COORDINATION COMPOUND

CO-ORDDINATION COMPOUNDS

"Acid and Base reacts to give salt and water this reaction is called as neutralisation reaction. "

When two or more such simple salts are combined in simple proportions, it forms a  new addition salt which have different properties as compared to parent salt.

Addition salts are further classified into two types -

A). Double salt

B). Co-ordination compound

A). Double salt  - The addition compound s which loose their identity in aqueous solution are called as double salts.

The solution of double salt gives test of similar and each ions in their simple salt .

They are crystalline in nature.

They contain only ionic bonds.

The properties of double salt are essentially the same as those of constituent ions.

 Example: Fe(NH4)2  (SO4)2 .6H2O

                        Mohr's salt .

B). Co-ordination compound -  The addition compound which retain their identity in aqueous solution are called as co-ordination compounds.

The solution of. Co-ordination compound gives test of new ions and does not give test of each ions.

They may or may not be crystalline in nature.

They contain ionic as well as co-ordination bonds.

The properties of co-ordination compound are different from the constituents ions.

 Example: K4 [Fe CN6]

                    Potassium ferrocyanide

 

Terms in co-ordination chemistry:

" A branch of chemistry which deals with the study of co-ordination compounds is called as co-ordination chemistry".

Following terminologies are used in co-ordination chemistry -

  

•  Central ion or central metal atom:

   " The positively charged metal ion or neutral metal atom which can accept electron pairs from ligands during complex formation is called as central metal ion or central metal atom."

 These are electron deficient species having sufficient vaccent orbitals to accommodate electron pairs from ligands.

These are called as Lewis acid.

  Example:

    K4[Fe CN6] has Fe2+  as central metal

• Ligands :

       " The molecules or ions which can donate their electron pairs to central metal during complex formation are called as Ligands".

These are electron Rich species . These are also called as Lewis base.

  Ligands can be negative ions , neutral molecules or positive ions.

  Example:.                                                                (NH3 , N2H5+ (hydrazinium ion ) etc.

Ligands can be classified into the following types –

• Monodentate ligands :

  "  The ligands in which only one donar atom can donate their electron pair to form only one coordinate bond with central metal is called as Monodentate ligands".
                       _                            _

            Ex:CN , NH3 ,H2O , Cl , etc. 

• Polydentate ligand :
    
     " The ligand in which more than one donar atoms can form more than one coordinate bonds with central metal is called as Polydentate ligand"
    These are further classified as—

1.Bidentate ligands : 
          " The ligand in which two donar atoms can form two co-ordinate bonds with central metal is called as bidentate ligand".
 Ex:  :NH2—CH2—CH2—H2N:
            Ethylene diammine

2. Tridentate ligand :
      " The ligand in which three donar atoms can form three co-ordinate bonds with central metal is called as tridentate ligand".
  Ex:.   
            :NH2—CH2—CH2—H2N:
              |
              CH2—CH2—H2N:
        Diethylene triammine ( dien)

3. Tetradentate ligand :
     " The ligand in which four donar atoms can form co-ordinate bonds with central metal is called as tetradentate ligand".

Ex:.       CH2—CH2— H2N:
               |                           |
              :NH                       CH2
               |                            |
              CH2                       CH2
               |                            |
              CH2                       :NH2
               |
              :NH2

           Triethylene tetramine
  
4. Hexadentate ligand :

   " The ligand in which six donar atoms can form six  coordinate bonds with central atoms is called as hexadentate ligand.        Ex.                                                                                2( _OOCCH2)–N(CH2)2–N(CH2COO_ )2

      Ethylene diammine tetra acetate.                                    (EDTA)              

                        

   • Ambidentate ligand:

    " The ligand in which more than one donar atoms are present but uses only one donar atom at a time to form one coordinate bond with a central metal is called as Ambidentate ligand."

 Ex:  NO2 group has two donar atoms (N,O) out of the two, only one donar atom is linked to the central metal as M—CN or         M—NO2

  Similarly,.    _ 

                    CN can form  bonds as M—CN 

                     Or M—NC

                                  

           

• Complex ion:

         "The charged or nutral species formed by the combination of central metal ion and ligand is called as complex ion". 

    

Depending upon the charge present, complex ion is of three types–

1. Cationic complex ion : It Carries positive charge.                               

Ex: [Co (NH3)6 ]³

2. Anionic complex ion : It Carries negative charge.

                         4-

Ex : [Fe CN6 ]

3. Neutral complex ion: It Carries no charge.

Ex: [ Ni (CO)4]

• Depending upon the type of ligands present , complex ion is of two types –

Homoleptic complex ion : 

          Here, central metal is attached to only one kind of ligand. 

                             4-

 Ex:  [ Fe ( CN )6]

Heteroleptic complex ion : 

              Here central metal is attached to more than one kind of ligands. 

                                       +

Ex:  [Co  (NH3 )4 CI2]

• Co-ordination  sphere : 

    "   The sphere formed by the ligands around the central metal is called as  Co-ordination sphere. 

It is denoted by 'Square bracket'

In this sphere, ligands are firmly attached to central metal and can not be separate easily. It is called as first inner zone or non-ionisable zone. 

• Coordination number: 

            " The number of co-ordinate bonds formed by central metal with ligands in a complex is called as Co-ordination number of central metal. "

Coordination number acts as the deciding factor for geometry of complex . 

   

Ex:  In K4[Fe CN6 ] , the Co-ordination number of Fe²+ ion is six and geometry is octahedral. 

• Counter ion : 

          " The oppositely charged ion associated with the complex ion  in Co-ordination compound is called as counter ion."

If the complex ion is positive then counter ion is negative or vise versa.

It is present outside the Co-ordination sphere and thus it is called as outer sphere or ionisable zone. 

Ex:   In K4 [Fe CN6 ],  K+ is a counter ion                where [Fe CN6]⁴- is complex ion .

• Coordination compound : 

     " A ionic compound formed by the combination of complex ion and oppositely charged counter ion is called as Co-ordination compound".

Ex :.   K4 [Fe CN6 ] ,   [Co (NH3)6 ] Cl3.