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Question: What is the chemical formula of iron\((III)\) hexacyanoferrate \((II)\)? (A) \(Fe[Fe{(CN)_6}]\) ...

What is the chemical formula of iron(III)(III) hexacyanoferrate (II)(II)?
(A) Fe[Fe(CN)6]Fe[Fe{(CN)_6}]
(B) Fe3[Fe(CN)6]F{e_3}[Fe{(CN)_6}]
(C) Fe3[Fe(CN)6]4F{e_3}{[Fe{(CN)_6}]_4}
(D) Fe4[Fe(CN)6]3F{e_4}{[Fe{(CN)_6}]_3}

Explanation

Solution

As we know that the coordination compounds are formulated and named according to the IUPAC nomenclature where the groups that surrounds the central atom must be identified first and the positive part is named first followed by the negative part along with the ligands in alphabetical order.

Complete answer: As we know that for writing the chemical formula of the given compounds we have to follow the rules assigned by the IUPAC for coordination compounds. So, the first point we need to keep in mind is to find out the central atom.
In the given question, we are given with ferrate (II)(II) which means iron is present in +2 + 2 oxidation state and it is the central atom. As the name of the central atom is ending with ‘ate’ it means that the central atom is present in the negative part of the complex compound.
Now we have, Iron which is present in +3 + 3 oxidation state so we can write the iron (III)(III) as Fe3+F{e^{3 + }} and as the name suggest the element is present in its original form so it must be present in positive part of the complex compound and it is the counter ion.
Then we have to find the ligand and in this case we can see the ligand is cyanate and it is present as hexacyanate meaning six cyano ions are surrounding the iron which can be written as 6  CN−6\;C{N^ - }.
So to balance the negative part of this complex compound we can write it as [Fe(CN)6]4−{[Fe{(CN)_6}]^{4 - }} and we know that counter ion is present as Fe3+F{e^{3 + }}.
We also know that the entire coordination compound entity is enclosed in square brackets whether charged or not. So after balancing the charge on the positive and negative part of our coordination complex compound, we can write the formula of the given name of the coordination compound as: Fe4[Fe(CN)6]3F{e_4}{[Fe{(CN)_6}]_3}.

Therefore, the correct answer is (D).

Note: Remember that when the formula of a given coordination complex compound is without the counter ion the charge is indicated outside the square bracket as a right subscript and the naming of anionic ligand ends with ‘o’ and cationic ligand ends with ‘ium’. When the coordination entity has negative charge then the name of the central atom ends with ‘ate’.