Synthesis of different cobalt compounds through ligand exchange was performed in this laboratory. Electroconductivities and Infrared spectrophotometry of the compounds were conducted to observe the effects of the exchanges.

2Co(NO3)2 + 6NH3 + 2(NH4)2CO3 + H2O2 → 2[Co(NH3)4CO3]NO3 + 2NH4NO3 +    

        2H2O

                The other cobalt III complex of interest is chloropentaamminecobalt (III) chloride, [Co(NH3)5Cl]Cl2.  PUT IN THIS STRUCTURE ONCE CHEM DRAW WORKS

The unreactive Co III ion, [Co(NH3)4CO3]1+ from above is used in this set of reactions also.  In this particular case, the chlorine ion is a monodentate ligand because it only has one binding site.  First, ligand exchange occurs by introduction of hydrochloric acid to the ion.  

[Co(NH3)4CO3]1+ + 2HCl → [Co(NH3)4(OH2)Cl]2+ + CO2 + Cl1-

        Ammonia is a better ligand than Cl-, so when it is present, it replaces the Cl in the complex to give:

[Co(NH3)4CO3]1+ + NH3(aq) → [Co(NH3)5(OH2)]3+ + Cl1-

Again, due to the greater electronegativity of the NH3 than that of Cl-, the Cl- associated with the cobalt is replaced by the ammonia.  

Next, HCl is added back into the solution of [Co(NH3)5(OH2)]3+ to give the desired product.  

[Co(NH3)5(OH2)]3+ + 3HCl → [Co(NH3)5Cl]Cl2 + H20 + 3H+

Once both products have been obtained, analysis can be accomplished.

The identity of unknown compounds can be partially revealed through determination of that compound’s electroconductivity.  By studying the electrical conductance of a particular substance, it is possible to relate that to the number of ions in that substance.  Electroconductivies will be found and analyzed for each of the products mentioned.

Resistance is the measured quantity of an electroconductor, but the particular machine used for this lab gave conductance readings.  Specific conductance, L, can then be determined by

C =

From the specific conductance, the conductance/mol (Λ) can be determined.

 Λ =  ,

where M is the molarity of the solution.   (White, 1999)

The Λ is related to the number of ions in a sample, determined by the following chart: (Angelici, 1969)

Procedure

Carbonatotetraamminecobalt (III) nitrate   [Co(NH3)4CO3]NO3

It was first necessary to make [Co(NH3)4CO3]NO3.  Twenty grams of (NH4)2CO3 were dissolved in 60 milliliters of water, to which 60 milliliters of concentrated aqueous ammonia was added. This solution and 15 grams of [Co(OH2)6](NO3)2 were combined in 30 milliliters of water.  A purple color is educed.  Carbon dioxide was released from the mixture after adding 8 milliliters of hydrogen peroxide.  While evaporating to about 75 milliliters without boiling, 2 doses of 2.5 grams (NH4)2CO3 were added.  The solid was then filtered off, and the filtrate was allowed to crystallize overnight.  The next day, red-purple crystals of [Co(NH3)4CO3]NO3 had formed.  The crystals were filtered once more then dried, followed by a calculation of percent yield.  Some [Co(NH3)4CO3]NO3  was set aside for later use.

Chloropentaamminecobalt (III) chloride  [Co(NH3)5Cl]Cl2

        To begin the second synthesis, 1.0 gram of the [Co(NH3)4CO3]NO3 was dissolved in 10 milliliters of water.  To this was added enough HCl, about 1 milliliter, to rid the solution of excess carbon dioxide.  Concentrated aqueous ammonia was added until the solution  was neutral according to a litmus paper test, followed by an additional 5 milliliters of NH3.  Upon evaporating for twenty minutes, without boiling, [Co(NH3)5(OH2)]3+ was formed in solution.  After cooling and adding 15 milliliters of HCl, the mixture was reheated.  During cooling this time, purple crystals appeared below a blue solution.  The complex was washed with cold water, and crystals of [Co(NH3)5Cl]Cl2 were filtered off using a glass-fritted filter.  The precipitate was washed with 3 milliliters of ethanol then allowed to dry in a heating oven at 120°C to ensure that all of the solvent was removed.  Percent yield was determined.  

Electroconductivity

To test the electroconductivity of [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2, a sample solution of KCl was first made by diluting 0.3728 grams of KCl to 250 mL with water.  Next, 0.001-M aqueous solutions of our test samples were made by diluting 0.1246 grams of the [Co(NH3)4CO3]NO3 to 500 mL with water and 0.1253 grams of the [Co(NH3)5Cl]Cl2 to 500 mL with water.  By using the KCl solution as the cell constant (k), the electroconductivities of the two synthesized compounds were measured.

Infrared Spectrophotometry

        To prove that the syntheses were successful, several IR samples were run for each compound in Nujol.  The compound [Co(NH3)4CO3]NO3 was also run in the IR with hexachloro-1,3-butadiene.

Co - 2,2-bipyridine Complex

        To further the investigation of cobalt complexes, it was attempted to substitute bipyridine for NH3 in the original equation.  The procedure for synthesis of [Co(NH3)4CO3]NO3 was followed with the following modifications.  The first difference was to conserve materials.  All reactants were cut by one-fifth of the original starting values.  Second, rather than adding 60 milliliters of NH3, 6.2516 grams of bipyridine were substituted.  Electroconductivity and IR were run on the synthesized compound to determine if it was in fact the alleged product, [Co(bipyridine)1 or 2CO3]NO3.

Results

Carbonatotetraamminecobalt (III) nitrate [Co(NH3)4CO3]NO3

A total of 8.2257g of [Co(NH3)4CO3]NO3 was synthesized after thrice filtering and drying the solution from our first reaction.  This compound was orange in color most likely due to the cobalt present.  Approximately 53.6% of the theoretical yield was obtained, resulting in 46.4% error. [Co(NH3)4CO3]NO3 was run in an IR with Nujol, and strong peaks were present at 3250, 1600, 1340, 1270, and 880 cm1.  It was also run in the IR with hexachloro-1,3-butadiene in place of the Nujol.  This IR showed almost identical peaks at 3250, 1590, 1350, 1270, and 850 cm1.  No additional peaks were seen due to the hexacloro-1,3-butadiene.  The electroconductivity results with a 0.02M solution of KCl as the standard at 23.0oC can be found below in Table 1.  

Chloropentaamminecobalt (III) chloride [Co(NH3)5Cl]Cl2

The synthesis of [Co(NH3)5Cl]Cl2 was began with 5.0110g of the [Co(NH3)4CO3]NO3.  This reaction produced 4.7835g of [Co(NH3)5Cl]Cl2,which is 94.5% of the 5.0412g expected under ideal conditions.  This was a much more productive reaction, in that there was only 5.1% error.  Again the compound was an orange color, having a similar look and texture to the [Co(NH3)4CO3]NO3.  Conductance of [Co(NH3)5Cl]Cl2 compound was measured under the same conditions with the same standard as the [Co(NH3)4CO3]NO3.  An IR for [Co(NH3)5Cl]Cl2 was run in Nujol.  The results showed that there were peaks at 3200, 1550, 1300, and 840 cm1.  

[Co(bip)nCO3]NO3

        The first attempt to produce the cobalt complex with either one or two bipyridine molecules attached proved unsuccessful.  Although a product was made, there were two different crystals; some were the orangish-red as in the previous syntheses and some were off-white colored.  The IR obtained for this unknown product showed peaks at 1650, 1630, and 1600 cm1.  The second time the compound was made, it showed several more peaks in the range of 1550 to 1650, and one at 1200 and 1150 cm1. In a third and final trial, sodium hydroxide was substituted for the ammonium carbonate.  This experiment resulted in an all white crystal.  An IR for the unknown substance was run, and peaks were found only at 860, 900, approximately 1800 to1450 (wide peak), and a wide peak spanning from 3000-3500 cm1.  An IR for 2,2 bipyridine, or dipyridil, was run for comparison purposes.  It showed peaks at 1550, 1250, 1080, 1060, 1030, and 980.

        Table 1. Results of Electroconductivity

*L value of KCl is known from resource. (Angelici, 1969)

Discussion

Synthesis of [Co(NH3)4CO3]NO3 produced a 53.6% yield.  Further analysis of this compound produced an IR that was analogous with what we had predicted.  The peak at 3250 can most likely be attributed to the ammonia groups of the cobalt complex, as N-H amine groups typically show up on IR around 3300 to 3500 cm1.  The peak at 1600 is one of the most important, as it signifies the presence of the carbonyl group on the compound.  The remaining peaks occur in connection with the two mentioned.  The [Co(NH3)4CO3]NO3 was run with hexachloro,1-3-to determine if a different solvent would have any effect on the infrared spectrum.  It turned out that although most peaks were identical, some were shifted a little.  Therefore, no significant reaction occurred between the [Co(NH3)4CO3]NO3 and the hexachloro-1,3-butadiene.  

Further analysis of Co(NH3)5Cl]Cl2 produced an IR that also compared with what we had predicted.  The first peak of interest is again for the N-H amine group.  In this compound, the peak showed at 3200 cm1.  No peak at 1600 was present, indicating that we had in fact produced a different compound than [Co(NH3)4CO3]NO3.   This compound contains no carbonyl group.  The peak at 1550 cm1 could also be due to the ammonia groups.  

By calculating the conductivity per mole of each substance and thus determining the number of ions, we can identify our products [Co(NH3)4CO3]NO3] and [Co(NH3)5Cl]Cl2] correctly.  By simply looking at the proposed structure and deciding how many ions it has, then comparing that number to the actual number of ions the synthesized compound contains, we see they are, in fact, our hypothesized products. [Co(NH3)4CO3]NO3] should have 2 ions, by looking at the charges.  We see from electroconductivity, that it does have 2 ions.  Likewise with [Co(NH3)5Cl]Cl2].  It supposedly should have 3 ions, and it was determined that it has 3 ions.

        Complications arose in the synthesis of what we expected to be

[Co(bip)1 or 2CO3]NO3.  The first trial yielded some sort of cobalt complex, but the identification of the peaks for the IR of this compound were different from any compound we had previously seen in our experimentation.  We are confident that the peak at 1600 cm1 represents a carbonyl group.  The remaining peaks could be due to the ammonia groups, but we are uncertain as to the substance’s true identity.  There was sufficient uncertainty in our procedures to prompt us to rerun the experiment.  At the idea that it may have been human error, we retried the synthesis and again produced similar results.  In this trial however, peaks were found in the same region as the prior alleged bipyridine complex, and additional peaks were seen at 1200 and 1150.  These peaks could have been due to C-O groups in our compound.  Although the IR of our supposed bipyridine did not look identical to our IR spectrum for [Co(NH3)4CO3]NO3, we began to wonder if we were ending up with a similar product.  In our reaction with bipyridine, we assumed that all other reactants would remain the same, and simply adjusted the amount of bipyridine to factor into the required molar equivalence.  Upon analysis of all of the other components in the reaction, we noticed that we had used ammonium carbonate ((NH4)2CO3).  We believed this to be the ultimate cause of our error.  The ammonium group in ammonium carbonate may have been exchanged instead of the bipyridine, possibly due to a greater affinity to cobalt.  

As a result, the experiment was redone once more with monohydrous sodium bicarbonate instead of ammonium carbonate to prevent interference from the ammonium ion.  In this attempt, white crystals were produced.  An orange  product, possibly containing cobalt did not filter through the filter paper.  So whatever product we obtained was not complexed with the cobalt unless it’s color was taken out in the process of reacting.  We believed the white product crystals to be 2,2 bipyridine, the product with which we began.  Analysis of IR spectrums disproved this theory.  The peaks were not the same for the two compounds of interest.  One possible explanation is that the initial reactant, cobaltous nitrate, underwent ligand exchange with some other reagent in the reaction; or possibly was never exchanged.  Exactly what occurred in these reactions with 2,2-bipridine is unclear.

After running the IRs, we tested electroconductivity.  We determined that, even without the IRs for our alleged bipyridine products that neither compound is what we assumed it to be.  The electroconductivities of the synthesized products were far below the expected values.  The X’s in the table signify the impossibility of determining number of ions.  Conductivity per mole must be greater than 100 to have a significance, which bipyridine complexes did not.

Conclusion

Synthesis of carbonatoetraamminecobalt (III) nitrate proved successful, corresponding with predicted results for infrared spectrophotometry data and electroconducitivity results.  Synthesis of chloropentamminecobalt (III) chloride was also a success.  The results for electroconductivity and IR were also as predicted.  Ligand exchange with bipyridine, however, was unsuccessful.  Further experimentation and most likely an altered procedure are needed to accomplish this exchange.  

Works Cited

Angelici, BLAH...Danielle has this one (the black book that we used like it was

the Bible)

Bailey, Philip S. Jr., and Christina A. Bailey.  Organic Chemistry: A Brief

        Survey of Concepts and Applications.  1978.  New Jersey: Prentice-Hall,

Inc., 1995.

Smith, Frank C.  The Practice of Ion Chromatography.  New York: John Wiley &

Sons, 1983.

Hamilton, Dorothy E.  “The Synthesis of Cobalt(III) Sepulchrate from

Tris(ethylenediamine)cobalt(III) and Its Purification by Ion-Exchange

Chromatography.”  Journal of Chemical Education 68 (1991): A144-A146.

Literature Cited

Angelici, Robert J. Synthesis and Technique in Inorganic Chemistry.

Philadelphia: W.B. Saunders Company, 1969.

Bailey PS, Bailey CA. Organic Chemistry: A Brief Summary of Concepts and

Applications. New Jersey: Prentice Hall, 1995.

Gahan LR, Healy P, Patch GJ. 1989. Synthesis of Cobalt (III) “Cage” 

Complexes. J Chem Ed 66:445-6.

Hop C, Bakhtiar R. 1996. Electrospray ionization mass spectrometry—Part III:

Applications in inorganic chemistry and synthetic polymer chemistry. J Chem Ed 73:A162-7.

White, James.  Personal Interview. November 1, 1999.