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Find the Formula of Iron(III) hexacyanidoferrate(II) | Coordination Compounds - CHEMCA

Find the Formula of Iron(III) hexacyanidoferrate(II) | Coordination Compounds - CHEMCA

Writing the Molecular Formula from IUPAC Name: Iron(III) hexacyanidoferrate(II)

Published by Abhishek Sengar | CHEMCA India

In the Coordination Compounds chapter, students often practice naming complexes from formulas, but examiners for JEE and NEET love testing the reverse process: deducing the molecular formula from a long, intimidating IUPAC name.

Let's break down one of the most famous and highly tested compounds in inorganic chemistry: Iron(III) hexacyanidoferrate(II). (Hint: This is the chemical name for a very famous deep blue pigment!)

Video Tutorial: The Criss-Cross Method

Watch Abhishek Sengar sir from CHEMCA expertly decode the IUPAC name and show you how to apply basic ionic bonding rules to complex coordination spheres.

Step-by-Step Decoding

The Core Principle: Even though it looks complicated, a coordination compound is ultimately an ionic compound. It consists of a Cation (positive) and an Anion (negative). You must find the charge of both, and then "criss-cross" their valencies just like you would for AlCl3 or Na2SO4.
  1. Identify the Counter Ion (Cation):
    The first part of the name is "Iron(III)". This tells us the cation outside the bracket is Fe3+.
  2. Decode the Complex Ion (Anion):
    The second part is "hexacyanidoferrate(II)".
    • "Hexacyanido": 6 Cyanide ligands → (CN)6. Each CN has a -1 charge.
    • "ferrate(II)": The central metal is Iron (Fe) with a +2 oxidation state.
    • Assemble the sphere: [Fe(CN)6]
  3. Calculate the Charge of the Complex Sphere:
    Total Charge = (Oxidation state of Fe) + (Charge of 6 CN ligands)
    Total Charge = (+2) + 6(-1) = 2 - 6 = -4.
    So, the complex anion is [Fe(CN)6]4-.
  4. Criss-Cross the Valencies:
    Now, treat them like simple ions. We have Fe3+ and [Fe(CN)6]4-. The 4 goes to the outer Iron, and the 3 goes to the entire complex bracket.
Fe3+ [Fe(CN)6]4- Fe4[Fe(CN)6]3

Fig: Criss-crossing the valencies between the counter ion and the coordination sphere.

Final Molecular Formula: Fe4[Fe(CN)6]3

Practice Questions for JEE & NEET

Ready to test your knowledge? Try these two questions covering the practical applications and variations of this complex.

Question 1: The compound we just derived, Fe4[Fe(CN)6]3, is famous for its intense deep blue color. What is the common name of this pigment, and in which qualitative analysis test is it formed?

Answer: Prussian Blue.

This compound is widely known as Prussian Blue. It is formed during the Lassaigne's Test for the detection of Nitrogen in organic compounds. When sodium fusion extract containing NaCN is treated with FeSO4 and FeCl3 in an acidic medium, this deep blue precipitate confirms the presence of Nitrogen.

Question 2: What is the molecular formula for a closely related compound named Potassium hexacyanidoferrate(III)?

Answer: K3[Fe(CN)6]

Reasoning:

  • Counter Ion: Potassium → K+ (valency +1).
  • Complex Sphere: Hexacyanidoferrate(III). Here, Iron is +3.
  • Charge of sphere = (+3) + 6(-1) = -3. So, the complex is [Fe(CN)6]3-.
  • Criss-cross K+ and [Fe(CN)6]3- → The 3 goes to Potassium.
  • Formula: K3[Fe(CN)6] (Commonly known as Potassium ferricyanide).

Ready for more Practice?

As Abhishek Sir mentioned, practice makes perfect! Visit www.chemca.in today for complete handwritten notes, mock tests, and a massive question bank tailored for JEE & NEET.

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