Figure 1 - The nitropentaamminecobalt(III) chloride complex is arranged in a low spin state with octahedral geometry. It’s composed of five ammine ligands and one nitro ligand attached to a cobalt center, with two chlorine counter ions to balance to charge. This compound is a yellow-brown color in solution or in solid state.
Figure 2 - Like the nitro isomer, the nitritopentaamminecobalt(III) chloride complex’s electrons are arranged low spin, and the ligands in an octahedral geometry. The instead of binding from the lone pair available on the nitrogen, in this state the cobalt is interacting with one of the terminal oxygens. This compound is less stable, and if left alone for a few months will isomerize into the more stable nitro complex. Nitritopentaamminecobalt(III) chloride is red in solid state and solution.
Procedure and observations:
Synthesis of nitritopentaamminecobalt(III) chloride:
The reaction was started by dissolving pentaamminechlorocobalt(III) chloride (2.510g) in concentrated ammonia (4.1mL, 14M) while heating and stirring to form a burgundy solution, matching the color of the pentaamminechlorocobalt(III) chloride solid. The solution was then filtered by gravity to remove any cobalt oxide impurities, then cooled in an ice bath. Hydrochloric acid (20mL, 3M) was added drop wise until the pH of the solution reached ~6. Sodium nitrite (2.503g) was added to this solution, followed by more hydrochloric acid (2.8mL, 6M) which formed a salmon-red precipitate. The precipitate was filtered by vacuum filtration to get the desired product. While in the Buchner funnel, the product was washed with water (12.0mL) and then dried with ethanol (12.2mL, 100%) due to its high viscosity. The product was obtained at 1.886g; 51.7% yield.
Synthesis of nitropentaamminecobalt(III) chloride
The synthesis was started by adding nitritopentaamminecobalt(III) chloride (0.978g) product produced above to a solution of hot water (26.0mL) and ammonia (1.7mL, 14M) to form a orange solution. While cooling, concentrated hydrochloric acid was added (10mL), which produced lots of white gas from the beaker. The solution was then vacuum filtered, and the product washed with ethanol (12.0mL, 100%) to try. The product was obtained at .223g; 22.8% yield.
Results and Data:
Balanced Equations:
[Co(NH3)5Cl]2+ + H2O → [Co(NH3)5(H2O)]3+ + Cl-
[Co(NH3)5(H2O)]3+ + NO2- → [Co(NH3)5ONO]2+ + H2O
[Co(NH3)5ONO]2+ → [Co(NH3)5NO2]2+
Theoretical Yields:
[Co(NH3)5ONO]Cl2:
* * =
[Co(NH3)5NO2]Cl2:
% Yields:
[Co(NH3)5ONO]Cl2:
= 51.7%
[Co(NH3)5NO2]Cl2:
Discussion:
Both nitropentaamminecobalt(III) chloride and nitritopentaamminecobalt(III) chloride were synthesized successfully according to the procedure/observations section. The infrared spectra’s measured showed all the expected stretches, bends and rockings.
Both compounds shared many similar peaks because their structure is very similar. In both compounds, an ammonia asymmetric stretch appeared around 3279 cm-1 and a degenerate asymmetric stretch around 1570 cm-1. A symmetric stretch appeared at around 3160 cm-1 - 3170 cm-1, and symmetric deformation stretch around 1315 cm-1. The last ammonia peak that was present was around 851 cm-1, where there was rocking.
As the compounds aren’t exactly the same, there are some differences in the infrared spectras also. In nitropentaamminecobalt(III) chloride, an asymmetric stretch, an asymmetric scissoring stretch, and a bend measured at 1609 cm-1, 1428 cm-1 and 595 cm-1 respectively. With the presence of these, and the lack of nitrito peaks, it can be concluded that nitropentaamminecobalt(III) chloride was purely synthesized. The spectra of the heated nitropentaamminecobalt(III) chloride also helps prove the purity by being virtually identical to the synthesized product; this means that all of the nitrito starting product isomerized to its nitro form.
Nitritopentaamminecobalt(III) chloride also had vibrations that did not appear on nitropentaamminecobalt(III) chloride. There were no NO2 peaks on the measured spectra, but two new peaks appeared at 1453 cm-1 and 1067 cm-1 corresponding to a Co-ONO bond and a ONO bond respectively.
The spectra for pure pentaamminechlorocobalt(III) chloride was used to compare the peaks with the two products obtained. With this, one is able to if any peaks are missing on the product IR data. Both nitropentaamminecobalt(III) chloride and nitritopentaamminecobalt(III) chloride, a had all the relevant peaks that appeared on the pure pentaamminechlorocobalt(III) chloride spectra, once again showing the purity of this experiment.
Through the course of the experiment, there were a few things that required special attention. First of all, when making nitritopentaamminecobalt(III) chloride, the pH of the filtrate after the gravity filtration had to be at an acetic level. This is because having a lower pH helps prevent the isomerism into the more stable nitro form. Aside from that, since the nitrito compound will isomerize by itself, the infrared spectra had to be taken immediately to insure a pure product. This required adding ethanol (100%) to wash the excess water from the sample. Ethanol works well for this because it is very viscous.
The nitropentaamminecobalt(III) had a lower than expected yield of 22.8%. This was most likely due to using a no desirable filtering method. Gravity filtration was used, where vacuum filtration should have, so lots of product was lost.
Conclusion:
The two linkage isomers, nitropentaamminecobalt(III) chloride and nitritopentaamminecobalt(III) chloride were successfully synthesized with remarkable purity according to the infrared spectra’s measured. Both spectras agree with the spectras of the two compounds used to compare - pure pentaamminechlorocobalt(III) chloride was used to observe amine peaks, and a nitropentaamminecobalt(III) chloride (heated in an oven at 110 degrees celcius overnight) was in good contrast with the freshly synthesized version.
As expected, the nitro compound does not contain any nitrito ligand frequencies, and vice versa, therefore complete isomerism was achieved during the nitro synthesis.
References:
1) D.M.L. Goodgame, D.M.L.; Hitchman, M.A. Inorg. Chem. 1964, 3, 1389
2) Pentland, R.B. et al., J. Amer. Chem. Soc, 1956, 78, 887.
3) Nakamoto, K. infrared and Raman Spectra of Inorganic and Coordination Compounds., third edition: Wiley Interscience
4) Shimanouchi, T., Tables of Molecular Vibrational Frequencies Consolidated Volume I,
National Bureau of Standards, 1972, 1-160. Obtained from NIST Chemistry WebBook, 2005.
5) Socrates, G.; Infrared and Raman Characteristic Group Frequencies, 3rd ed; John Wiley and Sons: New York, 2004; pp317