Effective Atomic Number Rule

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Effective Atomic Number Rule-

[ EAN – Rule]

The total number of electrons linked with central metal atom or ion in a complex plus the electrons donated by the ligand is known as effective Atomic Number.

Sidgwick suggested that the central atom or ion will accept electron pairs from ligands till the EAN of the metal equalize itself with the atomic number of the nearest noble gas.

EAN= Z – x + 2nL

Z= Atomic number of the metal in the complex

x = Oxidation state of the metal in complex

n= Number of the ligands

L = Number of co-ordinate bonds formed by ligand

For monodentate ligand,

L=1

For Bidentate ligand

L=2

For Tridentate ligand

L=3

For Tetra dentate ligand

L=4

For Ex.

(1)       EAN of Fe in K4[Fe(CN)6]

O.No. of Fe = x

4(+1)+x+6(-1)=0

x=+2

L=6

n=1 (Monodentate ligand)

z=26

EAN= Z–x+2nL

=26–2+2 x 1 x 6

=26–2+12

 EAN= 36

3.         [Fe(CO)5]

o no. of Fe =x

x+5×0=0

x= 0

z=26

L=5

n= 1

EAN = 26–0+2×1×5

=26+10

EAN= 36

5. [Ni(NH3)6]2+

o. no. of Ni =x

x+6(0)= +2

x= +2

L =6

z= 28

n=1

EAN= 28-2+2×1×6

= 26+12

EAN= 38

7. [Cr(C2O4)3]3-

oxalate is bidentate ligand

o. no. of Cr =x

x+3(-2)=-3

x=+3

L=3

n=2(bidentate)

EAN= 24-3+2×2×3

= 24-3+12

EAN = 33

9          [Co (EDTA)]

x-2= -1

x=+3

L=1

EDTA is hexa dentate ligand

n=6

z=27

EAN= 27-3+2×6×1

= 24+12

EAN= 36

2.         [Co(en)3]­­3+

en is bidentate ligand

Z=27

o.no.of Co =x

x+(3×0) = +3

x= +3

n= 2

EAN= Z–x+2nL

=27–3+2×2×3

=24+12

EAN= 36

4.         [Cr(NH3)6]3+

L=6

n=1

o. no. of Cr= x

x+6 x 0= +3

x= +3

EAN= Z-x+2nl

=24-3+2 x 1 x 6

=21+12

EAN= 33

6.         [PdCl4]2-

o. no. of Pd=x

x +4(-1)=-2

x=+2

L=4

n=1

z=46

EAN= 46-2+2×1×4

=44+8=52

8.         [ Cr Cl (H2O)(en)2]2+

x + 1 (-1)+0+2×0=+2

x= +3

Cl & H2O one monodentate ligand while ‘en’ is bidentate ligand

EAN= Z-x+2n L

=24-3+(2×1×2)+(2×2×2)

= 21+4+8

= 33

10.       [ Mn (en)2(NO2)]+

o. no. of Mn =x

x+2×0+2(–1)=+1

x+0–2=+1

x=+3

z = 25

EAN= 25-3+(2×2×2)+(2×1×1)

= 22+8+2

=32