Coordination compound

Definition of Coordination compound:

  • Ligand : Any species(ion or molecule) that have at least one lone pair of electron and can donate its lone pair of electron to a metal cation or atom is called a ligand. Since a ligand is an electron rich species it is also called Lewis base or nucleophile.
  • A metal cation is an electron deficient species and can accept a lone pair of electron, there fore it act as a Lewis acid or electrophile.
  • When a group of ligand donate its lone pair of electron to the metal cation or atom coordinate bonds are formed and the product formed is called a coordination compound
  • Thus the compound in which metal cation or atom is attached to a group of ligand by coordinate bond is called a coordination compound.
  • Or a complex ion in which a metal cation is attached to the ligand by coordinate bonds.

Structural formula of Coordination compound:

  • Here the central metal cation or atom and the ligand attached to it are written in a square bracket i.e.[ ] which is called coordination sphere.
  • The cation or anion outside the coordination sphere is called the ionization sphere or counter ion.
  • The atom in a ligand that is directly attached to a metal cation or atom is called the donor atom and the number of donor atom attached to a metal cation or atom is called the coordination number .
  • For example ,coordination number of   Ag+ in [Ag(NH3)2]2+   is 2
                                                                                 Cu+ in [Cu(NH3)4]2+ is 4

                                                                                 Co3+in [Co(NH3)6]3+  is 6

Properties of Coordination compound;

  • The coordination compound may be either neutral molecule or ionic compounds. In ionic coordination compound either the cation or anion or both may be complex ion.
  • The coordination compound retain their identity more or less even in solution though partial dissociation may occur.
  • These compound do not give the tests  of  all their constituent ion in aqueous solution i.e. some constituent ion lost their identity in aqueous solution. For example  [Co(NH3)6]Cl3 is a complex compound  and it does not give the test of all the constituent ions, Co3+and Cl–  instead it gives the    Co3+ as  [Co(NH3)6]3+ complex ion and  Cl–  ion.

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