Apply the concept of EAN rule on the complexes [Fe(CN)6]^(4-) and [Fe(CN)6]^(3-).

Question

Apply the concept of EAN rule on the complexes \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}\) and \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}\). What conclusion you draw about the validity of the EAN rule from these two well-known complexes? [Atomic number of \(\mathrm{Fe}=26\)].

Answer 1

(a) Let us take the case of \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}\)

Oxidation state* of Fe in \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}=+2\)

No. of electrons in iron = 26

\(\therefore\) No. of electrons in Fe (II) of the complex \(=26-2=24\)

Number of electrons donated by six \(\mathrm{CN}^{-}\) ligands \(=6 \times 2=12\)

\(\therefore\) EAN of Fe (II) in \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}=24+12=36\)

Since this number (36) = number of electrons in the atom of the nearest noble gas \((\mathrm{Kr}),\) the complex \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}\) follows the EAN rule or the EAN rule is valid on this complex.

(b) Now let us take the case of \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}\)

Oxidation state of iron \(=+3\)

\(\therefore \) Number of electrons in Fe (III) of the complex = 26 - 3 = 23

Number of electrons donated by six \(\mathrm{CN}^{-}\) ligands = 12

\(\therefore\) EAN of Fe (III) in \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}=23+12=35\)

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.