Question

How to calculate the oxidation number of Iron in ferric ferrocyanide${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3$?

Answer 1

Oxidation state of outer Iron

The oxidation state (oxidation number) is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic.

Step 1: Dissociation of Complex:

${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3$ is a coordination compound and dissociates as

${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3\to 4{\mathrm{Fe}}^{3+}+3[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{-4}$

Step 2: Calculation of Oxidation state:

The charge on outer iron is +3 so, the oxidation state of outer Fe is +3.

Oxidation State of iron in coordination complex

Step 1: Dissociation of Complex:

${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3$ is a coordination compound and dissociates as

${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3\to 4{\mathrm{Fe}}^{3+}+3[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{-4}$

Step 2: Calculation of oxidation state

Let the oxidation number of Fe be x.

Charge on CN = -1

For $[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{-4}$

$$\begin{gathered} \mathrm{x}\times 6(-1) \\ =-4 \\ \therefore \mathrm{x}=+2 \end{gathered}$$

the oxidation state of inner Fe is +2.

Hence, the oxidation state of outer iron is +3, and the one present inside the coordination sphere have +2 oxidation state.