Ex: Apply the concept of EAN rule on the complexes [Fe(CN)6]4− and [Fe(CN)6]3−. What conclusion you draw about the validity of the EAN rule from these two well–known complexes? [Atomic number of Fe = 26].
Solution : (a) Let us take the case [Fe(CN)6]4−
Oxidation state of Fe in [Fe(CN)6]4− = +2
No. of electrons in iron = 26
∴ No. of electrons in Fe(II) of the complex = 26−2 = 24
Number of electrons donated by six CN− ligands = 6×2 = 12
∴ EAN of Fe(II) in [Fe(CN)6]4− = 24+12 = 36
Since, this number (36) = number of electrons in the atom of the nearest noble gas (Kr), the complex [Fe(CN)6]4− follows the EAN rule or the EAN rule is valid on this complex.
(b) Now let us take the case of [Fe(CN)6]3−
Oxidation state of iron = +3
∴ Number of electrons in Fe(III) of the complex = 26−3 = 23
Number of electrons donated by six CN− ligands = 12
∴ EAN of Fe (III) in [Fe(CN)6]3− = 23+12 = 35
Since, this number (35) is different from the electrons present in the next noble gas (Kr = 36), we can say that the EAN rule is not valid on this complex.
Sidgwick suggested that the metal ion will continue accepting electron pairs till the total number of electrons in the metal ion and those donated by ligands is equal to that of the next higher noble gas.
This total number of electrons is called effective atomic number (EAN) of the metal.
This will become clear by taking the example of hexammine Rhodium (III) ion, [Rh(NH3)6]3+.
Atomic number of Rhodium = 45
In the present complex, Rhodium is present in the oxidation state of +3.
∴Total number of electrons in Rh+3 = 45 − 3 = 42
Since each NH3 ligand contributes two electrons to the Rhodium ion, electrons contributed by 6 NH3 ligands = 6×2 = 12
∴ The EAN of Rh+3 in the complex = 42 + 12 = 54
In general, EAN of metal = Atomic Number − Oxidation state + CN × 2
CN = Coordination number
In the said example since the number (54) corresponds to the atomic number of Xenon, according to Sidgwick the complex will be stable. Though EAN rule (i.e., those complexes are stable or those metal ions form complexes whose EAN is the same as the atomic number of the next noble gas) is applicable in many cases, there are several examples in which EAN rule is not obeyed.