Question

What is the formula for $Iron\left( {III} \right)$ cyanide?

Answer 1

$Iron\left( {III} \right)$ cyanide also known as ferric cyanide is anion. The other name for this is hexacyanoferrate. Potassium ferricyanide is the most common salt of this anion which is a red crystalline material which is used as an oxidant in organic chemistry.

Complete step by step answer:
When we say $Iron\left( {III} \right)$ that means iron exists as a ferric compound, that means it is present in trivalent state or existing as $Fe{}^{3 + }$
Cyanide $CN{}^ -$ has a charge of $- 1$ , so in order to make the molecule neutral, we need to balance the three cyanide molecules to irons $+ 3$ charge.
So the formula for $Iron\left( {III} \right)$ cyanide also known as ferric cyanide is; $Fe(CN){}^3$ .

Additional Information:
$[Fe(CN){}_6]{}^{3 - }$ has $Fe{}^{3 + }$ in the centre which is bound in an octahedral geometry to six ligands. The iron has low spin and is reduced easily to the related ferrocyanide ion $[Fe(CN){}_6]{}^{3 - }$ which is a ferrous derivative. This is a redox couple is a reversible and there is no breaking or making of $Fe - C$ bonds;
${[Fe{\left( {CN} \right)_6}]^{3 - }}\; + {\text{ }}{e^ - }\; \rightleftharpoons {[Fe{\left( {CN} \right)_6}]^{4 - }}$
This redox couple is a standard in electrochemistry.
It is used for the treatment of ferricyanide with iron salts affording the long lasting pigment known as Prussian blue, the traditional coloured of blueprints.

Note:
If compared to the main group of cyanides such as potassium cyanide, this form of cyanide that is ferricyanide is known to be very less toxic due to its very strong bond between the cyanide ion and the $Fe{}^{3 + }$ . This does not react with mineral acids, however to release highly toxic hydrogen cyanide gas.