Through a series of substitution reactions, different cobalt ammine complexes were created. These complexes were analyzed via, precipitation and gravimetric measures to determine that the substitution reactions that occurred. Introduction: Coordination chemistry is an important part of inorganic chemistry that involves the association and dissociation of ligands to a metal.
The size, shape, and nucleophilic strength of a ligand will determine if a substitution reaction will potentially take place. Also, the charge, size, and oxidation state of the metal will determine how well substitution occurs.Associative substitution occurs when the nucleophilic ligand coordinates before the replaced ligand leaves the complex. Dissociation occurs when the replaced ligand spontaneously breaks away from the metal leaving a vacancy that is filled by the replacement ligand. Under various conditions, Cobalt (III) is able to undergo substitution of its ligands. In this lab, carbonatotetraamminecobalt (III) nitrate was made.
Substitution reactions using chloride, nitrite, and water were performed to make other Cobalt (III) complexes. They were gravimetrically measured to determine yields and properties.Experimental Step 1: 20. 0053g (0. 2101mol) of (NH4)2CO3 was added to 60mL of concentrated (14. 8M) NH4OH solution.
This was stirred for several minutes before adding 15. 0035g (0. 05155mol) of Co(NO3)2-(H2O)6 that had been dissolved in 30mL of distilled H2O. To this resulting solution was added 8mL of a 30% solution of H2O2. H2O2 was handled with extreme care and with gloves due to its corrosive nature. This total solution was then concentrated by evaporation over a hot plate with a steady flow of N2 to increase evaporation speed.
This solution was kept just under boiling for about an hour during which time another 5 grams of (NH4)2CO3 added. At the end of the hour the solution, a dark violet color, was suction filtered and then cooled for an hour. The resulting red crystals were weighed, drying was avoided due to potential decomposition. Report Step 1: Carbonatotetraamminecobalt (III) became evident over the hour period of stirring. It became a dark violet color.
The crude product was weighed at 12. 97 grams of product giving 0. 0521 moles. The percent yield was calculated to be 101%.
The weight and yield are higher than expected due to water weight. The drying oven was avoided because the product could potentially decompose in the heat. The purple was likely to be the desired product due to the theory behind it. Cobalt would coordinate with the ammine groups in solution. Also, the carbonate would coordinate with the metal, donating two electrons to the complex.
Experimental Step 2: Formation of Chloro pentaminecobalt (III): 4. 992grams of our Carbonato tetra ammine cobalt (III) (0. 0200mol) was added to 50mL H2O. To this was slowly added 20mL of 6M HCl.This reacted CO3 into carbon dioxide and opened up the cobalt to reaction.
After all the bubbling had subsided. Any excess acid was neutralized with 14. 8M NH3 (5ml). Then an additional 5mL 14.
8M NH3 was added. This was heated for twenty minutes just under boiling and then cooled and 75ml of 12. 1 M HCl was added. This was heated again for 20 minutes and purple crystals were observed.
At the end of the 20min the liquid was decanted and the precipitate was collected by suction filtration. Care was taken not to add H2O which could potentially react.After the Chloropentaminecobalt (III) was created the ionizable chlorine was measured by adding 50mL of water and 15mL of 1M AgNO3. The precipitate was weighed and moles of Cl were determined per ole of Chloropentaminecobalt (III). Report Step 2: By adding the concentrated acid to carbonatotetraamminecobalt (III), carbonate was lost from the complex and released as carbon dioxide.
This allowed for ammonia to take its place and also freed up a space for chloride ion to coordinate. This gave the complex an 18 electron count, stabilizing the compound. The compound was a redish purple color.The step was performed twice due to a small amount of product (1. 7421 grams) the first time.
This may be because of not getting the reaction hot enough the first time to avoid making aquopentamminecobalt (III). The second time 3. 4552 grams product was made, giving a total of 5. 1973 grams for the two times the step was performed. Experimental Step 3: Preparation of Nitropentamminecobalt (III) Chloride [Co(NH3)5NO2]Cl2: 1. 4983 grams of chloropentamminecobalt (III) chloride was added to 15 mL water and 5 mL of 6M NH3.
The solution was warmed for about 10 minutes on and off a hot plate until the salt dissolved.The solution was then suction filtered to get rid of any remaining solid, then cooled and slightly acidified using dilute HCl. 1. 9986 grams of Na NO2 was added and heated slightly to dissolve the initial red precipitate that formed.
Again, the solution was cooled, and 20 mL of 12. 1 M HCl was added. Bubbling occurred and yellow-brown crystals were formed. These crystals were filtered off and washed with ethyl alcohol.
A wet crude product was weighed to be 0. 0817 grams. Report Step 3: By adding sodium nitrite, the substitution between chloride and nitrite could occur.After heating the reaction mixture and adding the final amount of hydrochloric acid, the nitrite had replaced the chloride. The final complex was measured with a crude weight of 0. 0817 grams.
The small amount of product may come from much of the original complex not substituting out the chloride for nitrite, and staying in solution. Perhaps by adding more nitrite and less HCl, the desired product would have been made in greater abundance. The product, nitropentamminecobalt (III), was a light purple when dried. Experimental Step 4: Preparation of Nitropentadimminecobalt (III): 1. 0036 grams of chloropentamminecobalt III (0. 0040mol) was dissolved in 16.
5mL of water and combined with 3. 4mL of concentrated ammonia, 14. 8M. This was warmed for ten minutes to dissolve everything and then filtered.To this was added 6. 5mL of 6M HCl, just enough to neutralize the base.
1. 5 grams of sodium nitrate were then added along with 1. 5mL HCl (6M) and left to stand in the cold for one hour. During this time a reddish pink (some would use the word salmon) precipitate began to form.
This was filtered and collected. The precipitate was then added to a small amount of water and heated. Upon heating a brown/yellow mixture was observed.Report Step 4: The nitrito complex was successfully made as could be observed by the salmon colored compound we collected. This was further demonstrated by the heated process which converted the salmon nitrito complex to a brown/ yellow nitro complex.
The difference being the point of complexation with in the nitrite molecule. Percent yield was unobtainable because of an overzealous jump to heat test to observe the color change. Experimental step 5: A procedure for the synthesis of aquopentamminecobalt (III) [Co(NH3)5(H20)]Cl3 was designed and carried out using the [Co(NH3)4(CO3)]NO3 product from the first reaction.Approximately. 6116 g of [Co(NH3)4(CO3)]NO3 was dissolved in 10 mL of H2O.
2mL of conc. HCl (CO2 expelled) and 2 mL of NH3 were then added to the solution. This provided for slightly acidic conditions. The solution was then placed on high heat until the solution had completely evaporated. The resultant blue residual product, [Co(NH3)5(H20)]Cl3 was weighed to be 1379 g and analyzed for ionizable Cl using excess AgNO3(aq).
The precipitated AgCl was weighed and the relative equivalents of Cl determined. .3284 g of AgCl was recovered, which equates to 2.7 equivalents of Cl ion.
Report Step 5: Based on the results of the AgNO3 test it can be concluded that of aquopentamminecobalt (III) [Co(NH3)5(H20)]Cl3 was synthesized. Within a reasonable amount error the achieved amount of ionizable chloride approached the theoretical amount of ionizable chloride. As the concentrated HCl is added to the starting solution gaseous CO2 is expelled. The octahedral starting complex undergoes a dissociative mechanism where CO2 is expelled and replaced with H2O, forming the Co3+ coordinated metal.
By heating the solution to boiling, water as well as other ions are removed and water is locked into the coordination thus avoiding anation to [Co(NH3)5Cl]Cl2.Conclusion: The preparation of carbonato tetra ammine cobalt (III)was successful. Substitution reactions of its various derivatives also occurred based on required reaction conditions. By having taken IR spectra of the various cobalt complexes formed, the metal-ammine stretches could be determined and compared to literature values to further prove the existence of the desired products.
Cobalt (III), a d6 ligand, will be happy when it is in an octahedral conformation where the ligands donate 2 electrons each. This leads to complex having the 18 electron noble gas conformation and stabilizes the compound. By allowing the products to sufficiently dry, the overall weights and yields would have been more accurate. Cobalt ammine complexes ultimately go through substitution reactions, whether dissociative or associative, depending on ligand properties and reaction conditions.
