3 3 4
5 5
Co NH NO SO Co NH NO SO
+
- ? ? ? ? ? +
? ? ? ?
()() 3 4 3 3 4 3
5 5
Co NH SO NO Co NH SO NO
+
- ? ? ? ? ? +
? ? ? ?
II. Hydrate Isomerism. The compounds which have the same molecular formula but differ in the number of water
molecules present as ligands or as molecules of hydration are called hydrate isomers. This type of isomerism is
similar to ionization isomerism and may occur inside and outside the coordination sphere as a coordinated group
or a water of hydration.
For example, there are three isomers for CrCl
3
.6H
2
O.
[Cr(H
2
O) 6
]Cl
3
; [Cr(H
2
O) 5
Cl]Cl
2
.H
2
O ; [Cr(H
2
O) 4
Cl
2
]Cl.2H
2
O
III. Coordination isomerism. This type of isomerism occurs in compounds containing both cationic and anionic
coordination entities and the isomers differ in the distribution of ligands in the coordination entity of cationic and
anionic parts, e.g.,
[Co(NH
3
) 6
] [Cr(CN) 6
] ; [Cr(NH
3
) 6
] [Co(CN) 6
]
IV. Coordination position isomerism. This type of isomerism occurs in polynuclear complexes in which coordination
groups may be present in the same number but may arrange themselves differently with respect to the different
metal ions present, i.e., it is a result of the exchange of ligands between the different metal ions present in the
same coordination sphere. An example of this is
(NH
3
) 4
Co
NH
2
Unsymmetric form
Co(NH
3
) 2
Cl
2
Cl
2
O
2
Cl(NH
3
) 3
Co
NH
2
Symmetric form
Co(NH
3
) 3
Cl Cl
2
O
2
COORDINATION COMPOUNDS
DAILY  PRACTICE  PROBLEMS - 3
2
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st
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V. Linkage Isomerism. The compounds which have the same molecular formula but differ in the mode of
attachment of ligand to the metal atom or ion are called linkage isomers. For example, in
2
NO
- ion, the nitrogen
atom as well as the oxygen atom can donate their lone pairs as shown in figure.
Co
NH
3
H
3
N
H
3
N
NH
3
NH
3
NO
2
Co
NH
3
H
3
N
H
3
N
NH
3
[Co(NH
3
) 5
NO
2
]Cl
2
ONO
2+ 2+
NH
3
(Red)(Yellow brown) Linkage isomers of pentamminenitrocobalt(III) chloride
VI. Polymerization isomerism. This term is used to describe compounds which have the same stoichiometric
composition but whose molecular composition are multiples of the simplest stoichiometric arrangements, e.g.,
I. [Pt(NH
3
) 2
Cl
2
] II. [Pt(NH
3
) 4
] [PtCl
4
] III. [Pt(NH
3
) 3
Cl]
2
 [PtCl
4
]
VII. Ligand Isomerism. Many ligands are themselves capable of existing in isomeric states. When such ligands are
associated to complexes, the complexes are isomers of each other. Such isomers are known as ligand isomers
and the phenomenon is known as ligand isomerism.
NH
2
Cl
NH
2
Cl
NH
2
Cl
o-Chloroaniline m-Chloroaniline p-Chloroaniline
(B) STEREO ISOMERISM
When the same molecular formula represents two or more compounds which differ in the spatial arrangement of atoms
or groups, then such compounds are called stereo isomers. The phenomenon is known as stereo  isomerism (stereo-
occupying space). It is further classified into two types
I. Geometrical Isomerism or Cis-Trans Isomerism
i. In cis isomers the two identical ligands occupy the positions adjacent to each other.
ii. In trans isomers, the two identical ligands occupy the positions diagonal to each other.
Geometrical isomers have different physical and chemical properties.
(a) Geometrical isomerism in Complexes of CN 4
The complexes having coordination number 4 acquire tetrahedral or square planar geometry. The geometrical
isomerism is not possible in tetrahedral geometry because any two positions are adjacent to each other.
However, square planar complexes show geometrical isomerism.
In square planar complexes the positions 1, 2 ; 2, 3 ; 3, 4 and 1, 4 are cis with respect to each other while the
positions 1, 3 and 2, 4 are trans to each other.
Square planar complexes of the type MA
2
X
2
, MA
2
XY or MABX
2
 can exist in cis trans isomers. Here A and B are
neutral ligands like H
2
O, NH
3
, Py, etc., while X and Y are anionic ligands like SCN
–
, Cl
–
, Br
–
, I, etc.
3
SCF-07, 1
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     DPP : 03 / Co-ordination compounds
(i) [ P t( N H
3
) 2
Cl
2
] exists in cis and trans forms. The cis isomer is used in medicine as an antitumour agent
called cisplatin.
(ii) [Pt(Py) 2
(NH
3
)Cl] exists in cis and trans forms.
(iii) Square planer complexes of the type M [ABCD] show three isomers. The structures of these isomers can
be written by fixing the position of one ligand (say A) and placing the other ligands B, C and D trans for it.
(iv) Geometrical isomerism cannot occur in complexes of the type MA
4
, MA
3
B or MAB
3
 because all possible
spatial arrangements for any of these complexes will be exactly equivalent.
(v) Chelate rings involving bidentate ligands are formed only in cis form because the chelating ligand is too
small to span the trans position. The distance across the two trans positions is too large for all but very
large ligands can span to trans positions an synthesis with such large rings is difficult.
(vi) The square planar complexes unsymmetrical bidentate ligands such as [M(AB) 2
] also exhibit geometrical
isomerism. For example, the complex [Pt(gly) 2
] where gly = NH
2
CH
2
COO
–
 (glycinate ion) exists in cis and
trans forms.
(vii) Geometrical isomerism is also shown by bridged binuclear complexes of the type M
2
A
2
X
4
. For example the
complex [PtCl
2
 P(C
2
H
5
) 3
]
2
 exhibits geometrical isomerism.
(b) Geometrical Isomerism in Complexes of CN 6
The complexes having coordination number 6 adopt octahedral geometry.
In the octahedral complexes, positions 1,6 ; 2,4 and 3,5 are trans while the others such as 1,2 ; 1,3 ; 1,4 ; 1,5; 2,5;
2,3; 3,4; 4,5; 2,6; 3,6; 4,6 and 5,6 are cis to each other.
1
M
2
3 4
5
6
(i) The octahedral complexes of the type MA
6
, MA
5
B or MAB
5
 type do not exhibit geometrical isomerism.
(ii) The octahedral complexes of the type MA
4
B
2
, MA
2
B
4
 and MA
4
BC, etc., exhibit geometrical isomerism, Some
common examples are [Co(NH
3
) 4
Cl
2
]
+
 ; [Pt(NH
3
) 4
Cl
2
]
2+
(iii) Octahedral complexes of the type MA
3
B
3
 also exist as two geometrical isomers. When the three ligands
(with same donor atoms) are on the same triangular face of the octahedron, the isomer is called facial or
fac isomer. When the three ligands are on the same equitorial plane the other in a plane bisecting the
molecule, this isomer is called meridional or mer isomer.
(iv) Octahedral complexes having bidentate ligands of the type [M(AA) 2
 X
2
] and [M(AA) 2
XY] can also exist as cis
and trans isomers where AA represents a symmetrical bidentate ligand such as ethylene diamine (en) oxalate ion (ox).
(v) Octahedral complexes having six different ligands of the type M(ABCDEF) would exhibit geometrical
isomerism. These isomers may be written by fixing a ligand at one position and then placing the other
ligands trans to it. Theoretically, 15 different isomers are possible for such type of complexes.
(vi) The complexes containing unsymmetrical bidentate ligands also show geometrical isomerism. For
example the complex triglycinato chromium(III) [Cr(gly) 3
] where gly is glycinate ion H
2
NCH
2
COO– exists in
cis and trans forms.
Page 4


1
SCF-07, 1
st
 floor, Sector -15, Panchkula
  9217610408,8699438881,8699438882
     DPP : 03 / Co-ordination compounds
ISOMERISM IN COORDINATION COMPOUNDS
Two or more molecular formula but different properties are called isomers and the phenomenon is called isomerism.
Isomerism can be broadly classified into two major categories
(A) STRUCTURAL ISOMERISM
The isomers have same molecular formula but different structural arrangement of atoms or groups of atoms around the
central metal ion are called structural isomers. Different types of structural isomerism are discussed here
I. Ionization Isomerism. Compounds which have same molecular formula but give different ions in solutions are
called ionization isomers. This type of isomerism occurs when the counter ion in a coordination compound is
itself a potential ligand. Some examples are given below
()() 2
2
3 2 4 3 3 4
5 5
Co NH NO SO Co NH NO SO
+
- ? ? ? ? ? +
? ? ? ?
()() 3 4 3 3 4 3
5 5
Co NH SO NO Co NH SO NO
+
- ? ? ? ? ? +
? ? ? ?
II. Hydrate Isomerism. The compounds which have the same molecular formula but differ in the number of water
molecules present as ligands or as molecules of hydration are called hydrate isomers. This type of isomerism is
similar to ionization isomerism and may occur inside and outside the coordination sphere as a coordinated group
or a water of hydration.
For example, there are three isomers for CrCl
3
.6H
2
O.
[Cr(H
2
O) 6
]Cl
3
; [Cr(H
2
O) 5
Cl]Cl
2
.H
2
O ; [Cr(H
2
O) 4
Cl
2
]Cl.2H
2
O
III. Coordination isomerism. This type of isomerism occurs in compounds containing both cationic and anionic
coordination entities and the isomers differ in the distribution of ligands in the coordination entity of cationic and
anionic parts, e.g.,
[Co(NH
3
) 6
] [Cr(CN) 6
] ; [Cr(NH
3
) 6
] [Co(CN) 6
]
IV. Coordination position isomerism. This type of isomerism occurs in polynuclear complexes in which coordination
groups may be present in the same number but may arrange themselves differently with respect to the different
metal ions present, i.e., it is a result of the exchange of ligands between the different metal ions present in the
same coordination sphere. An example of this is
(NH
3
) 4
Co
NH
2
Unsymmetric form
Co(NH
3
) 2
Cl
2
Cl
2
O
2
Cl(NH
3
) 3
Co
NH
2
Symmetric form
Co(NH
3
) 3
Cl Cl
2
O
2
COORDINATION COMPOUNDS
DAILY  PRACTICE  PROBLEMS - 3
2
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st
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V. Linkage Isomerism. The compounds which have the same molecular formula but differ in the mode of
attachment of ligand to the metal atom or ion are called linkage isomers. For example, in
2
NO
- ion, the nitrogen
atom as well as the oxygen atom can donate their lone pairs as shown in figure.
Co
NH
3
H
3
N
H
3
N
NH
3
NH
3
NO
2
Co
NH
3
H
3
N
H
3
N
NH
3
[Co(NH
3
) 5
NO
2
]Cl
2
ONO
2+ 2+
NH
3
(Red)(Yellow brown) Linkage isomers of pentamminenitrocobalt(III) chloride
VI. Polymerization isomerism. This term is used to describe compounds which have the same stoichiometric
composition but whose molecular composition are multiples of the simplest stoichiometric arrangements, e.g.,
I. [Pt(NH
3
) 2
Cl
2
] II. [Pt(NH
3
) 4
] [PtCl
4
] III. [Pt(NH
3
) 3
Cl]
2
 [PtCl
4
]
VII. Ligand Isomerism. Many ligands are themselves capable of existing in isomeric states. When such ligands are
associated to complexes, the complexes are isomers of each other. Such isomers are known as ligand isomers
and the phenomenon is known as ligand isomerism.
NH
2
Cl
NH
2
Cl
NH
2
Cl
o-Chloroaniline m-Chloroaniline p-Chloroaniline
(B) STEREO ISOMERISM
When the same molecular formula represents two or more compounds which differ in the spatial arrangement of atoms
or groups, then such compounds are called stereo isomers. The phenomenon is known as stereo  isomerism (stereo-
occupying space). It is further classified into two types
I. Geometrical Isomerism or Cis-Trans Isomerism
i. In cis isomers the two identical ligands occupy the positions adjacent to each other.
ii. In trans isomers, the two identical ligands occupy the positions diagonal to each other.
Geometrical isomers have different physical and chemical properties.
(a) Geometrical isomerism in Complexes of CN 4
The complexes having coordination number 4 acquire tetrahedral or square planar geometry. The geometrical
isomerism is not possible in tetrahedral geometry because any two positions are adjacent to each other.
However, square planar complexes show geometrical isomerism.
In square planar complexes the positions 1, 2 ; 2, 3 ; 3, 4 and 1, 4 are cis with respect to each other while the
positions 1, 3 and 2, 4 are trans to each other.
Square planar complexes of the type MA
2
X
2
, MA
2
XY or MABX
2
 can exist in cis trans isomers. Here A and B are
neutral ligands like H
2
O, NH
3
, Py, etc., while X and Y are anionic ligands like SCN
–
, Cl
–
, Br
–
, I, etc.
3
SCF-07, 1
st
 floor, Sector -15, Panchkula
  9217610408,8699438881,8699438882
     DPP : 03 / Co-ordination compounds
(i) [ P t( N H
3
) 2
Cl
2
] exists in cis and trans forms. The cis isomer is used in medicine as an antitumour agent
called cisplatin.
(ii) [Pt(Py) 2
(NH
3
)Cl] exists in cis and trans forms.
(iii) Square planer complexes of the type M [ABCD] show three isomers. The structures of these isomers can
be written by fixing the position of one ligand (say A) and placing the other ligands B, C and D trans for it.
(iv) Geometrical isomerism cannot occur in complexes of the type MA
4
, MA
3
B or MAB
3
 because all possible
spatial arrangements for any of these complexes will be exactly equivalent.
(v) Chelate rings involving bidentate ligands are formed only in cis form because the chelating ligand is too
small to span the trans position. The distance across the two trans positions is too large for all but very
large ligands can span to trans positions an synthesis with such large rings is difficult.
(vi) The square planar complexes unsymmetrical bidentate ligands such as [M(AB) 2
] also exhibit geometrical
isomerism. For example, the complex [Pt(gly) 2
] where gly = NH
2
CH
2
COO
–
 (glycinate ion) exists in cis and
trans forms.
(vii) Geometrical isomerism is also shown by bridged binuclear complexes of the type M
2
A
2
X
4
. For example the
complex [PtCl
2
 P(C
2
H
5
) 3
]
2
 exhibits geometrical isomerism.
(b) Geometrical Isomerism in Complexes of CN 6
The complexes having coordination number 6 adopt octahedral geometry.
In the octahedral complexes, positions 1,6 ; 2,4 and 3,5 are trans while the others such as 1,2 ; 1,3 ; 1,4 ; 1,5; 2,5;
2,3; 3,4; 4,5; 2,6; 3,6; 4,6 and 5,6 are cis to each other.
1
M
2
3 4
5
6
(i) The octahedral complexes of the type MA
6
, MA
5
B or MAB
5
 type do not exhibit geometrical isomerism.
(ii) The octahedral complexes of the type MA
4
B
2
, MA
2
B
4
 and MA
4
BC, etc., exhibit geometrical isomerism, Some
common examples are [Co(NH
3
) 4
Cl
2
]
+
 ; [Pt(NH
3
) 4
Cl
2
]
2+
(iii) Octahedral complexes of the type MA
3
B
3
 also exist as two geometrical isomers. When the three ligands
(with same donor atoms) are on the same triangular face of the octahedron, the isomer is called facial or
fac isomer. When the three ligands are on the same equitorial plane the other in a plane bisecting the
molecule, this isomer is called meridional or mer isomer.
(iv) Octahedral complexes having bidentate ligands of the type [M(AA) 2
 X
2
] and [M(AA) 2
XY] can also exist as cis
and trans isomers where AA represents a symmetrical bidentate ligand such as ethylene diamine (en) oxalate ion (ox).
(v) Octahedral complexes having six different ligands of the type M(ABCDEF) would exhibit geometrical
isomerism. These isomers may be written by fixing a ligand at one position and then placing the other
ligands trans to it. Theoretically, 15 different isomers are possible for such type of complexes.
(vi) The complexes containing unsymmetrical bidentate ligands also show geometrical isomerism. For
example the complex triglycinato chromium(III) [Cr(gly) 3
] where gly is glycinate ion H
2
NCH
2
COO– exists in
cis and trans forms.
4
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     DPP : 03 / Co-ordination compounds
(C) OPTICAL ISOMERS
There are certain substances which can rotate the plane of polarized light. These are called optically active substances.
These isomers which rotate the plane of polarized light equally but in opposite direction are called optically active
isomers. These are also called enantiomers. The optical isomers also possess chirality. The optical isomers have
identical physical and chemical properties.
(a) Optical isomerism in 4-Coordination Compounds
In tetrahedral and square planar complexes of the type MA
4
, MA
3
B and MAB
3
, stereoisomerism is not possible. The
reason for this is that all the possible arrangements of the ligands around the central metal ion M, are exactly
equivalent.
(b) Optical Isomerism in Square Planar Complexes
Optical isomerism is rarely observed in square planar complexes because all the four ligands and the central
metal ion are present in the same plane and hence possess a plane or axis of symmetry. These complexes
cannot exhibit optical isomerism even if all the four ligands are different.
(c) Tetrahedral Complexes
(i) Optical isomerism is expected in an asymmetric molecule with tetrahedral structure when the central atom
is surrounded by four different groups [M (ABCD)].
(ii) However, compounds containing two unsymmetric bidentate ligands are not easy to resolve into optical
isomers and are known for complexes of Be(II), Zn(II) and B(III), they have low heat of inversion.
(d) Optical Isomerism in CN-6 Compounds
This type of isomerism is more common in compounds with coordination number 6 than in coordination number
4. Optical isomerism is very common in the following type of octahedral complexes.
(i) Octahedral complexes containing only monodentate ligands
a. The [MA
2
B
2
C
2
] type complex has two optical isomers.
b. In the complexes [M ABCDEF] containing six different ligands the central atom is symmetrically
disposed. For each of the 15 that are possible for such compounds, the total optical isomers are 30
because each of 15 would exist in d- andl- forms, e.g., [Pt (Py) (NH
3
) (NO
2
) (Cl) (Br) (I)].
(ii) Octahedral complexes containing only symmetrical bidentate chelating ligands
a. Complexes of the type M(AA) 3
 (where AA is symmetrical bidentate ligand) such as [Co(en) 3
]
3+
,
[Cr(ox) 3
]
3–
 and [Pt(en) 3
]
4+
, etc., exist as optical isomers.
b. The complexes of the type [M(AA) 2
 (BB)] having different types of symmetrical bidentate ligands also
exist in two optical isomers, e.g., [Co(en) 2
(C
2
O
4
)]
+
.
(iii) Octahedral complexes containing monodentate and symmetrical bidentate chelating ligands
a. The complexes of the type() n
2
2
M AA B
±
? ?
? ?
 and() n
2
M AA BC
±
? ?
? ?
 (where AA = bidentate chelating
ligand and B and C are monodatate ligands) show optical isomerism, e.g., [Co(en) 2
Cl
2
]
+
 and
[Co(en) 2
Cl(NO
2
)]
+
.
The cis [Co(en) 2
Cl
2
]
+
 isomer has no plane of symmetry so it exhibits optical isomerism.
b. The complexes of the type [M(AA)B
2
C
2
]
n+
 exist in three forms; one is optically active and two are
inactive or symmetrical e.g., [Co(en) (NH
3
) 2
Cl
2
]
+
.
(iv) Octahedral complexes containing polydentate ligands
Octahedral complexes containing hexadentate ligands such as ethylenediamine tetracetate (EDTA) also
show optical isomerism. For example [Co(EDTA)]
–
 exists as two optical isomers.