Complexes Of Cobalt In Different Metal Oxidation State Biology Essay

Cobalt forms a complex since it has an empty valence shell orbital thus it is an electron pair acceptor. It can donate electrons from the ligands thus forming a coordination compound.

Cobalt usually exists in two oxidation stable states usually the +2 and +3 and can also be in the form of a tetrahedral arragement and octahedral in the case of the Cobalt(II) oxidation state . Co(III) oxidation state can only form an octahedral arragement. Co(II) is one of the transition metals which can form tetrahedral complexes more easily . The energy levels of octahedral and tetrahedral of Co(II) have the least difference in energy.The magnetic moments of tetrahedral ranges from 3.89 to 4.7BM and that of octahedral complexes ranges from 4.7 to 5.2BM

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Co(III) oxidation state is not very favourable, this is because when it reacts with water it

would quickly go back to the +2 oxidation state. When reacting with ammonia, cobalt(III) is more stable. The Co(III) can show also isomerism. Co(III) is expected to be paramagnetic but the cross over from high spin to low spin takes place at very long ligand field strengths and therefore it is more likely to be diamagnetic. Cobalt(III) complexes are described as kinetically inert and undergo ligand exchange very slowly. On the other hand cobalt(II) undergoes ligand exchange faster since it is liable.1

In this experiment ligand exchange is going to take place such that the ammonia molecule which is a stronger ligand replaces the water molecules in the cobalt(II) chloride hexahydrate. In the second experiment the coordination around cobalt is changing from an octahedral to a tetrahedral complex.

The different colours of these transition metals are caused by the excitation of the d electron to another d subshell. The 3d orbitals split in eg and t2g. This splitting enables the electron to excite from ground state to excited state. The size of energy gap of the excitation corresponds to the wavelength of the absorbance in the visible region of the spectra.

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Apparatus: Pasteur pipette, weighing boat, spatula, watch glass, measuring cyclinder, stirring rod, buchner funnel, stopper, heating mantle, weighing balance, beakers, thermometer, filter paper, ice-salt bath, flasks

Chemicals

Chemical

Grade

Brand

Cobalt(II) chloride hexahydrate

GPR

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N/A

Ammonium chloride

GPR

BDH

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Hydrogen peroxide

GPR

BDH

Ammonia

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GPR

Timstor

Hydrochloric acid

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BDH

Charcoal

GPR

N/A

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Acetone

GPR

BDH

Pyridine

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GPR

N/A

Ethanol

GPR

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Method:

Experiment A: Preparation of Hexamminecobalt(III) Chloride

6g of ammonium chlorideand 9g of cobalt(II) chloride hexahydrate were dissolved in 13cm3 boiling water and 0.5g of decolourising charcoal was added carefully.

The mixture was cooled to 0OC in an ice-bath and 20cm3 of ammonia was added, while keeping the temperature below 10OC.

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18cm3 of hydrogen peroxide was added while the solution was being stirred rapidly and the temperature was kept below 20OC.

When all the hydrogen peroxide was added, the mixture was heated to 60 OC until the pink colour disappeared.

The mixture was cooled in ice and the precipitate was collected by filtration on a Buchner funnel. The precipitate was dissolved in a boiling mixture of 80cm3 of water and 3cm3 of concentrated hydrochloric acid.

The charcoal was removed by filtration while it was still hot.

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10cm3 of concentrated hydrochloric acid was added to the filtrate and the mixture was cooled in ice when crystals of hexamminecobalt(II) chloride were deposited.

The crystalline product was collected on a Buchner funnel, then it was washed with acetone and dried in a vacuum dessicator.

The yield was measured and the product was kept for inspection.

Experiment B: Preparation of dichlorodipyridinocobalt

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1.2g of cobalt(II) chloride was dissolved in 6cm3 of hot absolute ethanol.

1cm3 of hot solution of pyridine in 3cm3 of absolute alcohol was added slowly to the solution.This was carried in a fume cupboard.

The solution was allowed to stand at room temperature for 15minutes and the product was collected by filtration on a Buchner funnel. The crystals were washed quickly with ice-cold absolute ethanol and the product was dried in a vacuum dessicator.

The yield was recorded and the product was kept for inspection.

Results:

Experiment A: Mass of ammonium chloride=5.850g

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Mass of cobalt chloride=8.960g

Mass of crystals of hexamminecobalt chloride=4.787g

Experiment B: Mass of cobalt(II) chloride=1.204g

Mass of crystals of dichlorodipyridinocobalt=0.000g

Calculation:

Moles of NH4Cl= 5.985 =0.112moles

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53.49 (RMM)

Moles of cobalt chloride= 8.960g= 0.038moles

237.9(RMM)

Moles of Hydrogen peroxide=

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6% of Hydrogen peroxide=20volumes

6 x 18= 1.08g

100

1.08g= 0.032moles

34(RMM)

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1 CoCl2.6H2O:2H2O2

0.038 : 0.076

Therefore: CoCl2.6H2O is the limiting reagent

1:1

CoCl2.6H2O:[Co(H2O)6]Cl3

1 mole=267.3g

0.038 moles=? 10.16g

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% yield= actual yield x100%

Theoritical yield

4.787gx100%=47.27% (percentage yield of hexamminecobalt(III) chloride)

10.16g

Precautions:

The vacuum at the flask was disconnected before turning off the water aspirator. This prevents water from being sucked into the vacuum flask.

The suction of the vacuum filtration was checked so that filtration would be a success.

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It was made sure that the crystals would not remain on the sides of the funnel since a low result would be obtained.

Prevention of excessive cooling during filtration was by suction through a flat piece of filter paper properly fitting a Buchner funnel.

The solution was cooled to room temperature and sometimes even colder with the aid of an ice-water bath.

Filtration was done using the Buchner funnel to increase the speed of filtration

A heating mantle was used instead of a bunsen burner because ethanol is flammable.

The hydrogen peroxide,ammonia, pyridine and absolute ethanol were quite dangerous and so they were performed in the fumehood.

The crystals that remained in the beaker were not rinsed by distilled water since some of the product would dissolve.

The solvent had to be cooled before washing the crystals since crystals could dissolve.

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Sources of error:

Transfer errors when collecting the crystals formed by suction filtraton, since some of them would remain with the filter paper.

Some of the substance was left with the glass rod during stirring which would cause loss of the product.

The mixture was contaminated and so the yields were not sufficiently pure.

Side reactions could have occurred beside the actual reaction which can lead to the generation of other products.

Error in the apparatus especially the weighing balance.

Discussion:

Experiment A: Preparation of Hexamminecobalt(III) chloride

When ammonium chloride is added to the cobalt(II) hexahydrate, it has a function to stabilize the ion.2 When dissolved in water the cobalt(II) chloride salt decomposes, resulting in the formation of the Co(H2O)62+ ion. Cobalt(II) can be oxidized by air oxidation to cobalt(III).5 When adding the ammonia solution the hexaammine complex is formed:

[Co(H2O)6]2+ +NH3 → [Co(NH3)6]2+ + 6 H2O

Ammonia is added to the solution to aid in this oxidation process. When adding ammonia the reaction would be exothermic and so the mixture is placed in a salt-bath to keep the mixture cooled.

Hydrogen peroxide is used as an oxidizing agent thus oxidizing the Cobalt(II) to Cobalt(III). The decolourising charcoal is used as the catalyst of the reaction to give high yields in a relatively short time. The activated charcoal increases the speed of the reaction by helping in the formation of the bonds between NH3 and conveniently, it also catalyzes the transformation of Co2+ into Co3+ by the hydrogen peroxide. Thus it is used to increase the reaction of the ligand exchange. 2 The charcoal is made from finely divided carbon sheets which provide a large surface area. The holes on the surface of the charcoal are used to allow the reaction of the ligand exchange to take place. 2

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The process for collecting the product involves the dissolving of the precipitate in the boiling water and adding concentrated hydrochloric acid to the precipitate, the hexamminecobalt(III) chloride. When heating the mixture directly on the hot plate while stirring, helps to dissolve the crystals. The filtrate should be orange, and it contains the dissolved product.

4 CoCl2·6H2O + 4 NH4Cl + 20 NH3 + O2 → 4 [Co(NH3)6]Cl3 + 26 H2O 2

CoCl2·6H2O + ½H2O2 + NH4Cl + 5 NH3 → [Co(NH3)6]Cl3 + 7 H2O 3

Acetone was used as the solvent so that impurities are removed from the crystals. When the solution was cooled, crystals of pure product were formed and the impurities remained dissolved in the solution. If the solvent was not cooled, the crystals may dissolve and result in a decrease of the percent yield. The solvent that is chosen has to have low solubility at low temperatures and high solubility at high temperatures. 4

Hexammincobalt(III) chloride is an ionic compound having three chloride ions with a charge of -1 and the cation having a charge of 3+ . Some years ago it was questioned whether the chlorine atoms in hexaamminecobalt(III) chloride were part of the complex or ionic thus they were free. Chlorine in this complex was indeed determined to be ionic. To verify this theory one can complex the cobalt iodometrically and so titrating the liberated iodine with sodium thiosulfate solution.5 The Co3+ is an electron deficient cation and so ammonia is capable of donating an electron pair to the metal ion in a coordinate covalent bond. NH3 is a strong field ligand thus there would be more splitting, ie. it is a low-spin complex.

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The yield of Hexamminecobalt(III) chloride was 4.787g which is a rather high yield if one is assuming that all of the cobalt(II) chloride hexahydrate turned into the hexamminecobalt(III)chloride. The yield could have been better if there were no losses during the synthesis and also during the recrystallisation process.

Hexamminecobalt(III) chloride absorbs light in the violet-blue-green region but reflects the orange wavelengths thus appearing orange. The ground state 5D would split into 5T2g and a 5Eg. 5 The ligand NH3 would have a weak field since it has four unpaired electrons. From the Tanabe sugano diagram one can verify that the ligand NH3 is of intermediate field strength having a Dq/B of 1.8.5

Experiment B: Preparation of Dichlorodipyridinocobalt

In the second experiment the octahedral complex of cobalt(II) chloride hexaydrate is going to take a tetrahedral form.6 This complex is found to exist in two forms: a monomer with a formula of [CoCl2(py)2] consisting of a tetrahedral with cobalt 2+ as the central metal ion and the other one is an octahedral polymer [CoCl2(py)2]n.6

Polymerizes on standing

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CoCl2+ 2py→[CoCl2(py)2] ↔ [CoCl2(py)2]n 6

Blue violet

Diagram of structure of dichlorodipyridinocobalt 6

Pyridine has an equatorial lone pair of electrons at the nitrogen atom in the benzene ring 7 and so it is able to donate the lone pairs to the metal ion cobalt(II). Pyridine and chlorine are monodentate ligands since they donate only one lone pair to the metal. When adding the hot absolute ethanol the cobalt(II) chloride would dissolve and pyridine would exchange with water ligands since it is a stronger ligand.

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Because a tetrahedral complex has fewer ligands and there are no ligands at the axis, the magnitude of the splitting is smaller when compared to the octahedral. Octahedral complexes would have large splitting because of higher repulsion.8 The difference between the energies of the t2g and eg orbitals in a tetrahedral complex is slightly less than half as large as the splitting in octahedral complexes. 8

From the results one can observe that the yield of the dichlorodipyridinocobalt resulted in 0g. This could be the cause of heating the alcohol and pyridine at high temperature thus evaporating some of them and so the synthesis would not occur as desired. The absolute ethanol could have melted the crystals so resulting in low yield of product. One source of error is that normal balances would not detect such low yields.

Conclusion

One can conclude that the aim of this experiment: To prepare two complexes of cobalt in different metal oxidation state was reached, such that relatively good yields were obtained for the hexamminecobalt(III) complex and dichlorodipyridinocobalt crystals were also obtained by in such a low yield that could not be measured.

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