Effective atomic number

 

Effective atomic number

it was proposed by Sidwick, also known as Noble gas rule.

is defined as the resultant number of electrons with the metal atom or Ion after gaining electrons from donor atoms of ligands.

EAN = atomic number of the metal - number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligand.

Example-

1)   EAN for  [Co(NH₃)₆]³⁺

Solution:- Atomic no of Co =27

no. of electron lost by cobalt = 3

no of electrons gained from ligand =12 

 (since one NH₃ donates two electrons so, 6  NH₃ donates 12electrons) 

EAN = atomic number of the metal - number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligand.

therefore, EAN = 27-3+12 =36

2). EAN for [Ni(CO)₄]⁰      

Solution:- Atomic no of Ni =28

no. of electron lost by Ni = 0

no of electrons gained from ligand = 8   

(since one CO   donates two electrons so, 6 CO  donates 8electrons) 

EAN = atomic number of the metal - number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligand.

therefore, EAN = 28-0+8=36