Synthesis of Nitro and Nitrito Pentaamminecobalt(III) Chloride | Linkage Isomerism in Coordination Compounds

Synthesis of Nitro and Nitrito Pentaamminecobalt(III) Chloride | A Demonstration of Linkage Isomerism

By Rizwan Ibn Ali Abdullah

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Introduction

In coordination chemistry, few laboratory experiments are as visually striking as the synthesis of the nitro and nitrito pentaamminecobalt(III) chloride complexes. These compounds are classical examples of linkage isomerism, where a single ligand — the nitrite ion (NO₂⁻) — can attach to the cobalt(III) center in two distinct ways. This simple yet elegant reaction demonstrates how small changes in pH and temperature can lead to visibly different products.

Starting from the pink precursor [Co(NH₃)₅Cl]Cl₂, treatment with sodium nitrite under controlled conditions produces two colorful coordination compounds: the dark orange nitro isomer (N-bonded) and the light orange nitrito isomer (O-bonded).

Table of Contents

• Overview of the Reaction
• Reaction Mechanism
• Synthesis of Nitro Isomer
• Synthesis of Nitrito Isomer
• Observations and Color Changes
• Characterization
• FAQ Section
• Conclusion

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Overview of the Reaction

The reaction involves substituting the chloride ligand in [Co(NH₃)₅Cl]Cl₂ with a nitrite ion (NO₂⁻), forming two possible products:

[Co(NH₃)₅Cl]Cl₂ + NaNO₂ → [Co(NH₃)₅(NO₂)]Cl₂ (Nitro) or [Co(NH₃)₅(ONO)]Cl₂ (Nitrito) + NaCl

Which isomer forms depends mainly on the pH and temperature of the solution.

Key Idea: The nitrite ion is ambidentate — it can coordinate via nitrogen or oxygen. This dual binding ability gives rise to two distinct linkage isomers.

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Reaction Mechanism

In acidic solutions, protonation of nitrite promotes bonding through nitrogen, leading to the nitro isomer. In neutral or basic media, oxygen coordination is favored, forming the nitrito isomer. Thus, the reaction demonstrates the interplay of kinetic and thermodynamic factors in coordination chemistry.

• Acidic + Heat → [Co(NH₃)₅(NO₂)]Cl₂ (Nitro, N-bonded)
• Neutral + Cool → [Co(NH₃)₅(ONO)]Cl₂ (Nitrito, O-bonded)

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Synthesis of Nitro-Pentaamminecobalt(III) Chloride ([Co(NH₃)₅(NO₂)]Cl₂)

Reagents:

• [Co(NH₃)₅Cl]Cl₂ – 0.9 g
• NaNO₂ – 0.5 g
• Dilute HCl (pH 1–2)
• Distilled water – 20 mL

Procedure:

1. Dissolve 0.9 g of the pink [Co(NH₃)₅Cl]Cl₂ in 10 mL warm water.
2. Add 0.5 g sodium nitrite solution slowly with stirring.
3. Acidify immediately with dilute HCl to maintain acidity.
4. Heat the mixture at 70–80°C for 15–20 minutes.
5. Cool in ice, filter the orange product, wash with cold water and ethanol.
6. Dry in a desiccator.

Observation: The mixture turns deep orange upon heating and forms orange crystals on cooling — the nitro isomer.

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Synthesis of Nitrito-Pentaamminecobalt(III) Chloride ([Co(NH₃)₅(ONO)]Cl₂)

Reagents:

• [Co(NH₃)₅Cl]Cl₂ – 0.6 g
• NaNO₂ – 0.35 g
• Distilled water – 15 mL (neutral)
• Ethanol for washing

Procedure:

1. Dissolve 0.6 g of the pink complex in 10 mL neutral water.
2. Add NaNO₂ solution and stir gently at 40–50°C for 10–15 minutes.
3. Cool rapidly in an ice bath to precipitate the product.
4. Filter, wash with cold water and ethanol, and dry.

Observation: A light orange-red product forms — the nitrito isomer. It slowly converts to the nitro form when heated or left standing.

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Observations and Color Changes

Both products show distinct colors and stability due to the different binding atoms of the nitrite ion.

Complex	Formula	Bonding Mode	Color	Conditions
Nitro	[Co(NH₃)₅(NO₂)]Cl₂	N-bonded	Dark orange	Acidic + Heat
Nitrito	[Co(NH₃)₅(ONO)]Cl₂	O-bonded	Light orange / Reddish	Neutral + Cool

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Characterization

• IR Spectra: Nitro shows N=O bands at 1430 & 825 cm⁻¹; Nitrito shows bands at 1460 & 1065 cm⁻¹.
• UV–Vis: Nitro complex absorbs at 457 nm (yellowish-orange), Nitrito at 486 nm (redder hue).
• Thermal Behavior: Heating the nitrito form in acid converts it into the nitro complex — a classical linkage isomerization.

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Frequently Asked Questions (FAQ)

1. What is linkage isomerism?

It is a type of structural isomerism where an ambidentate ligand bonds through different donor atoms, forming two distinct coordination compounds.

2. Why do these isomers differ in color?

The bonding atom (N or O) alters the electronic environment of cobalt, shifting the energy gap between d-orbitals and changing the visible color.

3. Which isomer is more stable?

The nitro isomer is thermodynamically more stable, while the nitrito isomer is kinetically formed and less stable.

4. What precautions should be taken?

Work in a fume hood, wear gloves and goggles. Cobalt salts are irritants, and sodium nitrite can form toxic gases in acid.

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Conclusion

This experiment beautifully demonstrates linkage isomerism in transition metal complexes. By adjusting reaction conditions, one can control whether the nitrite ligand binds through nitrogen or oxygen. The color difference — from light orange to deep orange — makes this reaction a classic example of how coordination geometry and electronic structure manifest visibly.

Beyond its beauty, this synthesis reinforces a key principle in chemistry: the delicate balance between kinetic control and thermodynamic stability.

About the Author

Rizwan Ibn Ali Abdullah — He writes educational articles combining scientific reasoning with clear, practical explanations to help students understand chemistry deeply and visually.

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