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[ALTSM 01/03 Q88]

The concept of Effective atomic number(EAN) was given by SIDWICK.EAN is the sum of electrons present in the metal ion and total number of electrons gained from the ligands.
It is given as -
EAN = (Z-x) + n * y
where,
Z = atomic no. of central metal atom
x = oxidation state of central metal ion
n = no. of ligands
y = no. of electrons donated by one ligand
For the above complexes
[Ni(CN)4]^2- = 28 – 2 + 4 *2 = 34
[Fe(CN)6]^3- = 26 -- 3 + 12 = 35
and for [Cu(CN)4]^3- = 29 --1 + 8 = 36
thus increasing order of EAN for the above complexes will be-
[Ni(CN)4]2- < [Fe(CN)6]3- < [Cu(CN)4]3-