Synthesis of Pentaamminechloro Cobalt(III) Chloride & Linkage Isomers (Nitrito–Nitro) | Full Practical Guide with IR & Yield

Pentaamminechloro-cobalt(III) Chloride & Its Linkage Isomers (Nitrito → Nitro): A Complete, Error-Free Practical with IR Proof

By Rizwan Ibn Ali Abdullah • Rizwan Chemistry Classes

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Introduction

Small shifts in ligand attachment can flip colours and spectra. Here we synthesize pentaamminechloro-cobalt(III) chloride, [Co(NH3)5Cl]Cl2, convert it to the nitrito complex [Co(NH3)5(ONO)]Cl2, and isomerize it to the more stable nitro complex [Co(NH3)5(NO2)]Cl2. You’ll get crystal-clear equations, a student-proof procedure, mechanism, observations, yield calculation, IR/UV markers, viva prep, and a safe mini-guide to preparing 3 M and 6 M HCl.

Table of Contents

Aim Theory & Background Chemical Reactions Involved Chemicals & Apparatus Preparation & Step-by-Step Procedure Reaction Mechanism Observations & Colour Changes IR/UV Characterization How to Prepare 3 M & 6 M HCl Percentage Yield (Worked Example) Viva Questions FAQ Conclusion & Call to Action

Aim

• Synthesize [Co(NH3)5Cl]Cl2 from CoCl₂·6H₂O in ammoniacal medium (H₂O₂ oxidant).
• Convert it to [Co(NH3)5(ONO)]Cl2 and isomerize to [Co(NH3)5(NO2)]Cl2.
• Confirm linkage isomerism by IR and support with UV-Vis.

Theory & Background

Co(III) ammine complexes are low-spin octahedral (d⁶) and substitution-inert. Nitrite, NO2⁻, is ambidentate, binding either via oxygen (–ONO, nitrito) or nitrogen (–NO2, nitro). Cold, near-neutral conditions favour the kinetic O-bound product; light or gentle heating yields the thermodynamic N-bound isomer. This experiment demonstrates kinetic vs thermodynamic control cleanly.

Key idea: Oxidation to Co(III) “locks” an octahedral ammine sphere so we can isolate both linkage isomers without breakdown.

Chemical Reactions Involved

1) Formation of the chloro complex

CoCl₂·6H₂O + 5 NH₃ + Cl^– + H₂O₂ ⟶ [Co(NH₃)₅Cl]Cl₂ + 2 H₂O

NH₄Cl buffers; conc. NH₃ provides ligand; H₂O₂ oxidizes Co(II) → Co(III).

2) Substitution to nitrito (O-bound)

[Co(NH₃)₅Cl]Cl₂ + NO₂^– ⇌ [Co(NH₃)₅(ONO)]Cl₂ + Cl^–

3) Linkage isomerization (nitrito → nitro)

[Co(NH₃)₅(ONO)]Cl₂ ⟶ [Co(NH₃)₅(NO₂)]Cl₂

Chemicals & Apparatus

Chemicals (bench-scale)

• CoCl₂·6H₂O — 6.0 g
• NH₄Cl — 18 g
• Conc. aqueous NH₃ — ~18 mL (for slurry; add as needed)
• H₂O₂ (30%) — 4.5 mL (ice-cold, add dropwise)
• HCl: 3 M (~75 mL) and 6 M (few mL for later steps)
• NaNO₂ — ~2.0 g (excess; near-neutral pH)
• Distilled water / ice-cold water; a few drops NH₃ for Method 2 isomerization

Quantities match ~0.025 mol cobalt scale from the lab notebook.

Apparatus

• 250 mL beaker and conical flask; measuring cylinders; glass rod or magnetic stirrer
• Ice bath; water bath; thermometer
• Büchner funnel + filter paper (or gravity filtration)
• Watch glasses; balance; pH paper

Safety Essentials

• Goggles, gloves, coat; work in a fume hood with NH₃/HCl.
• Always add acid to water, never water to acid.
• Keep 30% H₂O₂ ice-cold; add slowly to avoid violent oxidation.
• Maintain ≤10 °C during nitrite substitution to favour O-bound nitrito.

Preparation & Step-by-Step Procedure

A) Synthesis of [Co(NH3)5Cl]Cl2

1. Prepare Co(II) solution: Dissolve 6.0 g CoCl₂·6H₂O in ~8–10 mL warm distilled water.
2. Make ammoniacal slurry: In a chilled conical flask, mix 18 g NH₄Cl with ~18 mL conc. NH₃. Keep in an ice bath.
3. Combine slowly: Add the Co(II) solution slowly to the cold slurry with vigorous stirring.
4. Cold oxidation: While in ice, add 4.5 mL of 30% H₂O₂ dropwise (effervescence normal). Stir 5–10 min more.
5. Digest with acid: Transfer to a 250 mL beaker, add about 75 mL of 3 M HCl, and heat on a water bath for ~10 min. A yellow–orange solid forms.
6. Isolate: Cool to room temperature, filter, and wash the solid with ice-cold water (2–3 × 5 mL). Dry in air (or ≤50 °C).

B) Convert to Nitrito Isomer, [Co(NH3)5(ONO)]Cl2 (Compound B)

1. Substitute chloride: Suspend 1.0–2.0 g of the chloro complex in ~20 mL cold water (0–10 °C). Add excess NaNO₂ (~2.0 g) and stir 15–20 min at near-neutral pH.
2. Crystallize cold: With the mixture still below 10 °C, add 2–3 mL 6 M HCl dropwise. A red precipitate appears.
3. Work-up: Filter quickly; wash crystals with ice-cold water. Blot and air-dry to obtain the O-bound nitrito isomer.

C) Isomerize to Nitro Isomer, [Co(NH3)5(NO2)]Cl2 (Compound C)

Choose any one method:

1. Method 1 — Photochemical: Spread nitrito product thinly on a watch glass and expose to sunlight for several hours to a day. Colour turns yellow. Collect and dry.
2. Method 2 — Dissolve/Re-precipitate: Dissolve ~1.0 g nitrito in 10 mL hot water containing a few drops NH₃. Cool; while cooling, add ~10 mL 6 M HCl to precipitate the yellow nitro isomer. Filter, wash, dry.
3. Method 3 — Mild Thermal: Suspend nitrito in dilute HCl; warm on a water bath until yellow. Cool, filter, wash, dry.

Reaction Mechanism

1. Oxidative ammine complexation: In concentrated NH₃, Co(II) forms ammine species. Ice-cold 30% H₂O₂ oxidizes Co(II) → Co(III), giving [Co(NH3)5Cl]2+ with chloride as a ligand.
2. Associative substitution by NO₂^–: Under cold, near-neutral conditions, nitrite attacks via oxygen (kinetic control), displacing coordinated Cl⁻ to give the red nitrito complex.
3. Linkage isomerization: Light/heat (often with acid) promote rearrangement to the N-bound nitro complex, which has a stronger Co–N bond (thermodynamic product).

Observations & Colour Changes

• During oxidation: effervescence; mixture darkens and thickens.
• After 3 M HCl digestion: formation of a yellow–orange precipitate of [Co(NH3)5Cl]Cl2.
• Upon nitrite substitution (cold): red crystals — the nitrito isomer.
• Photolysis/heating: colour turns yellow — the nitro isomer.

IR/UV Characterization

Diagnostic IR bands (KBr or ATR)

Complex	Key IR bands (cm−1)	Assignment
[Co(NH3)5Cl]Cl2	~3300–3200 (br), 1600–1620	N–H stretch; δ(NH3)
Nitrito [Co(NH3)5(ONO)]Cl2	~1460, ~1060	νas(N–O), νs(N–O) for O-bound –ONO
Nitro [Co(NH3)5(NO2)]Cl2	~1310–1380, ~820–900	νas(N=O), νs(N=O) for N-bound –NO2

Notebook values: nitrito at ~1460 & ~1060 cm⁻¹; nitro near ~1330–1380 & ~825 cm⁻¹.

UV–Vis (aqueous)

• Colours dominated by ligand-field/charge-transfer bands.
• Recorded λ_max examples: nitrito (Compound B) ~408 nm; nitro (Compound C) ~457 nm. Chloro complex appears yellow-orange (≈400–500 nm region).

How to Prepare 3 M & 6 M HCl (Lab-Scale)

Safety first: Work in a fume hood; goggles + nitrile gloves. Always add acid to water. Keep the flask in an ice bath. Check the bottle assay of conc. HCl (typically 37% w/w, density ~1.19 g·mL⁻¹ → ≈ 12 M).

Formula

C₁V₁ = C₂V₂   →   V₁ (conc. HCl needed) = (C₂ × V₂) / C₁

Use C₁ = 12 M unless specified otherwise on your bottle.

To make 1.000 L

• 3 M HCl: V₁ = (3 × 1000) / 12 = 250 mL conc. HCl. Put ~700 mL water in a 1 L volumetric, add 250 mL acid slowly, cool, then make up to 1 L and mix.
• 6 M HCl: V₁ = (6 × 1000) / 12 = 500 mL conc. HCl. Put ~400 mL water in flask, add 500 mL acid slowly, cool, make up to 1 L.

Convenient 250 mL batches

• 3 M HCl (250 mL): V₁ = (3 × 250) / 12 = 62.5 mL conc. HCl. Procedure: 150 mL water → add 62.5 mL acid in ice → top to 250 mL.
• 6 M HCl (250 mL): V₁ = (6 × 250) / 12 = 125 mL conc. HCl. Procedure: 100 mL water → add 125 mL acid in ice → top to 250 mL.

Label bottles with molarity, date, and initials.

Percentage Yield (Worked Example)

Molar masses (approx.)

• M(CoCl₂·6H₂O) = 237.93 g·mol⁻¹
• M([Co(NH₃)₅Cl]Cl₂) ≈ 250.3 g·mol⁻¹

Example calculation

• CoCl₂·6H₂O taken = 6.00 g ⇒ n = 6.00 / 237.93 ≈ 0.0252 mol.
• Theoretical mass of chloro complex = 0.0252 × 250.3 ≈ 6.31 g.
• If isolated mass = 5.20 g, then % yield = (5.20 / 6.31) × 100 ≈ 82.4%.

Report separate conversion yields for nitrito and nitro based on actual masses.

Viva Questions (with crisp answers)

1. What is linkage isomerism? Isomerism from ambidentate ligands binding through different atoms (–ONO vs –NO₂).
2. Why is Co(III) substitution-inert? Low-spin d⁶, large Δ_o, strong metal–ligand bonds → slow exchange.
3. Role of H₂O₂? Oxidizes Co(II) → Co(III) during ammine complexation.
4. Why keep the nitrite step cold? To obtain the kinetic O-bound nitrito product and avoid side reactions.
5. Colour differences? Chloro: yellow-orange; nitrito: red; nitro: yellow.
6. IR markers? Nitrito: ~1460 & 1060 cm⁻¹; Nitro: ~1310–1380 & 820–900 cm⁻¹.
7. Coordination number & geometry? 6; octahedral.
8. Why add NH₄Cl? Buffers, provides Cl⁻/ionic strength, aids crystallization.
9. How to drive nitrito → nitro? Photolysis (sunlight) or mild heating in dilute acid.

FAQ

My oxidation turned tarry/black. What went wrong?

H₂O₂ was added too fast or warm. Keep the flask in an ice bath and add oxidant dropwise.

Nitrito yield is low.

The substitution was too warm or too acidic. Keep ≤10 °C and near-neutral; acidify only at the end for crystallization.

Which sample should I run for IR?

Run dry powders (KBr pellet or ATR). Compare nitrito’s 1460/1060 cm⁻¹ with nitro’s ~1330/830 cm⁻¹.

Do I need to recrystallize?

Optional, but a quick dissolve/precipitate often sharpens IR bands and improves colour purity.

Conclusion & Call to Action

We prepared [Co(NH3)5Cl]Cl2, isolated the nitrito isomer, and converted it to the nitro isomer. The colour transitions and diagnostic IR bands provide a vivid demonstration of ambidentate ligand chemistry and kinetic vs thermodynamic control.

Try this protocol and share your spectra or questions with me. For more guides, follow Rizwan Chemistry Classes everywhere:

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About the Author

Rizwan Ibn Ali Abdullah — Student of Islam and Science | Researcher | Founder of Rizwan Chemistry Classes (@Rizwanchemistryclasses).

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