Question

Which of the following compounds is brown coloured?
(A) $Fe[Fe{(CN)_4}]$
(B) $Fe[Fe{(CN)_6}]$
(C) $F{e_4}[Fe{(CN)_6}]$
(D) ${K_2}Fe[Fe{(CN)_6}]$

Answer 1

Hint: To check whether the compound is brown coloured or not, there must be the availability of $F{e^{3 + }}$ion in an aqueous solution of a complex compound hence, we will first check whether the complex has $F{e^{3 + }}$ ion or not. If the complex can dissociate and has $F{e^{3 + }}$ions, then choose the suitable option from the given question.

Complete Step by Step Solution:
Let us consider all the four options given and write the oxidation state of $Fe$ in all the complex compounds one by one. There are the combinations of the oxidation state of atoms in such type of complex compound such as $C|M$ which the first one is of a counter atom and the second one is of central metal atom:


In $Fe[Fe{(CN)_6}]$ , the combination of oxidation state of $Fe$ is $ + 3| + 3$. In such a combination, brown colour is imparted to the compound.

In $F{e_4}[Fe{(CN)_6}]$ , the combination of oxidation state of $Fe$ is $ + 3| + 2$. Such a combination imparts dark blue or Prussian blue colour to the compound.

In $Fe[Fe{(CN)_4}]$ and ${K_2}Fe[Fe{(CN)_6}]$ , the combination of oxidation state of $Fe$ is $ + 2| + 2$. Such kind of combination imparts white colour to the compound.

Thus, In the given complex compounds, only $Fe[Fe{(CN)_6}]$ compound is brown coloured.
Hence, the correct option is (B) $Fe[Fe{(CN)_6}]$.

Note: Since this is a conceptual-based problem hence, it is essential that the options given in the question are analysed very carefully to give an accurate solution. Also, remember the lone pair availability while dissociating complex compounds. Identify the oxidation state of $Fe$ in complex compounds carefully.🪀🎱🎱⚽️⚽️