Identification of Iron Cyanide Complex Compound Z

To determine the identity of compound $\mathbf{Z}$:

In the qualitative analysis of iron, the reaction of iron(III) ions ($\mathrm{Fe}^{3+}$) with potassium hexacyanoferrate(III) (potassium ferricyanide, $\mathrm{K}_3[\mathrm{Fe}(\mathrm{CN})_6]$) yields a brown-colored complex of iron(III) hexacyanoferrate(III):

$\mathrm{Fe}^{3+}(\mathrm{aq}) + [\mathrm{Fe}(\mathrm{CN})_6]^{3-}(\mathrm{aq}) \longrightarrow \mathrm{Fe}[\mathrm{Fe}(\mathrm{CN})_6](\mathrm{s}/\mathrm{aq})$

In this neutral coordination compound:

• The coordination entity carries a charge of $-3$, i.e., $[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_6]^{3-}$.
• The counter cation is $\mathrm{Fe}^{3+}$.
• Combining these in a $1:1$ stoichiometric ratio gives the neutral formula $\mathrm{Fe}[\mathrm{Fe}(\mathrm{CN})_6]$.

Therefore, the compound $\mathbf{Z}$ is $\mathrm{Fe}[\mathrm{Fe}(\mathrm{CN})_6]$.

Hence, the correct option is (B).