NCERT Textbook: Coordination Compounds | Chemistry Class 12 - NEET PDF Download

 Page 1


118 Chemistry
In the previous Unit we learnt that the transition metals
form a large number of complex compounds in which
the metal atoms are bound to a number of anions or
neutral molecules by sharing of electrons. In modern
terminology such compounds are called coordination
compounds. The chemistry of coordination compounds
is an important and challenging area of modern
inorganic chemistry. New concepts of chemical bonding
and molecular structure have provided insights into
the functioning of these compounds as vital components
of biological systems. Chlorophyll, haemoglobin and
vitamin B
12 
are coordination compounds of magnesium,
iron and cobalt respectively. Variety of metallurgical
processes, industrial catalysts and analytical reagents
involve the use of coordination compounds.
Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
Coordination
Compounds
After studying this Unit, you will be
able to
• appreciate the postulates of
Werner’s theory of coordination
compounds;
• know the meaning of the terms:
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination
polyhedron, oxidation number,
homoleptic and heteroleptic;
• learn the rules of nomenclature
of coordination compounds;
• write the formulas and names
of mononuclear coordination
compounds;
• define different types of isomerism
in coordination compounds;
• understand the nature of bonding
in coordination compounds in
terms of the Valence Bond and
Crystal Field theories;
• appreciate the importance and
applications of coordination
compounds in our day to day life.
Objectives
Coordination Compounds are the backbone of modern inorganic
and bio–inorganic chemistry and chemical industry.
Coordination
Compounds
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
and characterised a large number of coordination compounds and
studied their physical and chemical behaviour by simple experimental
techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl
3
, CoCl
2
 or PdCl
2
 have primary valence of 3, 2 and 2 respectively.
In a series of compounds of cobalt(III) chloride with ammonia, it was
found that some of the chloride ions could be precipitated as AgCl on
adding excess silver nitrate solution in cold but some remained in
solution.
5.1 5.1 5.1 5.1 5.1 Werner’ Werner’ Werner’ Werner’ Werner’s s s s s
Theory of Theory of Theory of Theory of Theory of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
5
Unit Unit Unit Unit Unit
5
Reprint 2024-25
Page 2


118 Chemistry
In the previous Unit we learnt that the transition metals
form a large number of complex compounds in which
the metal atoms are bound to a number of anions or
neutral molecules by sharing of electrons. In modern
terminology such compounds are called coordination
compounds. The chemistry of coordination compounds
is an important and challenging area of modern
inorganic chemistry. New concepts of chemical bonding
and molecular structure have provided insights into
the functioning of these compounds as vital components
of biological systems. Chlorophyll, haemoglobin and
vitamin B
12 
are coordination compounds of magnesium,
iron and cobalt respectively. Variety of metallurgical
processes, industrial catalysts and analytical reagents
involve the use of coordination compounds.
Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
Coordination
Compounds
After studying this Unit, you will be
able to
• appreciate the postulates of
Werner’s theory of coordination
compounds;
• know the meaning of the terms:
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination
polyhedron, oxidation number,
homoleptic and heteroleptic;
• learn the rules of nomenclature
of coordination compounds;
• write the formulas and names
of mononuclear coordination
compounds;
• define different types of isomerism
in coordination compounds;
• understand the nature of bonding
in coordination compounds in
terms of the Valence Bond and
Crystal Field theories;
• appreciate the importance and
applications of coordination
compounds in our day to day life.
Objectives
Coordination Compounds are the backbone of modern inorganic
and bio–inorganic chemistry and chemical industry.
Coordination
Compounds
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
and characterised a large number of coordination compounds and
studied their physical and chemical behaviour by simple experimental
techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl
3
, CoCl
2
 or PdCl
2
 have primary valence of 3, 2 and 2 respectively.
In a series of compounds of cobalt(III) chloride with ammonia, it was
found that some of the chloride ions could be precipitated as AgCl on
adding excess silver nitrate solution in cold but some remained in
solution.
5.1 5.1 5.1 5.1 5.1 Werner’ Werner’ Werner’ Werner’ Werner’s s s s s
Theory of Theory of Theory of Theory of Theory of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
5
Unit Unit Unit Unit Unit
5
Reprint 2024-25
119 Coordination Compounds
1 mol CoCl
3
.6NH
3 
(Yellow) gave 3 mol AgCl
1 mol CoCl
3
.5NH
3 
(Purple) gave 2 mol AgCl
1 mol CoCl
3
.4NH
3 
(Green) gave 1 mol AgCl
1 mol CoCl
3
.4NH
3 
(Violet) gave 1 mol AgCl
These observations, together with the results of conductivity
measurements in solution can be explained if (i) six groups in all,
either chloride ions or ammonia molecules or both, remain bonded to
the cobalt ion during the reaction and (ii) the compounds are formulated
as shown in Table 5.1, where the atoms within the square brackets
form a single entity which does not dissociate under the reaction
conditions. Werner proposed the term secondary valence for the
number of groups bound directly to the metal ion; in each of these
examples the secondary valences are six.
Note that the last two compounds in Table 5.1 have identical empirical
formula, CoCl
3
.4NH
3
, but distinct properties. Such compounds are
termed as isomers. Werner in 1898, propounded his theory of
coordination compounds. The main postulates are:
1. In coordination compounds metals show two types of linkages
(valences)-primary and secondary.
2. The primary valences are normally ionisable and are satisfied by
negative ions.
3. The secondary valences are non ionisable. These are satisfied by
neutral molecules or negative ions. The secondary valence is equal to
the coordination number and is fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have
characteristic spatial arrangements corresponding to different
coordination numbers.
In modern formulations, such spatial arrangements are called
coordination polyhedra. The species within the square bracket are
coordination entities or complexes and the ions outside the square
bracket are called counter ions.
He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals. Thus, [Co(NH
3
)
6
]
3+
, [CoCl(NH
3
)
5
]
2+
 and [CoCl
2
(NH
3
)
4
]
+
are octahedral entities, while [Ni(CO)
4
] and [PtCl
4
]
2–
 are tetrahedral and
square planar, respectively.
Colour Formula Solution conductivity
corresponds to
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Yellow [Co(NH
3
)
6
]
3+
3Cl
–
1:3 electrolyte
Purple [CoCl(NH
3
)
5
]
2+
2Cl
–
1:2 electrolyte
Green [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Violet [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Reprint 2024-25
Page 3


118 Chemistry
In the previous Unit we learnt that the transition metals
form a large number of complex compounds in which
the metal atoms are bound to a number of anions or
neutral molecules by sharing of electrons. In modern
terminology such compounds are called coordination
compounds. The chemistry of coordination compounds
is an important and challenging area of modern
inorganic chemistry. New concepts of chemical bonding
and molecular structure have provided insights into
the functioning of these compounds as vital components
of biological systems. Chlorophyll, haemoglobin and
vitamin B
12 
are coordination compounds of magnesium,
iron and cobalt respectively. Variety of metallurgical
processes, industrial catalysts and analytical reagents
involve the use of coordination compounds.
Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
Coordination
Compounds
After studying this Unit, you will be
able to
• appreciate the postulates of
Werner’s theory of coordination
compounds;
• know the meaning of the terms:
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination
polyhedron, oxidation number,
homoleptic and heteroleptic;
• learn the rules of nomenclature
of coordination compounds;
• write the formulas and names
of mononuclear coordination
compounds;
• define different types of isomerism
in coordination compounds;
• understand the nature of bonding
in coordination compounds in
terms of the Valence Bond and
Crystal Field theories;
• appreciate the importance and
applications of coordination
compounds in our day to day life.
Objectives
Coordination Compounds are the backbone of modern inorganic
and bio–inorganic chemistry and chemical industry.
Coordination
Compounds
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
and characterised a large number of coordination compounds and
studied their physical and chemical behaviour by simple experimental
techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl
3
, CoCl
2
 or PdCl
2
 have primary valence of 3, 2 and 2 respectively.
In a series of compounds of cobalt(III) chloride with ammonia, it was
found that some of the chloride ions could be precipitated as AgCl on
adding excess silver nitrate solution in cold but some remained in
solution.
5.1 5.1 5.1 5.1 5.1 Werner’ Werner’ Werner’ Werner’ Werner’s s s s s
Theory of Theory of Theory of Theory of Theory of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
5
Unit Unit Unit Unit Unit
5
Reprint 2024-25
119 Coordination Compounds
1 mol CoCl
3
.6NH
3 
(Yellow) gave 3 mol AgCl
1 mol CoCl
3
.5NH
3 
(Purple) gave 2 mol AgCl
1 mol CoCl
3
.4NH
3 
(Green) gave 1 mol AgCl
1 mol CoCl
3
.4NH
3 
(Violet) gave 1 mol AgCl
These observations, together with the results of conductivity
measurements in solution can be explained if (i) six groups in all,
either chloride ions or ammonia molecules or both, remain bonded to
the cobalt ion during the reaction and (ii) the compounds are formulated
as shown in Table 5.1, where the atoms within the square brackets
form a single entity which does not dissociate under the reaction
conditions. Werner proposed the term secondary valence for the
number of groups bound directly to the metal ion; in each of these
examples the secondary valences are six.
Note that the last two compounds in Table 5.1 have identical empirical
formula, CoCl
3
.4NH
3
, but distinct properties. Such compounds are
termed as isomers. Werner in 1898, propounded his theory of
coordination compounds. The main postulates are:
1. In coordination compounds metals show two types of linkages
(valences)-primary and secondary.
2. The primary valences are normally ionisable and are satisfied by
negative ions.
3. The secondary valences are non ionisable. These are satisfied by
neutral molecules or negative ions. The secondary valence is equal to
the coordination number and is fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have
characteristic spatial arrangements corresponding to different
coordination numbers.
In modern formulations, such spatial arrangements are called
coordination polyhedra. The species within the square bracket are
coordination entities or complexes and the ions outside the square
bracket are called counter ions.
He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals. Thus, [Co(NH
3
)
6
]
3+
, [CoCl(NH
3
)
5
]
2+
 and [CoCl
2
(NH
3
)
4
]
+
are octahedral entities, while [Ni(CO)
4
] and [PtCl
4
]
2–
 are tetrahedral and
square planar, respectively.
Colour Formula Solution conductivity
corresponds to
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Yellow [Co(NH
3
)
6
]
3+
3Cl
–
1:3 electrolyte
Purple [CoCl(NH
3
)
5
]
2+
2Cl
–
1:2 electrolyte
Green [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Violet [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Reprint 2024-25
120 Chemistry
(i) Secondary 4 (ii) Secondary 6
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4
On the basis of the following observations made with aqueous solutions,
assign secondary valences to metals in the following compounds:
Solution Solution Solution Solution Solution
Difference between a double salt and a complex
Both double salts as well as complexes are formed by the combination
of two or more stable compounds in stoichiometric ratio. However, they
differ in the fact that double salts such as carnallite, KCl.MgCl
2
.6H
2
O,
Mohr’s salt, FeSO
4
.(NH
4
)
2
SO
4
.6H
2
O, potash alum, KAl(SO
4
)
2
.12H
2
O, etc.
dissociate into simple ions completely when dissolved in water. However,
complex ions such as [Fe(CN)
6
]
4– 
of K
4
[Fe(CN)
6
] do not dissociate into
Fe
2+
 and CN
–
 ions.
Formula Moles of AgCl precipitated per mole of
the compounds with excess AgNO
3
(i) PdCl
2
.4NH
3
2
(ii) NiCl
2
.6H
2
O 2
(iii) PtCl
4
.2HCl 0
(iv) CoCl
3
.4NH
3
1
(v) PtCl
2
.2NH
3
0
Example 5.1 Example 5.1 Example 5.1 Example 5.1 Example 5.1
Werner Werner Werner Werner Werner was born on December 12, 1866, in Mülhouse,
a small community in the French province of Alsace.
His study of chemistry began in Karlsruhe (Germany)
and continued in Zurich (Switzerland), where in his
doctoral thesis in 1890, he explained the difference in
properties of certain nitrogen containing organic
substances on the basis of isomerism. He extended vant
Hoff’s theory of tetrahedral carbon atom and modified
it for nitrogen. Werner showed optical and electrical differences between
complex compounds based on physical measurements. In fact, Werner was
the first to discover optical activity in certain coordination compounds.
He, at the age of 29 years became a full professor at Technische
Hochschule in Zurich in 1895. Alfred Werner was a chemist and educationist.
His accomplishments included the development of the theory of coordination
compounds. This theory, in which Werner proposed revolutionary ideas about
how atoms and molecules are linked together, was formulated in a span of
only three years, from 1890 to 1893. The remainder of his career was spent
gathering the experimental support required to validate his new ideas. Werner
became the first Swiss chemist to win the Nobel Prize in 1913 for his work
on the linkage of atoms and the coordination theory.
(1866-1919) (1866-1919) (1866-1919) (1866-1919) (1866-1919)
Reprint 2024-25
Page 4


118 Chemistry
In the previous Unit we learnt that the transition metals
form a large number of complex compounds in which
the metal atoms are bound to a number of anions or
neutral molecules by sharing of electrons. In modern
terminology such compounds are called coordination
compounds. The chemistry of coordination compounds
is an important and challenging area of modern
inorganic chemistry. New concepts of chemical bonding
and molecular structure have provided insights into
the functioning of these compounds as vital components
of biological systems. Chlorophyll, haemoglobin and
vitamin B
12 
are coordination compounds of magnesium,
iron and cobalt respectively. Variety of metallurgical
processes, industrial catalysts and analytical reagents
involve the use of coordination compounds.
Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
Coordination
Compounds
After studying this Unit, you will be
able to
• appreciate the postulates of
Werner’s theory of coordination
compounds;
• know the meaning of the terms:
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination
polyhedron, oxidation number,
homoleptic and heteroleptic;
• learn the rules of nomenclature
of coordination compounds;
• write the formulas and names
of mononuclear coordination
compounds;
• define different types of isomerism
in coordination compounds;
• understand the nature of bonding
in coordination compounds in
terms of the Valence Bond and
Crystal Field theories;
• appreciate the importance and
applications of coordination
compounds in our day to day life.
Objectives
Coordination Compounds are the backbone of modern inorganic
and bio–inorganic chemistry and chemical industry.
Coordination
Compounds
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
and characterised a large number of coordination compounds and
studied their physical and chemical behaviour by simple experimental
techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl
3
, CoCl
2
 or PdCl
2
 have primary valence of 3, 2 and 2 respectively.
In a series of compounds of cobalt(III) chloride with ammonia, it was
found that some of the chloride ions could be precipitated as AgCl on
adding excess silver nitrate solution in cold but some remained in
solution.
5.1 5.1 5.1 5.1 5.1 Werner’ Werner’ Werner’ Werner’ Werner’s s s s s
Theory of Theory of Theory of Theory of Theory of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
5
Unit Unit Unit Unit Unit
5
Reprint 2024-25
119 Coordination Compounds
1 mol CoCl
3
.6NH
3 
(Yellow) gave 3 mol AgCl
1 mol CoCl
3
.5NH
3 
(Purple) gave 2 mol AgCl
1 mol CoCl
3
.4NH
3 
(Green) gave 1 mol AgCl
1 mol CoCl
3
.4NH
3 
(Violet) gave 1 mol AgCl
These observations, together with the results of conductivity
measurements in solution can be explained if (i) six groups in all,
either chloride ions or ammonia molecules or both, remain bonded to
the cobalt ion during the reaction and (ii) the compounds are formulated
as shown in Table 5.1, where the atoms within the square brackets
form a single entity which does not dissociate under the reaction
conditions. Werner proposed the term secondary valence for the
number of groups bound directly to the metal ion; in each of these
examples the secondary valences are six.
Note that the last two compounds in Table 5.1 have identical empirical
formula, CoCl
3
.4NH
3
, but distinct properties. Such compounds are
termed as isomers. Werner in 1898, propounded his theory of
coordination compounds. The main postulates are:
1. In coordination compounds metals show two types of linkages
(valences)-primary and secondary.
2. The primary valences are normally ionisable and are satisfied by
negative ions.
3. The secondary valences are non ionisable. These are satisfied by
neutral molecules or negative ions. The secondary valence is equal to
the coordination number and is fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have
characteristic spatial arrangements corresponding to different
coordination numbers.
In modern formulations, such spatial arrangements are called
coordination polyhedra. The species within the square bracket are
coordination entities or complexes and the ions outside the square
bracket are called counter ions.
He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals. Thus, [Co(NH
3
)
6
]
3+
, [CoCl(NH
3
)
5
]
2+
 and [CoCl
2
(NH
3
)
4
]
+
are octahedral entities, while [Ni(CO)
4
] and [PtCl
4
]
2–
 are tetrahedral and
square planar, respectively.
Colour Formula Solution conductivity
corresponds to
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Yellow [Co(NH
3
)
6
]
3+
3Cl
–
1:3 electrolyte
Purple [CoCl(NH
3
)
5
]
2+
2Cl
–
1:2 electrolyte
Green [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Violet [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Reprint 2024-25
120 Chemistry
(i) Secondary 4 (ii) Secondary 6
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4
On the basis of the following observations made with aqueous solutions,
assign secondary valences to metals in the following compounds:
Solution Solution Solution Solution Solution
Difference between a double salt and a complex
Both double salts as well as complexes are formed by the combination
of two or more stable compounds in stoichiometric ratio. However, they
differ in the fact that double salts such as carnallite, KCl.MgCl
2
.6H
2
O,
Mohr’s salt, FeSO
4
.(NH
4
)
2
SO
4
.6H
2
O, potash alum, KAl(SO
4
)
2
.12H
2
O, etc.
dissociate into simple ions completely when dissolved in water. However,
complex ions such as [Fe(CN)
6
]
4– 
of K
4
[Fe(CN)
6
] do not dissociate into
Fe
2+
 and CN
–
 ions.
Formula Moles of AgCl precipitated per mole of
the compounds with excess AgNO
3
(i) PdCl
2
.4NH
3
2
(ii) NiCl
2
.6H
2
O 2
(iii) PtCl
4
.2HCl 0
(iv) CoCl
3
.4NH
3
1
(v) PtCl
2
.2NH
3
0
Example 5.1 Example 5.1 Example 5.1 Example 5.1 Example 5.1
Werner Werner Werner Werner Werner was born on December 12, 1866, in Mülhouse,
a small community in the French province of Alsace.
His study of chemistry began in Karlsruhe (Germany)
and continued in Zurich (Switzerland), where in his
doctoral thesis in 1890, he explained the difference in
properties of certain nitrogen containing organic
substances on the basis of isomerism. He extended vant
Hoff’s theory of tetrahedral carbon atom and modified
it for nitrogen. Werner showed optical and electrical differences between
complex compounds based on physical measurements. In fact, Werner was
the first to discover optical activity in certain coordination compounds.
He, at the age of 29 years became a full professor at Technische
Hochschule in Zurich in 1895. Alfred Werner was a chemist and educationist.
His accomplishments included the development of the theory of coordination
compounds. This theory, in which Werner proposed revolutionary ideas about
how atoms and molecules are linked together, was formulated in a span of
only three years, from 1890 to 1893. The remainder of his career was spent
gathering the experimental support required to validate his new ideas. Werner
became the first Swiss chemist to win the Nobel Prize in 1913 for his work
on the linkage of atoms and the coordination theory.
(1866-1919) (1866-1919) (1866-1919) (1866-1919) (1866-1919)
Reprint 2024-25
121 Coordination Compounds
(a) Coordination entity
A coordination entity constitutes a central metal atom or ion bonded
to a fixed number of ions or molecules. For example, [CoCl
3
(NH
3
)
3
]
is a coordination entity in which the cobalt ion is surrounded by
three ammonia molecules and three chloride ions. Other examples
are [Ni(CO)
4
], [PtCl
2
(NH
3
)
2
], [Fe(CN)
6
]
4–
, [Co(NH
3
)
6
]
3+
.
(b) Central atom/ion
In a coordination entity, the atom/ion to which a fixed number
of ions/groups are bound in a definite geometrical arrangement
around it, is called the central atom or ion. For example, the
central atom/ion in the coordination entities: [NiCl
2
(H
2
O)
4
],
[CoCl(NH
3
)
5
]
2+ 
and [Fe(CN)
6
]
3–
 are Ni
2+
, Co
3+
 and Fe
3+
, respectively.
These central atoms/ions are also referred to as Lewis acids.
(c) Ligands
The ions or molecules bound to the central atom/ion in the
coordination entity are called ligands. These may be simple ions
such as Cl
–
, small molecules such as H
2
O or NH
3
, larger molecules
such as H
2
NCH
2
CH
2
NH
2
 or N(CH
2
CH
2
NH
2
)
3
 or even macromolecules,
such as proteins.
When a ligand is bound to a metal ion through a single donor
atom, as with Cl
–
, H
2
O or NH
3
, the ligand is said to be unidentate.
When a ligand can bind through two donor atoms as in
H
2
NCH
2
CH
2
NH
2
 (ethane-1,2-diamine) or C
2
O
4
2–
 (oxalate), the
ligand is said to be didentate and when several donor atoms are
present in a single ligand as in N(CH
2
CH
2
NH
2
)
3
, the ligand is said
to be polydentate. Ethylenediaminetetraacetate ion (EDTA
4–
) is
an important hexadentate ligand. It can bind through two
nitrogen and four oxygen atoms to a central metal ion.
When a di- or polydentate ligand uses its two or more donor
atoms simultaneously to bind a single metal ion, it is said to be a
chelate ligand. The number of such ligating groups is called the
denticity of the ligand. Such complexes, called chelate complexes
tend to be more stable than similar complexes containing unidentate
ligands. Ligand which has two different donor atoms and either of
the two ligetes in the complex is called ambidentate
ligand. Examples of such ligands are the NO
2
–
 and
SCN
–
 ions. NO
2
–
 ion can coordinate either through
nitrogen or through oxygen to a central metal
atom/ion.
Similarly, SCN
–
 ion can coordinate through the
sulphur or nitrogen atom.
(d)Coordination number
The coordination number (CN) of a metal ion in a complex can be
defined as the number of ligand donor atoms to which the metal is
directly bonded. For example, in the complex ions, [PtCl
6
]
2–
 and
[Ni(NH
3
)
4
]
2+
, the coordination number of Pt and Ni are 6 and 4
respectively. Similarly, in the complex ions, [Fe(C
2
O
4
)
3
]
3–
 and
[Co(en)
3
]
3+
, the coordination number of both, Fe and Co, is 6 because
C
2
O
4
2–
 and en (ethane-1,2-diamine) are didentate ligands.
5.2 5.2 5.2 5.2 5.2 Definitions of Definitions of Definitions of Definitions of Definitions of
Some Some Some Some Some
Important Important Important Important Important
Terms Terms Terms Terms Terms
Pertaining to Pertaining to Pertaining to Pertaining to Pertaining to
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
Reprint 2024-25
Page 5


118 Chemistry
In the previous Unit we learnt that the transition metals
form a large number of complex compounds in which
the metal atoms are bound to a number of anions or
neutral molecules by sharing of electrons. In modern
terminology such compounds are called coordination
compounds. The chemistry of coordination compounds
is an important and challenging area of modern
inorganic chemistry. New concepts of chemical bonding
and molecular structure have provided insights into
the functioning of these compounds as vital components
of biological systems. Chlorophyll, haemoglobin and
vitamin B
12 
are coordination compounds of magnesium,
iron and cobalt respectively. Variety of metallurgical
processes, industrial catalysts and analytical reagents
involve the use of coordination compounds.
Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
Coordination
Compounds
After studying this Unit, you will be
able to
• appreciate the postulates of
Werner’s theory of coordination
compounds;
• know the meaning of the terms:
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination
polyhedron, oxidation number,
homoleptic and heteroleptic;
• learn the rules of nomenclature
of coordination compounds;
• write the formulas and names
of mononuclear coordination
compounds;
• define different types of isomerism
in coordination compounds;
• understand the nature of bonding
in coordination compounds in
terms of the Valence Bond and
Crystal Field theories;
• appreciate the importance and
applications of coordination
compounds in our day to day life.
Objectives
Coordination Compounds are the backbone of modern inorganic
and bio–inorganic chemistry and chemical industry.
Coordination
Compounds
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
and characterised a large number of coordination compounds and
studied their physical and chemical behaviour by simple experimental
techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl
3
, CoCl
2
 or PdCl
2
 have primary valence of 3, 2 and 2 respectively.
In a series of compounds of cobalt(III) chloride with ammonia, it was
found that some of the chloride ions could be precipitated as AgCl on
adding excess silver nitrate solution in cold but some remained in
solution.
5.1 5.1 5.1 5.1 5.1 Werner’ Werner’ Werner’ Werner’ Werner’s s s s s
Theory of Theory of Theory of Theory of Theory of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
5
Unit Unit Unit Unit Unit
5
Reprint 2024-25
119 Coordination Compounds
1 mol CoCl
3
.6NH
3 
(Yellow) gave 3 mol AgCl
1 mol CoCl
3
.5NH
3 
(Purple) gave 2 mol AgCl
1 mol CoCl
3
.4NH
3 
(Green) gave 1 mol AgCl
1 mol CoCl
3
.4NH
3 
(Violet) gave 1 mol AgCl
These observations, together with the results of conductivity
measurements in solution can be explained if (i) six groups in all,
either chloride ions or ammonia molecules or both, remain bonded to
the cobalt ion during the reaction and (ii) the compounds are formulated
as shown in Table 5.1, where the atoms within the square brackets
form a single entity which does not dissociate under the reaction
conditions. Werner proposed the term secondary valence for the
number of groups bound directly to the metal ion; in each of these
examples the secondary valences are six.
Note that the last two compounds in Table 5.1 have identical empirical
formula, CoCl
3
.4NH
3
, but distinct properties. Such compounds are
termed as isomers. Werner in 1898, propounded his theory of
coordination compounds. The main postulates are:
1. In coordination compounds metals show two types of linkages
(valences)-primary and secondary.
2. The primary valences are normally ionisable and are satisfied by
negative ions.
3. The secondary valences are non ionisable. These are satisfied by
neutral molecules or negative ions. The secondary valence is equal to
the coordination number and is fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have
characteristic spatial arrangements corresponding to different
coordination numbers.
In modern formulations, such spatial arrangements are called
coordination polyhedra. The species within the square bracket are
coordination entities or complexes and the ions outside the square
bracket are called counter ions.
He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals. Thus, [Co(NH
3
)
6
]
3+
, [CoCl(NH
3
)
5
]
2+
 and [CoCl
2
(NH
3
)
4
]
+
are octahedral entities, while [Ni(CO)
4
] and [PtCl
4
]
2–
 are tetrahedral and
square planar, respectively.
Colour Formula Solution conductivity
corresponds to
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Yellow [Co(NH
3
)
6
]
3+
3Cl
–
1:3 electrolyte
Purple [CoCl(NH
3
)
5
]
2+
2Cl
–
1:2 electrolyte
Green [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Violet [CoCl
2
(NH
3
)
4
]
+
Cl
–
1:1 electrolyte
Reprint 2024-25
120 Chemistry
(i) Secondary 4 (ii) Secondary 6
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4
On the basis of the following observations made with aqueous solutions,
assign secondary valences to metals in the following compounds:
Solution Solution Solution Solution Solution
Difference between a double salt and a complex
Both double salts as well as complexes are formed by the combination
of two or more stable compounds in stoichiometric ratio. However, they
differ in the fact that double salts such as carnallite, KCl.MgCl
2
.6H
2
O,
Mohr’s salt, FeSO
4
.(NH
4
)
2
SO
4
.6H
2
O, potash alum, KAl(SO
4
)
2
.12H
2
O, etc.
dissociate into simple ions completely when dissolved in water. However,
complex ions such as [Fe(CN)
6
]
4– 
of K
4
[Fe(CN)
6
] do not dissociate into
Fe
2+
 and CN
–
 ions.
Formula Moles of AgCl precipitated per mole of
the compounds with excess AgNO
3
(i) PdCl
2
.4NH
3
2
(ii) NiCl
2
.6H
2
O 2
(iii) PtCl
4
.2HCl 0
(iv) CoCl
3
.4NH
3
1
(v) PtCl
2
.2NH
3
0
Example 5.1 Example 5.1 Example 5.1 Example 5.1 Example 5.1
Werner Werner Werner Werner Werner was born on December 12, 1866, in Mülhouse,
a small community in the French province of Alsace.
His study of chemistry began in Karlsruhe (Germany)
and continued in Zurich (Switzerland), where in his
doctoral thesis in 1890, he explained the difference in
properties of certain nitrogen containing organic
substances on the basis of isomerism. He extended vant
Hoff’s theory of tetrahedral carbon atom and modified
it for nitrogen. Werner showed optical and electrical differences between
complex compounds based on physical measurements. In fact, Werner was
the first to discover optical activity in certain coordination compounds.
He, at the age of 29 years became a full professor at Technische
Hochschule in Zurich in 1895. Alfred Werner was a chemist and educationist.
His accomplishments included the development of the theory of coordination
compounds. This theory, in which Werner proposed revolutionary ideas about
how atoms and molecules are linked together, was formulated in a span of
only three years, from 1890 to 1893. The remainder of his career was spent
gathering the experimental support required to validate his new ideas. Werner
became the first Swiss chemist to win the Nobel Prize in 1913 for his work
on the linkage of atoms and the coordination theory.
(1866-1919) (1866-1919) (1866-1919) (1866-1919) (1866-1919)
Reprint 2024-25
121 Coordination Compounds
(a) Coordination entity
A coordination entity constitutes a central metal atom or ion bonded
to a fixed number of ions or molecules. For example, [CoCl
3
(NH
3
)
3
]
is a coordination entity in which the cobalt ion is surrounded by
three ammonia molecules and three chloride ions. Other examples
are [Ni(CO)
4
], [PtCl
2
(NH
3
)
2
], [Fe(CN)
6
]
4–
, [Co(NH
3
)
6
]
3+
.
(b) Central atom/ion
In a coordination entity, the atom/ion to which a fixed number
of ions/groups are bound in a definite geometrical arrangement
around it, is called the central atom or ion. For example, the
central atom/ion in the coordination entities: [NiCl
2
(H
2
O)
4
],
[CoCl(NH
3
)
5
]
2+ 
and [Fe(CN)
6
]
3–
 are Ni
2+
, Co
3+
 and Fe
3+
, respectively.
These central atoms/ions are also referred to as Lewis acids.
(c) Ligands
The ions or molecules bound to the central atom/ion in the
coordination entity are called ligands. These may be simple ions
such as Cl
–
, small molecules such as H
2
O or NH
3
, larger molecules
such as H
2
NCH
2
CH
2
NH
2
 or N(CH
2
CH
2
NH
2
)
3
 or even macromolecules,
such as proteins.
When a ligand is bound to a metal ion through a single donor
atom, as with Cl
–
, H
2
O or NH
3
, the ligand is said to be unidentate.
When a ligand can bind through two donor atoms as in
H
2
NCH
2
CH
2
NH
2
 (ethane-1,2-diamine) or C
2
O
4
2–
 (oxalate), the
ligand is said to be didentate and when several donor atoms are
present in a single ligand as in N(CH
2
CH
2
NH
2
)
3
, the ligand is said
to be polydentate. Ethylenediaminetetraacetate ion (EDTA
4–
) is
an important hexadentate ligand. It can bind through two
nitrogen and four oxygen atoms to a central metal ion.
When a di- or polydentate ligand uses its two or more donor
atoms simultaneously to bind a single metal ion, it is said to be a
chelate ligand. The number of such ligating groups is called the
denticity of the ligand. Such complexes, called chelate complexes
tend to be more stable than similar complexes containing unidentate
ligands. Ligand which has two different donor atoms and either of
the two ligetes in the complex is called ambidentate
ligand. Examples of such ligands are the NO
2
–
 and
SCN
–
 ions. NO
2
–
 ion can coordinate either through
nitrogen or through oxygen to a central metal
atom/ion.
Similarly, SCN
–
 ion can coordinate through the
sulphur or nitrogen atom.
(d)Coordination number
The coordination number (CN) of a metal ion in a complex can be
defined as the number of ligand donor atoms to which the metal is
directly bonded. For example, in the complex ions, [PtCl
6
]
2–
 and
[Ni(NH
3
)
4
]
2+
, the coordination number of Pt and Ni are 6 and 4
respectively. Similarly, in the complex ions, [Fe(C
2
O
4
)
3
]
3–
 and
[Co(en)
3
]
3+
, the coordination number of both, Fe and Co, is 6 because
C
2
O
4
2–
 and en (ethane-1,2-diamine) are didentate ligands.
5.2 5.2 5.2 5.2 5.2 Definitions of Definitions of Definitions of Definitions of Definitions of
Some Some Some Some Some
Important Important Important Important Important
Terms Terms Terms Terms Terms
Pertaining to Pertaining to Pertaining to Pertaining to Pertaining to
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
Reprint 2024-25
122 Chemistry
It is important to note here that coordination number of the central
atom/ion is determined only by the number of sigma bonds formed by
the ligand with the central atom/ion. Pi bonds, if formed between the
ligand and the central atom/ion, are not counted for this purpose.
(e) Coordination sphere
The central atom/ion and the ligands attached to it are enclosed in
square bracket and is collectively termed as the coordination
sphere. The ionisable groups are written outside the bracket and
are called counter ions. For example, in the complex K
4
[Fe(CN)
6
],
the coordination sphere is [Fe(CN)
6
]
4–
 and the counter ion is K
+
.
(f) Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly
attached to the central atom/ion defines a coordination
polyhedron about the central atom. The most common
coordination polyhedra are octahedral, square planar and
tetrahedral. For example, [Co(NH
3
)
6
]
3+
 is octahedral, [Ni(CO)
4
] is
tetrahedral and [PtCl
4
]
2–
 is square planar. Fig. 5.1 shows the
shapes of different coordination polyhedra.
5.3 5.3 5.3 5.3 5.3 Nomenclature Nomenclature Nomenclature Nomenclature Nomenclature
of of of of of
Coordination Coordination Coordination Coordination Coordination
Compounds Compounds Compounds Compounds Compounds
(g) Oxidation number of central atom
The oxidation number of the central atom in a complex is defined
as the charge it would carry if all the ligands are removed along
with the electron pairs that are shared with the central atom. The
oxidation number is represented by a Roman numeral in parenthesis
following the name of the coordination entity. For example, oxidation
number of copper in [Cu(CN)
4
]
3–
 is +1 and it is written as Cu(I).
(h) Homoleptic and heteroleptic complexes
Complexes in which a metal is bound to only one kind of donor
groups, e.g., [Co(NH
3
)
6
]
3+
, are known as homoleptic. Complexes in
which a metal is bound to more than one kind of donor groups,
e.g., [Co(NH
3
)
4
Cl
2
]
+
, are known as heteroleptic.
Nomenclature is important in Coordination Chemistry because of the
need to have an unambiguous method of describing formulas and
writing systematic names, particularly when dealing with isomers. The
formulas and names adopted for coordination entities are based on the
recommendations of the International Union of Pure and Applied
Chemistry (IUPAC).
Fig. 5.1: Shapes of different coordination polyhedra. M
represents the central atom/ion and L, a unidentate
ligand.
Reprint 2024-25