Linkage Isomerism Practical: Preparing Nitro and Nitrito Cobalt(III) Ammine Complexes

Linkage Isomerism Practical: Preparing Nitro vs. Nitrito Cobalt(III) Ammine Complexes (Complete A–Z Guide)

A clean, stepwise laboratory write-up for coordination chemistry: objective, theory, reagents, full procedure, calculations, spectroscopy, safety, troubleshooting, viva, and FAQs.

Labels: linkage isomerism cobalt(III) ammines nitro vs nitrito coordination chemistry IR UV-Vis M.Sc. inorganic practical

Short, clear, and lab-ready. This practical shows how to prepare a cobalt(III) ammine precursor and convert it into two linkage isomers using the ambidentate nitrite ligand: O-bound nitrito and N-bound nitro. You’ll control conditions to favor kinetic vs. thermodynamic products, verify each isomer with simple spectral cues, and calculate yields with confidence.

Key Idea — One ligand, two binding modes: The nitrite ion (NO₂⁻) can coordinate via oxygen (–ONO, nitrito, typically red-orange; kinetic) or via nitrogen (–NO₂, nitro, yellow-orange; thermodynamic). pH and temperature steer which one dominates.

Table of Contents

1. Aim & Learning Outcomes
2. Theory & Background
3. Chemicals, Apparatus & Hazards
4. Key Equations & Mechanistic Notes
5. Procedure (A–Z)
6. Observations & Expected Results
7. Characterization Cheatsheet (IR / UV-Vis)
8. Sample Calculations & Yield
9. Troubleshooting & Good Practices
10. Waste, Cleanup & Storage
11. Viva-Voce Questions
12. FAQ
13. Conclusion & Next Steps

Aim & Learning Outcomes

Aim: To synthesize chloropentaamminecobalt(III) chloride, [Co(NH₃)₅Cl]Cl₂, and convert it into nitrito and nitro pentaamminecobalt(III) chlorides, demonstrating linkage isomerism of NO₂⁻.

Outcomes: After completing this practical, you should be able to:

• Explain ambidentate coordination and linkage isomerism.
• Control conditions to obtain kinetic (O-bound) vs. thermodynamic (N-bound) products.
• Record observations, calculate percentage yields, and confirm isomers using IR/UV-Vis cues.

Theory & Background

NO₂⁻ has resonance forms enabling coordination either through O (nitrito, –ONO) or N (nitro, –NO₂). Low-spin d⁶ cobalt(III) in octahedral fields is kinetically inert, allowing isolation of both forms. Mild, neutral conditions favor the nitrito isomer (faster O-coordination), while acidic or warmer conditions promote intramolecular rearrangement to the more stable nitro isomer.

Chemicals, Apparatus & Hazards

Chemicals (typical scale)

• Cobalt(II) chloride hexahydrate, CoCl₂·6H₂O (2–3 g)
• Aqueous ammonia, conc. (~25–30 mL total)
• Hydrochloric acid, conc. (10–15 mL) and dilute (1–2 M)
• Sodium nitrite, NaNO₂ (~0.4 g per 0.5–1.0 g precursor)
• Activated charcoal (~0.5 g), ice-cold ethanol (washing), distilled water

Glassware: beakers, stirring rod/magnetic stirrer, thermometer, Buchner funnel + vacuum, ice bath, filter papers.

Hazards & PPE

• Ammonia: Pungent, irritant. Use in fume hood.
• Acids (HCl): Corrosive. Add acid to water, not vice versa.
• Cobalt salts: Harmful if swallowed; skin/eye irritants.
• Sodium nitrite: Oxidizer; avoid contact with acids outside prescribed steps.

Wear lab coat, gloves, goggles; ensure ventilation. Label waste appropriately.

Key Equations & Mechanistic Notes

1. Formation of ammine Co(III): Co²⁺ + NH₃ + O₂ → Co³⁺ ammine complexes (aerial oxidation; gentle heat accelerates).
2. Precursor isolation: [Co(NH₃)₆]³⁺ + Cl⁻ ⇌ [Co(NH₃)₅Cl]²⁺ + NH₃; isolate as [Co(NH₃)₅Cl]Cl₂ (pink).
3. Nitrite substitution (kinetic control): [Co(NH₃)₅Cl]²⁺ + NO₂⁻ → [Co(NH₃)₅(ONO)]²⁺ + Cl⁻.
4. Linkage isomerization (thermodynamic control): [Co(NH₃)₅(ONO)]²⁺ ⇌ [Co(NH₃)₅(NO₂)]²⁺ (favored by heat/acid).

Procedure (A–Z)

A. Synthesis of Chloropentaamminecobalt(III) Chloride, [Co(NH₃)₅Cl]Cl₂ (Pink)

1. Dissolution & Ammine Formation: Dissolve 2–3 g CoCl₂·6H₂O in ~10 mL distilled water. Add conc. NH₃ slowly with stirring until persistent odor of NH₃ remains (~25–30 mL total).
2. Aerial Oxidation: Bubble air (pipette/air line) 30–60 min at room temperature or warm to ~50 °C. The solution deepens in color as Co(III) forms.
3. Chloride Introduction & Isolation: Add conc. HCl (~10–15 mL) dropwise with stirring. A pink solid of [Co(NH₃)₅Cl]Cl₂ precipitates.
4. Clarification (optional): Add a pinch of activated charcoal, warm briefly, and hot-filter.
5. Cooling & Washing: Cool the filtrate/mixture in an ice bath 10–15 min, collect the pink product by vacuum filtration, wash with a little cold water then ice-cold ethanol, and air-dry.

B. Nitrito-Pentaamminecobalt(III) Chloride, [Co(NH₃)₅(ONO)]Cl₂ (Red-Orange)

1. Dissolve 0.5–1.0 g of the pink precursor in ~10 mL warm water (≈40 °C).
2. Dissolve ~0.4 g NaNO₂ in 5 mL water and add to the precursor solution at neutral pH.
3. Stir at room temperature or warm gently to 40–50 °C for 10–15 min. Allow to develop a red-orange color.
4. Cool in an ice bath, filter, wash with ice-cold water then ethanol, and dry.

C. Nitro-Pentaamminecobalt(III) Chloride, [Co(NH₃)₅(NO₂)]Cl₂ (Yellow-Orange)

Either convert the nitrito product or start from the precursor under the following conditions:

• Acid route: Adjust the reaction mixture to pH ≈ 2 with 1–2 M HCl and warm 10–20 min.
• Heat route: Warm at 70–80 °C for ~20–25 min to drive linkage isomerization.

Cool, filter, wash as above, and dry to obtain a yellow-orange solid.

Observations & Expected Results

Species	Appearance	Typical Yield	Notes
[Co(NH3)5Cl]Cl2	Pink crystalline solid	40–70%	Forms after oxidation + acidification.
[Co(NH3)5(ONO)]Cl2	Light red-orange	60–70%	Kinetic product (O-bound); avoid long heating.
[Co(NH3)5(NO2)]Cl2	Yellow-orange	50–80%	Thermodynamic product (N-bound); more stable.

Both isomers typically decompose on heating near ~250 °C rather than showing a sharp melt.

Characterization Cheatsheet (IR / UV-Vis)

Infrared (ATR/KBr)

• Nitro (N-bound): strong asymmetric NO₂ stretch at ~1380–1400 cm⁻¹.
• Nitrito (O-bound): characteristic M–O–N features around ~1050–1100 cm⁻¹; compare side-by-side.

UV-Vis (in water)

• Nitro: λ_max ≈ 460 nm (appears more yellow-orange).
• Nitrito: λ_max ≈ 480 nm (slightly red-shifted, more orange-red).

Sample Calculations & Yield

Example (illustrative numbers)

• M(CoCl₂·6H₂O) ≈ 237.93 g·mol⁻¹
• M([Co(NH₃)₅Cl]Cl₂) ≈ 250–260 g·mol⁻¹ (use your exact value)

1. Moles of Co(II) input: n = mass / M. For 2.50 g CoCl₂·6H₂O → n ≈ 0.0105 mol.
2. Theoretical moles of product: Assume 1:1 → 0.0105 mol precursor.
3. Theoretical mass: m_theor = n × M(product).
4. % Yield: (m_obs / m_theor) × 100.

Report yields for each stage separately (precursor, nitrito, nitro). Attach working in your lab book.

Troubleshooting & Good Practices

• Slow oxidation: Increase aeration surface, extend time, or warm gently (~50 °C). Avoid strong oxidants unless instructed.
• Muddy/indistinct colors: Keep ionic strength moderate; avoid overheating during nitrito formation; maintain neutral pH.
• Product too fine to filter: Cool longer; add a little ethanol to induce crystallization; use a fine-porosity frit.
• Mixed IR signatures: Re-prepare under strictly neutral (nitrito) or mildly acidic/warm (nitro) conditions; recrystallize quickly and wash cold.

Waste, Cleanup & Storage

• Collect cobalt-containing liquids in a labeled heavy-metal waste container.
• Neutralize acidic/alkaline rinses as per institutional SOPs before disposal.
• Store dry products in labeled vials, away from heat and moisture.

Viva-Voce Questions

1. Define linkage isomerism with an example other than nitrite.
2. Why is Co(III) termed “kinetically inert” and how does that help this experiment?
3. How do pH and temperature influence nitro vs. nitrito formation?
4. Which IR bands distinguish the two isomers and why?
5. Explain the slight bathochromic shift observed between the two UV-Vis spectra.

FAQ

Do I need strictly anhydrous conditions?

No. Aqueous conditions are standard here; just control pH and temperature carefully.

Can the nitrito isomer slowly convert to nitro on storage?

Yes, especially if warm or slightly acidic. Store cool and dry; handle promptly.

Is melting point useful?

Both often decompose rather than melt cleanly. Use IR (primary) and UV-Vis (supporting) for identification.

What typical yields should I expect?

Precursor 40–70%; nitrito 60–70%; nitro 50–80%, highly dependent on technique.

Conclusion & Next Steps

With one ligand and two binding sites, this practical turns abstract coordination theory into something you can see. Set neutral, mild conditions to capture the nitrito isomer; add heat or acid to tip the equilibrium toward nitro. Confirm with IR bands and support with UV-Vis. As an extension, compare reactivity with other ambidentate ligands (e.g., SCN⁻), or explore kinetics of the isomerization step.

Call to action: If you want printable handouts, spectra worksheets, or a quick-grade observation sheet, let me know and I’ll add downloadable resources.

About the Author

Rizwan Ibn Ali Abdullah — Student of Islam and Science | Researcher.

Previous posts: Synthesis of Pentaamminechloro-cobalt(III) Chloride: A Complete Guide How to Read IR Spectra: Fast Field Guide for Students

Labels: linkage isomerism, cobalt(III), ammine complexes, nitro, nitrito, IR spectroscopy, UV-Vis, inorganic chemistry lab, coordination compounds

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