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Coordination Compounds

Addition Compounds

When solutions containing two (or) more simple stable compounds in molecular proportions are allowed to evaporate, crystals of new substances are obtained. These substances are termed molecular (or) addition compounds. Some common examples are

KCl + MgCl2 +6H2O →  KCl. MgCl2.6H2O

K2SO4 + Al2(SO4)3 + 24H2O  →  K2SO4×Al2(SO4)3×24H2O

CuSO4 + 4NH3 → CuSO4×4NH3

The molecular (or) addition compounds are of two types

Double salts:

The addition compounds which are stable in solid state only but are broken down into individual constituents when dissolved in water are called double salts. Their solutions have the same properties as the mixture of individual compounds.

For example,When Mohr’s salt, [FeSO4×(NH4)2SO4×6H2O] is dissolved in water it exhibits the properties of FeSO4 and (NH4)2SO4 i.e. they produce Fe2+, NH4+ and SO42– ions in solution. Thus each ion has its identity in double salt.

Co-ordination compounds:

The addition compounds in which some of the constituent ions (or) molecules lose their identity and when dissolved in water they do not break up completely into individual ions are called co-ordination compounds. The properties of their solutions of coordination compounds are different than those of their constituents. When CuSO4.4NH3 is dissolved in water there is no evidence for the presence of Cu2+ ions (or) NH3 molecules. This is because a new ion [Cu(NH3)4]2+ is furnished in which NH3 molecules are directly linked with the metal ion

CuSO4×4NH3 →  [Cu(NH3)4]2+ + SO42–

Thus, a co-ordination compound is a molecular compound that is formed by combination of two (or) more simple stable molecular compounds and retains its identity in the solid as well as in dissolved state. The properties of coordination compounds are totally different than their individual constituents. A co-ordination compound contains very often but not always a complex ion.

A complex  ion may is an electrically charged radical which consists a central metal atom (or) ion surrounded by a group of ions (or) neutral molecules (or) both.

We will discuss the basic of coordination compounds in more detail under following sub topics

Terms used in Coordination Chemistry

The important terms used in the study of coordination compounds are mentioned below.

Central Metal Ion

Central metal ion is an electron acceptor metal atom which has vacant orbitals to which a fixed number of ligands are attached via co-ordinate bonds in definite geometrical arrangement.

Co-ordination Sphere:

It is the complex ion enclosed in square bracket. Co-ordination sphere behaves as a single unit.

Ionization Sphere:

Ionization sphereis the part of compound present outside co-ordination sphere, e.g

Terms used in Coordination Chemistry

Co-ordination Number:

It is the total number of ligands attached to the central metal atom through co-ordinate bonds or the number of atoms of a ligand attached to the same central atom, e.g. hexadentate ligand should be counted as forming six co-ordinate bonds. 

Oxidation number:

It is the charge which the central atom appears to have if all the ligands are removed along with the electron pairs that are shared with the central atom.


x + (4 × 0) + (-1 × 2) = +1    [because the ligand H2O is neutral and 2Cl carries - 2 charge]

x + 0 - 2 = +1

x = 3


It is an ion or molecule capable of donating a pair of electrons to the central atom via a donor atom.

Types of ligands

  • Unidentate ligands:  These are the ligands with only one donor atom, e.g. NH3, Cl-, F- etc.

  • Bidentate ligands: These are the ligands with two donor atoms, e.g. ethylenediamine, C2O42-(oxalate ion) etc. 

  • Tridentate ligands: These are the ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.

  • Hexadentate ligands: These are the ligands which have six donor atoms per ligand, e.g. EDTA. 

  • Chelating Ligands: Multidentate ligand simultaneously co-ordinating to a metal ion through more than one site is called chelating ligand. These ligands produce a ring like structure called chelate. Chelation increases the stability of complex. This effect is called chelation effect.

Werner’s Co-ordination Theory

  • Metals possess two types of valencies i.e. primary (ionizable) valency and secondary (nonionizable) valency.

  • Secondary valency of a metal is equal to the number of ligands attached to it i.e. coordination number.

  • Primary valencies are satisfied by negative ions, while secondary valencies may be satisfied by neutral, negative or positive ions.

  • Secondary valencies have a fixed orientation around the metal in space.


  Primary Valencies = 3 Cl-

  Secondary Valencies = 6 NH3

  Coordination Sphere =  [Co(NH3)6]3-

Refer to the following video for Werner’s co-ordination theory

Whenever the complex is formed the secondary valency  should always be satisfied. This he proved with the following example.

  • CoCl3.6NH3 + AgNO3 → 3AgCl

  • CoCl3.5NH3 + AgNO3 → 2AgCl

  • CoCl3.4NH3 + AgNO3 → 1AgCl

Werner concluded that in CoCl3. 6NH3 the 3 Cl- acted as primary valencies while  the 6 NH3 as secondary valencies. In modern terms the complex can be written as [Co(NH3)6]Cl3. The 3 Cl  are ionic and hence are precipitated as AgCl by AgNO3

In the second complex 2 moles of AgCl is precipitated indicating the presence of  2 Cl-  in the outer sphere i.e., in order to fulfill the secondary valency  one chlorine from the outer sphere drifts into the inner sphere. Similar is the case with the third complex. So actual structure becomes [Co(NH3)6]Cl3, [Co(NH3)5 Cl]Cl2,  [Co(NH3)4 Cl2]Cl,

Question 1: Mohr’s salt is

a. double salt

b. coordination compound

c. both

d. none

Question 2: Secondary valancy of [Cu(NH3)4]2+ is

a. 1

b. 2

c. 3

d. 4

Question 3: Total number of legand in [Cr(H2O)4Cl2]+ is

a. 6

b. 4

c. 2

d. 1

Question 4: EDTA.  is

a. unidentate ligands

b. bidentate ligands

c. ridentate ligands

d. hexadentate ligands

Question 5: Carbonate ion(CO32–) acts as a

a.  tridentate ligand

b. bidentate ligand

c. monodentate ligand

d. tetradentate ligand

Question 6:  Which of the following co-ordination number is unknown for Co3+?

a. 5

b. 4

c. 6

d. zero













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