Chelating effect-EAN rule-application of EAN rule

What is chelating effect in co-ordination chemistry ?

Chelating effect is related to the stability of complex in co-ordination chemistry.

It has been experimentally found that chelate complex which made by chelating ligand, are more stable than complex compound made by mono dentate ligand.

This contribution of chelating ligand to enhance the stability of chelating complex, is known as chelating effect.

For example, let us consider the formation of complexes from the hydrated cobalt ion ,[ Co ( H₂O )₆ ] ²⁺ with ammine [ mono dentate ligand] and ethylene diamine [a bidentate and also chelating ligand ] ligands in aqueous medium is as follows,

From the above two equation it is seen that , in case of second equation, the number of particle increase after reaction. Hence entropy increases.

That is, in second cases, free energy of the system, decreases [ ΔGᵒ = ΔHᵒ –TΔSᵒ ].

Consequently, it is clear that chelate complex which made by chelating ligand, are more stable than complex compound made by mono dentate ligand.

What is EAN rule in co-ordination chemistry ?  

Sidgwick introduces a rules, depending on the effective atomic number of the central 

metal atom to explain the stability of co-ordination complexes, are known as effective atomic number rules or EAN  rule.

Sidgwick stated that, in co-ordination complex, the value of effective atomic number is equal to the atomic number in the next noble gas element.

In the formation of a complex, each ligand is considered to be donating one electron pair to the central metal ion.

The total number of electrons which the central metal ion appears to possess in the complex including those gained by it in bonding, is called ( EAN ) or effective atomic number of the central metal ion .

It has been found that in many cases ligands are added until the central metal ion gets the same number of electrons as are present in the next noble gas.

Accordingly, the effective atomic number in a complex should be equal to 36(Kr) ,54(Xe) or 86( Rn ).

The mathematical expression of effectiveatomic number ( EAN ) is ,

EAN = [ atomic number of metal atom – oxidation number of metal atom ] + 2 x number of ligands.

Calculation of effective atomic number( EAN ) of some metal atoms are given below.

Application of EAN rule in co-ordination chemistry .

The number of ligands that attached to the central metal ion in co-ordination complex, can be calculated with the help of this theory.

For example, calculation of ligand numbers in nickel carbonyl compound .

Atomic number of nickel is 28 . The atomic number of the next inert gas is 36( Kr ).

The number of electrons are contributed by the ligand due to formation of co-ordinate with the central metal atom is ( 36 – 28 ) = 8 .

Now, each ligand contributes two electrons in co-ordinate bond formation .

Hence, the number of carbonyl ( CO ) ligand = 8 /2 = 4 .

In the similar way, calculation of ligand numbers in iron carbonyl compound .

Atomic number of Fe is 26 . The atomic number of the next inert gas is 36( Kr ).

The number of electrons are contributed by the ligand due to formation of co-ordinate with the central metal atom is ( 36 – 26 ) = 10 .

Now, each ligand contributes two electrons in co-ordinate bond formation .

Hence, the number of carbonyl ( CO ) ligand = 10 /2 = 5 .

Summary:

What is chelating effect in co-ordination chemistry ?

What is EAN rule in co-ordination chemistry ? 

Application of EAN rule in co-ordination chemistry .

 Fajan's rule and its application in chemistry.