Unit
Objectives
9
Coordination
After studying this Unit, you will be
able to Compounds
• appreciate the postulates of
Werner’s theory of coordination
compounds;
Coordination Compounds are the backbone of modern inorganic
• know the meaning of the terms: and bio–inorganic chemistry and chemical industry.
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination In the previous Unit we learnt that the transition metals
polyhedron, oxidation number, form a large number of complex compounds in which
homoleptic and heteroleptic; the metal atoms are bound to a number of anions or
• learn the rules of nomenclature neutral molecules by sharing of electrons. In modern
of coordination compounds;
terminology such compounds are called coordination
• write the formulas and names compounds. The chemistry of coordination compounds
of mononuclear coordination
is an important and challenging area of modern
compounds;
inorganic chemistry. New concepts of chemical bonding
• define different types of isomerism
and molecular structure have provided insights into
in coordination compounds;
the functioning of these compounds as vital components
• understand the nature of bonding
of biological systems. Chlorophyll, haemoglobin and
in coordination compounds in
terms of the Valence Bond and vitamin B12 are coordination compounds of magnesium,
Crystal Field theories; iron and cobalt respectively. Variety of metallurgical
• appreciate the importance and processes, industrial catalysts and analytical reagents
applications of coordination involve the use of coordination compounds.
compounds in our day to day life. Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
9.1 Werner’
Werner’ss Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
Theory of and characterised a large number of coordination compounds and
Coordination studied their physical and chemical behaviour by simple experimental
Compounds techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl3, CoCl2 or PdCl2 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.
Chemistry 244
2022-23
, 1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (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 9.1, 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.
Table 9.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Colour Formula Solution conductivity
corresponds to
3+ –
Yellow [Co(NH3)6] 3Cl 1:3 electrolyte
2+ –
Purple [CoCl(NH3)5] 2Cl 1:2 electrolyte
+ –
Green [CoCl2(NH3)4] Cl 1:1 electrolyte
+ –
Violet [CoCl2(NH3)4] Cl 1:1 electrolyte
Note that the last two compounds in Table 9.1 have identical empirical
formula, CoCl3.4NH3, 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(NH3)6]3+, [CoCl(NH3)5]2+ and [CoCl2(NH3)4]+
2–
are octahedral entities, while [Ni(CO)4] and [PtCl4] are tetrahedral and
square planar, respectively.
245 Coordination Compounds
2022-23
, On the basis of the following observations made with aqueous solutions, Example 9.1
assign secondary valences to metals in the following compounds:
Formula Moles of AgCl precipitated per mole of
the compounds with excess AgNO3
(i) PdCl2.4NH3 2
(ii) NiCl2.6H2O 2
(iii) PtCl4.2HCl 0
(iv) CoCl3.4NH3 1
(v) PtCl2.2NH3 0
(i) Secondary 4 (ii) Secondary 6 Solution
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4
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.MgCl2.6H2O,
Mohr’s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc.
dissociate into simple ions completely when dissolved in water. However,
4–
complex ions such as [Fe(CN)6] of K4 [Fe(CN)6] do not dissociate into
2+ –
Fe and CN ions.
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
(1866-1919) substances on the basis of isomerism. He extended vant
Hoff’s theory of tetrahedral carbon atom and modified
it for nitrogen. Wer ner 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.
Chemistry 246
2022-23
Objectives
9
Coordination
After studying this Unit, you will be
able to Compounds
• appreciate the postulates of
Werner’s theory of coordination
compounds;
Coordination Compounds are the backbone of modern inorganic
• know the meaning of the terms: and bio–inorganic chemistry and chemical industry.
coordination entity, central atom/
ion, ligand, coordination number,
coordination sphere, coordination In the previous Unit we learnt that the transition metals
polyhedron, oxidation number, form a large number of complex compounds in which
homoleptic and heteroleptic; the metal atoms are bound to a number of anions or
• learn the rules of nomenclature neutral molecules by sharing of electrons. In modern
of coordination compounds;
terminology such compounds are called coordination
• write the formulas and names compounds. The chemistry of coordination compounds
of mononuclear coordination
is an important and challenging area of modern
compounds;
inorganic chemistry. New concepts of chemical bonding
• define different types of isomerism
and molecular structure have provided insights into
in coordination compounds;
the functioning of these compounds as vital components
• understand the nature of bonding
of biological systems. Chlorophyll, haemoglobin and
in coordination compounds in
terms of the Valence Bond and vitamin B12 are coordination compounds of magnesium,
Crystal Field theories; iron and cobalt respectively. Variety of metallurgical
• appreciate the importance and processes, industrial catalysts and analytical reagents
applications of coordination involve the use of coordination compounds.
compounds in our day to day life. Coordination compounds also find many applications
in electroplating, textile dyeing and medicinal chemistry.
9.1 Werner’
Werner’ss Alfred Werner (1866-1919), a Swiss chemist was the first to formulate
his ideas about the structures of coordination compounds. He prepared
Theory of and characterised a large number of coordination compounds and
Coordination studied their physical and chemical behaviour by simple experimental
Compounds techniques. Werner proposed the concept of a primary valence and
a secondary valence for a metal ion. Binary compounds such as
CrCl3, CoCl2 or PdCl2 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.
Chemistry 244
2022-23
, 1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (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 9.1, 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.
Table 9.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes
Colour Formula Solution conductivity
corresponds to
3+ –
Yellow [Co(NH3)6] 3Cl 1:3 electrolyte
2+ –
Purple [CoCl(NH3)5] 2Cl 1:2 electrolyte
+ –
Green [CoCl2(NH3)4] Cl 1:1 electrolyte
+ –
Violet [CoCl2(NH3)4] Cl 1:1 electrolyte
Note that the last two compounds in Table 9.1 have identical empirical
formula, CoCl3.4NH3, 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(NH3)6]3+, [CoCl(NH3)5]2+ and [CoCl2(NH3)4]+
2–
are octahedral entities, while [Ni(CO)4] and [PtCl4] are tetrahedral and
square planar, respectively.
245 Coordination Compounds
2022-23
, On the basis of the following observations made with aqueous solutions, Example 9.1
assign secondary valences to metals in the following compounds:
Formula Moles of AgCl precipitated per mole of
the compounds with excess AgNO3
(i) PdCl2.4NH3 2
(ii) NiCl2.6H2O 2
(iii) PtCl4.2HCl 0
(iv) CoCl3.4NH3 1
(v) PtCl2.2NH3 0
(i) Secondary 4 (ii) Secondary 6 Solution
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4
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.MgCl2.6H2O,
Mohr’s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc.
dissociate into simple ions completely when dissolved in water. However,
4–
complex ions such as [Fe(CN)6] of K4 [Fe(CN)6] do not dissociate into
2+ –
Fe and CN ions.
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
(1866-1919) substances on the basis of isomerism. He extended vant
Hoff’s theory of tetrahedral carbon atom and modified
it for nitrogen. Wer ner 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.
Chemistry 246
2022-23
