UNIT 3 Preparation of [Co(NH ) ]Cl and [Co(en) ]Cl – INORGANIC CHEMISTRY PRACTICALS

UNIT 3 Preparation of [Co(NH ) ]Cl and [Co(en) ]Cl

3 6 3 3 3

(Hexaamminecolbalt(III) Chloride and Tris(ethylenediamine)

1.0 Introduction

2.0 Objectives

1. Main Content
2. Preparation of [Co(NH ) ]Cl

3 6 3

3.2 Preparation of [Co(en) ]Cl

1. 3

3.3 Properties

4.0 Conclusion

5.0 Summary

6.0 Tutor Marked Assignments

7.0 References/Further Reading

1.0 INTRODUCTION

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Cobalt has the outer electronic configuration 3d 4s and its highest significant oxidation state is +4. This reflects the trend towards decreased stability of the very high oxidation state on moving across the transition metal series. Cobalt (I) forms some complexes, most with p-bonding ligands. The chemistry of cobalt (I) is better characterized than any other unipositive oxidation state of the first transition series except copper. The two most important oxidation states are

7 6 cobalt(II) [Ar]3d and cobalt(III) [Ar]3d , and in an aqueous solution containing no complexing agent cobalt(III) is easily reduced to cobalt(II). However, cobalt (III) is more stable in the presence of a complexing agent such as ammonia as shown by the electrode potentials.

3+ – 2+ o 3+ – 2+ o

[Co(H O) ] + e = [Co(H O) ] E = 1.85V. [Co(NH ) ] + e = [Co(NH ) ] E = 0.1V

2 6 2 6 3 6 3 6

Cobalt(II) forms both octahedral and tetrahedral complexes but they are labile and they have a strong tendency to be oxidized by molecular oxygen. The complexes are usually prepared in an inert atmosphere. In aqueous solution the cobalt (II) ion is pale pink because the absorption is weak and occurs in the blue region of the visible. Tetrahedral cobalt (II) complexes are often highly colored owing to their lower order of symmetry relative to the octahedral complexes. The

2-

spectrum of [CoCl ] shows a large absorption in the visible part of the spectrum, which

4

accounts for its deep-blue color.

Cobalt (III) salts are difficult to prepare because the ion is a strong oxidizing agent and the chemistry of this oxidation state is largely that of coordination compounds. Cobalt (III) usually forms octahedral complexes and it has a strong affinity for nitrogen donors such as ammonia, amines(e.g. ethylenediamine), nitro groups and nitrogen bonded-SCN groups as well as water molecules and halide ions

2.0 OBJECTIVE

At the end of this unit you be able

• Identify the most important oxidation states
• Describe three methods by which Co(III) salt can be

1. MAIN CONTENT
2. Preparation of [Co(NH ) ]Cl

3 6 3

Hexaamminecobalt(III) salts may be prepared by any of three methods that depend on oxidation of cobalt(II) ion in ammoniacal solution:

1. Air oxidation, with formation of the pentamine ion, which is converted to the hexamine by heating with aqueous ammonia under pressure;
2. Oxidation with an agent such as hydrogen peroxide, iodine, potassium permanganate, lead oxide, or hypochlorite solution;
3. Oxidation in the presence of a catalyst that allows equilibrium between the pentamine and hexamine ions to be established at room temperature and atmospheric pressure. These compounds may also be prepared indirectly from other hexaminecobalt(III) salts.

In the best of the catalytic methods diamminesilver ion or decolorizing charcoal is used as a catalyst. The method devised by J.Bjerrum in which decolorizing charcoal is catalyst is simple, gives high yields of pure product, and is not time-consuming. A high concentration of ammonium salt is sufficient to stabilize the hexaminecobalt(III) ion, and the carbon serves only to establish the equilibrium. Air is used as oxidant except when the cobalt (II) compound is slightly soluble in the ammoniacal solution, as in the preparation of [Co(NH ) ]Br , for which

3 6 3 hydrogen peroxide is preferable.

4CoCl + 4NH Cl + 20NH + O → 4[Co(NH ) ]Cl + 2H O

2 4 3 2 3 6 3 2

.

3.1.1 Materials required CoCl 6H O, NH OH, NH Cl, HCl, charcoal

2 2 4 4

3.1.2 Procedure 240g (1mol) of cobalt(II) chloride 6-hydrate and 160g (3mol) of ammonium chloride are added to 200ml of water. The mixture is shaken until almost of the salts are dissolved. Then 4g of activated decolorizing charcoal and 500ml of concentrated ammonia are added. Air is bubbled vigorously through the mixture until the red solution becomes yellowish brown (usually about 4hours). The air inlet tube is of fairly large bore (10mm) to prevent clogging with the precipitated hexamminecobalt(III) salt.

The crystals and carbon are filtered on a Buchner funnel and then added to a solution of 15 to 30ml. of concentrated hydrochloric acid in 1500ml of water; sufficient acid reaction. The mixture is heated on a hot plate to effect complete solution and is filtered hot. The hexamminecobalt(III) chloride is precipitated by adding 400ml of concentrated hydrochloric acid

• and slowly cooling to 0 C . The precipitate is filtered, washed first with 60% and then with 95%
• alcohol, and dried at 80 to 100 C.

3.2 Preparation of [Co(en) ]Cl

3 3

Ethylenediamine coordinates with metallic ions through both nitrogen atoms. The fivemembered chelate rings that are thus formed are very stable. Many cobalt (III) ammines are converted by aqueous ethylenediamine to ethylenediamine cobalt (III) chloride. Thus, Jorgensen prepared the salt by heating [Co(NH ) Cl]Cl with aqueous ethylenediamine. Grossman and

3 5 2

Schuck obtained the salt by oxidizing a mixture of cobalt(II) chloride, ethylenediamine, and water. The method described below has been developed from the latter suggestion.

.

3.2.1 Material required en, HCl, CoCl 6H O

1. 2

3.2.2 Procedure 61g of 30% ethylenediamine is partly neutralized with 17 ml of 6N hydrochloric

. acid and the resulting mixture poured into a solution of 24 g of CoCl 6H O in 75 ml. of water. The

1. 2 cobalt is oxidized by bubbling a vigorous stream of air through the solution for three hours. The solution allowed to evaporate on a steam bath until a crust begins to form over the surface (the volume will be about 15 to 20 ml.); then 15 ml. of concentrated hydrochloric acid and 30 ml.of ethyl alcohol are added. After cooling, the crystals of [Co(en) ]Cl are filtered and washed with alcohol
2. 3

until the washings are colorless. They are then washed with ether or dried in an oven. Properties [Co(en) ]Cl crystallizes in orange-yellow needles, which are readily soluble in water but

1. 3

insoluble in the usual organic sovents. Its solubility in 6 N hydrochloric acid is about 3 %. It is stable

o at temperatures as high as 200 and is decomposed only slowly by hydrogen sulfide and sodium hydroxide.

4.0 CONCLUSION

The preparation of hexammine cobalt(III) salts is prepared from cobalt(II) chloride hexahydrate, CoCl₂·6H₂O, which is both stable and inexpensive..

Ethylenediamine cobalt (III) chloride is prepared from an aqueous solution of ethylenediamine and virtually any cobalt(II) salt, such as cobalt(II) chloride.

5.0
SUMMARY

In this unit you have been learnt how to perform two experiments to involving the transition element cobalt

6.0 TUTOR MARKED ASSIGNMENTS (TMA)

1. [CoCl₆]^3- is orange while [Co(NH₃)₆]³⁺ is yellow. Which of the following

statements is correct?

1. [CoCl₆]^3- absorbs orange light.
2. The different colors are due to the different numbers of unpaired electrons on the complexes.
3. The two sets of d orbitals are split further apart in [Co(NH₃)₆]³⁺ than they are in CoCl₆]^3-
   
4. The different colors are due to the different charges on the complexes.

2. Consider the complex compound [Co(NH₃)₆]Cl₃ .
What is the oxidation state of cobalt in this compound?