what is effective atomic number rule(EAN) ?sir can you explain with eg......

Effactive Atomic Number (EAN) Rule: This theory was proposed by Sidgwick to explain the stability of complexes on the basis of  effactive atomic no. According to the Rule; "A stable complex is formed if the EAN is equal to the atomoc no. of next noble gas to the metal." 

It means , the EAN of a metal which undergo complexation, should be equal to the atomic no. of that noble gas which is just near to the metal.

Before to understand the rule, we need to understand that of EAN; actually the effective atomic no. of a metal in acomplex is considered as the Total no. of electron present in metal ion + no. of electrons doneted by ligands to the metal.

Thus, EAN may be calculated as follows:

EAN of a metal in a complex: = At. No.- oxidation state + 2 X Co -ordination no.

For ex: To find the stability of [Co(NH₃)₆]⁺³ we get the EAN as

O.S of Co= +3, At, No. of Co= 27, Co-ordination no= 6

Thus EAN= 27-3+2x6= 36 that is equal to [Kr] the noble gas.

Hence the complex is stable.

but we cant use, this rule in many cases as for ex, 

to find the no of CO ligand to be attached to Ni- metal;

At. No. of Ni:- 28

At. No. of next noble gas = 36

No. of electrons to be provided by ligand= 36 - 28= 8

Since, each CO provides 2 electrons so no. of CO group to be attached to Ni is = 8/2= 4

Hence formula is [Ni(CO)₄]