Sidwick also 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 hexamminecobalt (III) ion, \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\).
Atomic number of cobalt = 27
In the present complex, cobalt is present in the oxidation state of +3 .
\(\therefore\) Total number of electrons in \(\mathrm{Co}^{3+}=27-3=24\)
Since each \(\mathrm{NH}_3\) ligand contributes two electrons to the cobal: ion.
Electrons contributed by \(6 \mathrm{NH}_3\) ligands \(=6 \times 2=12\)
\(\therefore\) The EAN of \(\mathrm{Co}^{3+}\) in the complex \(=24+12=36\)
In general, EAN of metal = At. No. - Oxi. state + CN x 2
In the said example since the number (36) corresponds to the atomic number of krypton, according to Sidwick 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.
