Preparation of some Werner Complexes

Experimental

Cobalt sulfate

cis-tetraamminecarbonatocobalt(III) ion

7g of ammonium carbonate is dissolved in 20 mL of water and 20 mL of concentrated ammonia solution is added. While stirring, this is then added to a solution containing 5g cobalt sulfate in 10 mL of water. 3 mL of 30% hydrogen peroxide is then slowly added. (Handle H₂O₂ carefully. If spilled on the skin, wash the affected area immediately with water; hydrogen peroxide can cause severe skin burns).Next, pour the solution into an evaporating dish in a fumehood and concentrate it over a gas burner to 30-35 mL. Do not allow the solution to boil. During the evaporation add, in small portions, 2g of ammonium carbonate. Suction filter (with water aspirator) the hot solution and then cool the filtrate in an ice bath. Under suction, filter off the red product crystals of [CoCO₃(NH₃)₄]₂SO₄.3H₂O. Wash the product in the filtration apparatus using a saturated solution prepared from a small portion of the precipitate.
Record the yield and show the product to your demonstrator.

cis-tetraamminediaquacobalt(III) ion

2g of the previously prepared tetraamminecarbonatocobalt(III) complex is dissolved in 55mL of 0.3M sulfuric acid. Evolution of carbon dioxide should occur. The clear solution is then treated with 20-25 mL of ethanol, added in small portions. The precipitate is filtered off, washed with 50% ethanol/water until free of acid. (Check with litmus paper.) Finally, the product is dried in air. Record the yield and show the product to your demonstrator.

"hexol"

hexol.

2g of the previously prepared tetraamminediaquacobalt(III) complex is dissolved in 25-30mL of 0.2M ammonia solution. After 24 hour (can be left until the next session) purple-black crystals precipitate. These are suction filtered. Record the yield and show the product to your demonstrator.

Cobalt chloride

hexaamminecobalt chloride

3g of cobalt(II) chloride hexahydrate is dissolved in a boiling solution of 2g of ammonium chloride in 5mL of water. This is then added to 0.15g of decolourising charcoal in a 100 mL flask and cooled by running water on the outside of the flask. The original vessel is rinsed with 8 mL of concentrated ammonia and these washings are added to the flask with the charcoal. The flask is then cooled to 10°C or below.
8 mL of "20 volume" hydrogen peroxide is then slowly added in portions and the falsk is briskly shaken. When all the oxidising agent has been added, the mixture is heated gradually to 60°C and maintained at this temperature until the pinkish tint in the liquid disappears (about 30 minutes). During this time the flask should be shaken frequently. At the end of the heating period, crystals should start to separate.
The flask is then cooled in an ice-bath for about 10 minutes and the crude solid filtered off via a Buchner funnel. Portions of the filtrate are used to rinse and solid remaining in the flask on to the filter. The product is allowed to drain for 5 minutes and then is recrystallised as follows:
Transfer the product to a beaker containing a boiling mixture of 25 mL of water and 1 mL of concentrated hydrochloric acid. When all the solid (except the charcoal) has dissolved, filter the hot liquid, via a Buchner funnel. 3 mL of concentrated hydrochloric acid is then added to the filtrate and the solution cooled in an ice-bath.
The golden brown crystal of the hexaamminecobalt(III) ion should be obtained in high yield. Filter off the crystals on a sintered glass filter funnel and wash them with ethanol (NO FLAMES). Transfer the crystals to a small weighed beaker and dry them in an oven at 110-120 for 30 minutes. (NOTE that higher temperatures will decompose the product). Allow the crystals to cool in a dessicator and then record the yield and show the product to your demonstrator.

pentaamminechlorocobalt(III) chloride

In a fume hood, dissolve 1 g of ammonium chloride in 9 ml concentrated aqueous ammonia in a 100-ml Erlenmeyer flask. (The combination of NH₄Cl and NH₃ (aq.) guarantees a large excess of the NH₃ ligand.) Stir the ammonium chloride solution vigorously while adding 2 g of finely divided CoCl₂.6H₂O in small portions. A yellow-pink precipitate of the hexaammine Co(II) salt forms on slight warming as the reddish starting material dissolves. Any air oxidation that occurs during this exothermic stage is ignored since the solution is going to be fully oxidised by adding hydrogen peroxide.

Caution: 30% hydrogen peroxide is a strong oxidizing agent that will cause severe burns and bleaching of skin and clothing.

Slowly add 2 mL 30% hydrogen peroxide to the brown Co slurry, using a burette. An addition rate of about 2 drops per second is usually sufficient, but care should be taken to avoid excessive effervescence. (If the reaction shows signs of excessive effervescence, stopping the stirring momentarily will usually prevent overflow of the solution.)

You should notice that all the Co(II) ammine dissolves to form a deep red solution. (This corresponds to the formation of the pentaammineaquacobalt(III) salt.) When the effervescence has virtually ceased, cautiously add 6 mL of concentrated HCl in small portions and with continuous stirring. This operation needs to be carried out in a fume hood since fumes of ammonium chloride will be produced during the neutralisation. After this point the reaction may be removed from the hood. A purple product should then precipitate from the hot reaction mixture leaving a pale green-blue supernatant liquid.

While occasionally stirring, use a steam bath or hot plate to heat the solution to 60°C. Hold the temperature between 55°C and 65°C for 15 min; this incubation period is necessary to allow complete displacement of all aqua ligands.

Collect the purple product by filtration through a No. 3 sinter glass crucible. The mother liquor may be discarded.

When the product has been drained well it is washed with 4 mL ice-cold deionised water in small portions, followed by 5 mL ice-cold 95% ethanol. (The solutions must be cold to prevent undue loss of product by redissolving.) Transfer the product to a crucible and dry in an oven at 100°C for one hour. This helps complete the conversion of any remaining pentaammineaquacobalt(III) salt. Submit your sample to the demonstrator.

pentaamminenitritocobalt(III) ion

Dissolve 1g of pentaamminechlorocobalt(III) chloride, by gently warming and stirring, in a mixture of 20 mL water and 1.5 mL of concentrated ammonia solution (CARE! EYES!). Filter off any slight precipitate of cobalt oxide that may form, and then cool the filtrate in ice to about 10°C. When cool, add 2M HCl dropwise until the solution is just neutral to litmus.
Next, dissolve 1g of sodium nitrite in the solution before adding 0.5 mL 11 M HCl. Allow the solution to stand in an ice-bath for 1 hour and then filter off the salmon-pink crystals. Wash with 5 mL of ice-water, followed by 5 mL of alcohol, and air dry on filter paper at room temperature. Record the yield and show the product to your demonstrator.

pentaamminenitrocobalt(III) ion

Dissolve 0.5g of the above nitrito-complex in 4 mL of hot water containing a few drops of concentrated ammonia solution and then add, while cooling, 4 mL of concentrated HCl. Cool the solution in an ice-bath and filter off the yellow-brown crystals. Wash the product with 2 mL of alcohol and air dry on filter paper at room temperature. Record the yield and show the product to your demonstrator.

linkage isomerism

These latter two complexes are examples of what is described as linkage isomerism. The initial reaction requires the formation of the aqua complex as intermediate and then the replacement of the OH₂ by the O-bonded nitrito ligand. This replacement is unexpectedly labile considering the usual inert reactions of Co(III). A detailed isotopic study revealled that the Co-O bond remained intact with the bonds on the O being substituted rather than at the Co.
The second reaction where the O-bonded group is replaced by the N-bonded nitro group does proceed relatively slowly as expected for a Co-O to Co-N substitution.
The IR of these linkage isomers is available. The O-bonded isomer will slowly convert to the more stable N-bonded isomer with time. It is possible to study this reaction via IR by preparing a KBr pellet and recording the IR spectrum over a period of a few days.

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Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.

Created Sept 1995. Links checked and/or last modified 10th September 2006.
URL http://wwwchem.uwimona.edu.jm/lab_manuals/Werner2.html