Question

Write the formula of the following coordination compound.
Iron(III) hexacyanoferrate(II).

Answer 1

The formula of the complex anion hexacyanoferrate(II) is $\left[Fe\right(CN{)}_6{]}^{-4}$. The oxidation number of iron is $+2$ and each cyanide ion has $-1$ charge. Hence, the net charge on the complex anion is $+2+6(-1)=-4$.
-4 charge on complex anion will be balanced by +3 charge on $F{e}^{3+}$ cation when 4 $F{e}^{3+}$ cations combine with 3 $\left[Fe\right(CN{)}_6{]}^{-4}$ complex anions.
The formula of the coordination compound Iron(III) hexacyanoferrate(II) is $F{e}_4\left(\right[Fe\left(CN{)}_6\right]{)}_3$
Note: In the name Iron(III) hexacyanoferrate(II), the numbers in parenthesis in roman numerals indicates the oxidation number of iron atoms.