On complete reaction of FeCl3 with oxalic acid in aqueous solution containing KOH, resulted in the f

Q: Why do we need KOH in this reaction?

Oxalic acid is a weak acid and doesn't dissociate completely. KOH is a strong base that neutralizes the acid, forcing it to completely deprotonate and form oxalate ions ($\text{C}_2\text{O}_4^{2-}$), which are the actual ligands that bind to iron.

Q: What is the difference between primary and secondary valency?

According to Werner's theory, primary valency corresponds to the oxidation state of the metal (which is ionizable), while secondary valency corresponds to the coordination number (the number of coordinate bonds formed by ligands), which is non-ionizable and directional.

Q: How do we know oxalate is a bidentate ligand?

The oxalate ion ($\text{C}_2\text{O}_4^{2-}$) has two negatively charged oxygen atoms that can simultaneously donate lone pairs of electrons to the same central metal atom, forming a stable 5-membered chelate ring.

Q: Does this complex show geometrical isomerism?

No, complexes of the type $[\text{M}(\text{AA})_3]$ (where AA is a symmetrical bidentate ligand like oxalate) do not show geometrical isomerism because all positions are equivalent. However, they do show optical isomerism!

Q: Why is the coordination number 6 and not 3?

Coordination number is the total number of coordinate bonds, not just the number of ligand molecules. Since there are 3 oxalate molecules and each forms 2 bonds, the total coordination number is $3 \times 2 = 6$.

Question

On complete reaction of $	ext{FeCl}_3$ with oxalic acid in aqueous solution containing KOH, resulted in the formation of product A. The secondary valency of Fe in the product A is ........ . (Round off to the nearest integer).

Sigma Percentile · Accepted Answer

The journey to finding the secondary valency of a coordination complex often begins with a seemingly simple chemical reaction. In this problem, we are tasked with reacting iron(III) chloride ($	ext{FeCl}_3$) with oxalic acid ($	ext{H}_2	ext{C}_2	ext{O}_4$) in the presence of potassium hydroxide ($	ext{KOH}$).

At first glance, you might wonder, "Why do we need KOH?" The answer lies in the nature of oxalic acid.

### The Role of the Base

Oxalic acid is a weak acid. On its own, it doesn't fully dissociate into oxalate ions. However, when we introduce a strong base like KOH, it completely neutralizes the oxalic acid.

This neutralization reaction strips the protons away, leaving behind the fully deprotonated oxalate ions ($	ext{C}_2	ext{O}_4^{2-}$). These oxalate ions are the true stars of the show—they are fantastic ligands eager to bind to a metal center.

### The Formation of the Complex

Once the oxalate ions are generated, they immediately seek out the $	ext{Fe}^{3+}$ ions from the iron(III) chloride. The reaction proceeds to form a highly stable coordination complex.

When we balance the stoichiometry, we find that three oxalate molecules coordinate with a single iron atom. The overall balanced chemical equation looks like this:

$$ 	ext{FeCl}_3 + 3	ext{H}_2	ext{C}_2	ext{O}_4 + 6	ext{KOH} \longrightarrow 	ext{K}_3[	ext{Fe}(	ext{C}_2	ext{O}_4)_3] + 3	ext{KCl} + 6	ext{H}_2	ext{O} $$

Our "Product A" is the complex salt potassium trioxalatoferrate(III), $	ext{K}_3[	ext{Fe}(	ext{C}_2	ext{O}_4)_3]$.

### Decoding Secondary Valency

Now, we arrive at the core of the question: What is the secondary valency of iron in this complex?

According to Alfred Werner's groundbreaking theory of coordination compounds, the **secondary valency** is synonymous with the **coordination number** of the central metal atom. It represents the total number of coordinate covalent bonds formed between the ligands and the metal.

To find this, we must look closely at our ligand. The oxalate ion is a **bidentate ligand**. This means that a single oxalate molecule has two donor oxygen atoms that can simultaneously bite onto the iron center, forming a stable five-membered chelate ring.

Since our complex contains three of these bidentate oxalate ligands, we can calculate the total coordination number easily:

$$ 	ext{Secondary Valency} = 	ext{Number of Ligands} 	imes 	ext{Denticity} $$

$$ 	ext{Secondary Valency} = 3 	imes 2 = 6 $$

The iron atom is surrounded by six coordinate bonds, giving the complex a perfect **octahedral geometry**.

### A Hidden Layer: Optical Isomerism

Before we wrap up, let's appreciate a beautiful hidden concept in this molecule. Because the $[	ext{Fe}(	ext{C}_2	ext{O}_4)_3]^{3-}$ complex features three symmetrical bidentate ligands arranged octahedrally, it completely lacks a plane of symmetry.

This asymmetry means the complex is chiral! It exists as two non-superimposable mirror images, known as the $d$ (dextrorotatory) and $l$ (levorotatory) optical isomers. Recognizing these subtle details is what transforms a good chemistry student into a great one.

Animated Solution for Chemistry - Coordination Compounds: On complete reaction of $FeCl_{3}$ with oxalic acid in aqueous solution containing KOH, resulted in the formation of product A. The secondary valency of Fe in the product A is ........ . (Round off to the nearest integer).

Enter Numerical Value:

Visualized Solution

$The Chemical Reaction$

$Role of KOH$

$Formation of Product A$

$Analyzing the Ligand$

$Calculating Secondary Valency$

$Optical Isomerism$

The Sigma Insight: Nomenclature, Isomerism, Importance and Werner's Theory

The Role of the Base

The Formation of the Complex

Decoding Secondary Valency

A Hidden Layer

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Visualized Solution

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The Chemical Reaction

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Role of KOH

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Formation of Product A

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Analyzing the Ligand

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Calculating Secondary Valency

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Optical Isomerism