Preparation of Tetraamminecarbonatocobalt(III) Nitrate and Its Conversion to Pentaamminechlorocobalt(III) Chloride – Full Practical Guide

Preparation of Tetraamminecarbonatocobalt(III) Nitrate and Its Conversion to Pentaamminechlorocobalt(III) Chloride

Aim

To prepare [Co(NH₃)₄CO₃]NO₃ (Tetraamminecarbonatocobalt(III) nitrate) from cobalt(II) nitrate and then convert it to [Co(NH₃)₅Cl]Cl₂ (Pentaamminechlorocobalt(III) chloride), studying each step’s color change, reaction mechanism, and percentage yield.

Theory

Cobalt forms stable ammine complexes, especially in the +3 oxidation state. In aqueous solution, Co²⁺ ions (pink) form octahedral complexes with ammonia ligands. When oxidized to Co³⁺, the complexes become kinetically inert — meaning ligand substitution occurs slowly.

1. In Part A, Co²⁺ is oxidized to Co³⁺ in an ammoniacal carbonate medium by hydrogen peroxide, forming the complex [Co(NH₃)₄CO₃]NO₃. The carbonate ion acts as a bidentate ligand, binding through both oxygen atoms.
2. In Part B, the carbonate ligand is replaced by chloride ions from concentrated hydrochloric acid, forming [Co(NH₃)₅Cl]Cl₂ through an intermediate [Co(NH₃)₅(H₂O)]³⁺ complex.

Because Co(III) is a low-spin d⁶ system, the trans effect is negligible. Ligand exchange proceeds by an associative substitution mechanism rather than through strong trans-directing influence.

Chemical Reactions Involved

Part A: Formation and Oxidation

1. Co(NO₃)₂·6H₂O + 6NH₃ → [Co(NH₃)₆]²⁺ + 2NO₃⁻ + 6H₂O
2. [Co(NH₃)₆]²⁺ + CO₃²⁻ → [Co(NH₃)₄CO₃] + 2NH₃
3. 2[Co(NH₃)₄CO₃]²⁺ + H₂O₂ → 2[Co(NH₃)₄CO₃]⁺ + 2OH⁻
4. [Co(NH₃)₄CO₃]⁺ + NO₃⁻ → [Co(NH₃)₄CO₃]NO₃ (orange-brown solid)
  

Part B: Conversion to [Co(NH₃)₅Cl]Cl₂

1. [Co(NH₃)₄CO₃]NO₃ + 2HCl → [Co(NH₃)₄(H₂O)₂]³⁺ + CO₂↑
2. [Co(NH₃)₄(H₂O)₂]³⁺ + NH₃ → [Co(NH₃)₅(H₂O)]³⁺ + H₂O
3. [Co(NH₃)₅(H₂O)]³⁺ + Cl⁻ → [Co(NH₃)₅Cl]²⁺ + H₂O
4. [Co(NH₃)₅Cl]²⁺ + 2Cl⁻ → [Co(NH₃)₅Cl]Cl₂ (brown crystals)
  

Requirements

Chemicals

• Co(NO₃)₂·6H₂O – 7.5 g
• (NH₄)₂CO₃ – 10 g (plus 2.5 g extra during concentration)
• Conc. NH₃ (30%) – 30 mL
• 30% H₂O₂ – 4 mL
• Conc. HCl – 50–75 mL
• Distilled water and ethanol (for washing)

Apparatus

• Beakers (100 & 250 mL), glass rod, thermometer, measuring cylinders
• Hotplate or water bath, ice bath, filter paper and funnel
• China dish, watch glass, drying oven (≤60°C)

Part A: Preparation of [Co(NH₃)₄CO₃]NO₃

Step-by-Step Procedure

1. Dissolve 10 g (NH₄)₂CO₃ in 30 mL of water. Add 30 mL conc. NH₃ (30%) → label as Solution A.
2. Dissolve 7.5 g Co(NO₃)₂·6H₂O in 15 mL water → label as Solution B.
3. Slowly add A to B while stirring to prevent CoCO₃ precipitation.
4. Add 4 mL 30% H₂O₂ dropwise in an ice bath. Color changes from pink → orange (oxidation).
5. Warm gently to 60–70°C to complete oxidation.
6. Evaporate to 50 mL (avoid boiling). Add 2.5 g (NH₄)₂CO₃ alternately to maintain excess ligand.
7. Cool, allow crystallization, then filter and wash crystals with ice water and ethanol.
8. Dry the product below 60°C and calculate yield.

Part B: Conversion to [Co(NH₃)₅Cl]Cl₂

Step-by-Step Procedure

1. Dissolve 2.5 g [Co(NH₃)₄CO₃]NO₃ in 25 mL hot water (~60°C).
2. Add 2.5–5.0 mL conc. HCl slowly — CO₂ gas evolves.
3. Neutralize with 2–3 mL conc. NH₃ (30%) to keep slightly basic medium.
4. Add extra 20–30 mL HCl, heat for 20–30 min at 60°C (avoid boiling).
5. Cool slightly, then add 40 mL conc. HCl and reheat for 30 min.
6. Cool, allow brown crystals of [Co(NH₃)₅Cl]Cl₂ to form.
7. Filter, wash with ice water and ethanol, and dry below 60°C.

Reaction Mechanism (Simplified)

1. Coordination and oxidation: Co²⁺ + NH₃ → [Co(NH₃)₆]²⁺ → oxidation by H₂O₂ → [Co(NH₃)₆]³⁺ → partial replacement by CO₃²⁻ → [Co(NH₃)₄CO₃]⁺

2. Substitution stage: CO₃²⁻ is protonated and released as CO₂, replaced sequentially by H₂O then Cl⁻ → [Co(NH₃)₅Cl]²⁺.

3. Mechanism type: Associative, since NH₃ has weak trans-directing ability.

Observations

Stage	Observation	Interpretation
Addition of NH₃	Pink → deep pink	Formation of Co(II)-ammine complex
Addition of H₂O₂	Pink → brown/orange	Oxidation to Co(III) complex
Addition of HCl	Effervescence	CO₂ released; carbonate removed
Heating with HCl	Brown solution	Formation of [Co(NH₃)₅Cl]²⁺
Cooling	Brown crystals	Crystallization of final product

Percentage Yield Calculation

Molar mass of [Co(NH₃)₅Cl]Cl₂ = 267.4 g/mol
If 2.5 g [Co(NH₃)₄CO₃]NO₃ (M = 245 g/mol) used:
Moles = 2.5 / 245 = 0.0102 mol
Theoretical yield = 0.0102 × 267.4 = 2.73 g
Percentage yield = (Experimental yield / 2.73) × 100
  

Troubleshooting

• Precipitate forms early → add NH₃ first; maintain basic pH.
• No oxidation color change → cool mixture and add H₂O₂ slowly.
• Low yield → avoid overheating; maintain carbonate excess.
• Pale product → incomplete substitution; extend heating.

Viva Questions and Answers

1. Why is ammonia used? It acts as both ligand and base, stabilizing Co(III).
2. How is oxidation confirmed? Color changes from pink to orange/brown.
3. Why add H₂O₂ in an ice bath? The oxidation is exothermic; cooling prevents decomposition.
4. What is the role of conc. HCl? It replaces carbonate with chloride ligands.
5. What mechanism is followed? An associative ligand substitution.
6. Why are Co(III) complexes inert? Due to low-spin d⁶ configuration and strong CFSE.
7. Which gas evolves in Part B? Carbon dioxide (CO₂).
8. Why avoid boiling? To prevent complex decomposition and ammonia loss.

Conclusion

The experiment demonstrates oxidation and ligand substitution in Co(III) complexes. Tetraamminecarbonatocobalt(III) nitrate is prepared first, then converted to Pentaamminechlorocobalt(III) chloride. This highlights key coordination chemistry principles: ligand strength, oxidation states, substitution mechanisms, and complex stability.

Author: Rizwan Ibn Ali Abdullah
Student of Islam and Science | Researcher | Rizwan Chemistry Classes