Coordination compounds
Coordination compounds

Coordination sphere

When a certain number of ions or ligands are attached to the central atom present in a compound and written inside a bracket, they are known as co-ordination spears.

                             NH3
                                |                          2-
       [    NH3 ___     Cu  ____   NH3 ]
                                 |
                             NH3

  • Cu_______  Central atom
  • NH3______ ligand

What is Ligand

The natural molecules are ions which surround the central metal atom or ion in a complex are called as ligand.

Classification of ligand

1)Monodentate or Unidentate liands

These ligands can donate only one pair of electrons to the central metal atom or ion .
for example H2O ,NH3 ,CO,OH- etc

2) Bidentate ligand

These ligands have to donor atoms and  donate to pair of electrons to the central metal atom or ion .
For example CH2-NH2
                         |
                        CH2-NH2
(Ethylene diamine)

3) Polydentate ligands

These lagand which contain more than two donor atom in the molecule are called as polydentate ligands.
For example
                        CH2COO-
          CH2-N
                        CH2COO-
EDTA(ETHYLENE DIIAMINE TETRA ACETATE)

Coordination number

The ligands are attached to the central metal atom or ion though covalent and co-ordinate bonds the number of covalent and co-ordinate bonds formed by the ligands with the central metal atom or ion is known as co-ordination number of that atom .
for example
compound________________[Ag(NH3)2]Cl

  • Central Metal ion  ______         Ag+
  • Ligand    ________________        NH3
  • Coordination number___         2

Complex compound ___       K2[Ni(CN4)]

  • Central metal ion _______        Ni2+
  • Ligand    _________________      CN-
  • Coordination number ___       4


 What is Chelate compound

The cyclic complex compounds formed when a multidentate ligand (ligand having more than one doner sites) attached through two or more doner atoms to the central metal ion are called as chelate compounds .
Figure of chelate compound.

Coordination compound
Coordination compound

Distinction between chelate compound and complex compound

In chelate compound,the metel combines with organic or inorganic or ions which May use two or more doner atoms resulting in the formation of cyclic rings.thus,the donor  groups are firmly attached to the metal ions present in the ring and so chelate compound are very stable.

Importance of chelate compounds


  •  Metal chelate constituted Theconstitut institute the minerals plants and also present in animals for example chlorophyll is a magnesium chelete whereas hemoglobin is an iron chelete . 
  •  Metals chelate are used in the qualitative and quantitative estimation of metal ion.
  • The hardness of water is estimated by simple titration against EDTA solution. EDTA forms stable chelate with the Ca2+and Mg2+ ions present in the hard water.
  • Metal chelate are used in the preservation of fruits juices and food stuffs etc .
  • The chelating agent EDTA in the form of Na2Ca EDTA  is used to remove harmful radioactive metals such as lead  from the body.



Ambidentate ligands

The unidentate ligands which can bind to the central metal atom though two donor atoms are called as ambidentate ligands.


Polynuclear complexes


  • The complexes containing two or More than central metal atoms are called as polynuclear complexes. 
  • In these complexes ,the bridging group (the group which joins the two central metal atom) is indicated in the formula of the complex by seprating it from rest of the complex by hyphens and by adding μ befour it's name.
  • Its important figure is given below. 
Coordination compound
Coordination compound