Seminar Assignments - Lab Report For Cobalt Lab. PDF

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Lab report for Cobalt Lab. ...

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CH 431 – Inorganic Chemistry

Experiment VI: Coordination Complexes of Cobalt Quynh Sam Siri Manjunath, Rafael Williams

Abstract In this experiment, the cobalt complex of nitritopentaamminecobalt (III) chloride, or [Co(NH3)5ONO]Cl2 was synthesized from chloropentamminecobalt chloride, [Co(NH3)5Cl]Cl2, and sodium nitrite. The product was obtained at 122% yield. A UV/vis spectrum of the product showed a λmax value of approximately 480 nm. This was compared with that of [Co(NH3)5Cl]Cl2, [Co(NH3)5(H2O)]Cl2, [Co(NH3)5NO2]Cl2, and [Co(NH3)6]Cl2. The approximate λmax values were 530 nm, 500 nm, 475 nm, and 470 nm, respectively. From these results the following spectrochemical series was created: Cl- < H2O < ONO- < NO2- < NH3.

Introduction The development of crystal field theory took place in the 1920s and 1930s to interpret the electronic and magnetic structure of crystalline solids. In crystal field theory, a central metal ion is surrounded by anions that are treated simply as point charges. The arrangement of these point charges affects the energy levels of the metal ion. The difference in energy levels between valence d-orbitals is the main focus when examining metal ions.1 While there are several different possible geometries for metal complexes, the focus of this experiment was on high spin pseudo-octahedral complexes. The octahedral field contains triply

degenerate t2g orbitals and doubly degenerate eg orbitals. Nitritopentaamminecobalt (III) chloride, or [Co(NH3)5ONO]Cl2, is an octahedral complex that can be synthesized from chloropentamminecobalt chloride and sodium nitrite as in the following equation:2 [Co(NH3)5Cl]Cl2 + NO2- → [Co(NH3)5NO2]Cl2 + ClIn transition metal complexes such as nitritopentaamminecobalt (III) chloride, the energy differences between the valence d-orbitals lie in the ultra-violet and visible light spectrum ranges. For this reason, it is possible to measure the energy differences using UV/vis spectroscopy. In this type of spectroscopy, a sample of a compound is exposed to UV and visible light. When light of certain wavelengths correspond to the energy difference between the d orbitals, the light is absorbed and electrons within the molecule are excited to a higher energy state. The wavelength of light absorbed is related to the color that the compound appears to be. This can be determined by looking at the λmax value on a UV/vis spectrum. The wavelength at maximal light absorption by the sample is indicated by λmax. The perceived color of a compound is complimentary to the color that it absorbs. With transition metal complexes, the ligands surrounding the central metal ion greatly affect the energy levels and therefore the color of the complex.3 In addition to nitritopentaamminecobalt (III) chloride, the UV/vis spectrum of several other cobalt complexes will be plotted. This data will be used to determine a spectrochemical series containing several different ligands: Cl-, H2O, ONO-, NO2-, and NH3. In the following procedure, [Co(NH3)5ONO]Cl2 will be synthesized from chloropentamminecobalt chloride and sodium nitrite. Additionally, a UV/vis spectrum of the product along with several other cobalt complexes will be obtained and analyzed.

Experimental Method, Calculations, and Yield Hazards Ammonia, hydrochloric acid, sodium nitrate, and [Co(NH3)5Cl]Cl2 are all irritants. Ammonia, hydrochloric acid, and sodium nitrate will cause serious burns to skin and eyes. Concentrated ammonia and hydrochloric acid release toxic vapors, which will cause serious damage to the respiratory tract if inhaled. Method To a 250 mL beaker, 1.5010 g [Co(NH3)5Cl]Cl2, 20 mL deionized water, and 10 mL concentrated aqueous ammonia were added. The beaker was covered with a watch glass and heated at 110 °C on a hot plate with stirring. After the solution turned merlot in color, the reaction solution was cooled in an ice bath. While stirring, 6 M HCl was added slowly until the solution was indicated as neutral by litmus paper. After the desired pH was reached, 1.5129 g sodium nitrate and 2 mL 6 M HCl were added. The reaction mixture was cooled in an ice bath for 30 minutes. The product was isolated via vacuum filtration. The product was then washed with 15 mL ice-cold water and 15 mL ice-cold ethanol.

Calculations

Results and Discussion In this experiment, nitritopentaamminecobalt (III) chloride was synthesized from chloropentamminecobalt chloride and sodium nitrite. Concentrated aqueous ammonia and heat were necessary to dissolve the starting cobalt compound. Dilute hydrochloric acid was added to neutralize the ammonia as well as prevent any salt from precipitating out of solution. The addition of sodium nitrite resulted in the formation of the desired compound. A UV/vis spectrum of the product was obtained and then compared with that of several other cobalt complexes in order to create a spectrochemical series featuring several different ligands: Cl-, H2O, ONO-, NO2-, and NH3. The complexes containing these ligands were [Co(NH3)5Cl]Cl2,

[Co(NH3)5(H2O)]Cl2,

[Co(NH3)5ONO]Cl2,

[Co(NH3)5NO2]Cl2,

and

[Co(NH3)6]Cl2, respectively. The geometry and structure of the complexes can be seen in the following illustrations:

The color and approximate λmax values of the different complexes are provided in the table below: Cobalt Complex

Physical Color

Approximate λmax

[Co(NH3)5Cl]Cl2

purple

(nm) 530

[Co(NH3)5(H2O)]Cl2

bright red

500

[Co(NH3)5ONO]Cl2

salmon

480

[Co(NH3)5NO2]Cl2

burnt orange

475

[Co(NH3)6]Cl2

orange

470

The color that the complex appears to be is complimentary to the color absorbed by the complex. This can be seen by looking at the colors that correspond to the λmax values and comparing to the actual color of the complex. Because wavelength is inversely related to energy, the complexes with maximal absorbance at higher wavelengths signify ligands with weaker field strengths. Consequently, the following spectrochemical series was obtained from the results: Cl- < H2O < ONO- < NO2- < NH3 Many factors influence ligand field strength, including the characteristics of the ligand. Smaller ligands tend to have greater field strengths because their size allows them to create better bonds with the metal. This is a likely reason for the chloride ligand being as weak as it is. Basicity has also been shown to affect ligand field strength. Bases are better able to form strong bonds with

metals, which helps explain the field strength of ammonia. Pi-acceptor ligands such as NO 2- also produce strong fields where as pi donor ligands such as ONO - and H2O tend to produce smaller field values.1 Another factor that influences field strength is metal oxidation state. The oxidation state and electronic configuration of cobalt as well as the electron count of the cobalt complexes are shown in the following table: Cobalt Complex

Oxidation state of

Electronic

Electron count

[Co(NH3)5Cl]Cl2

cobalt +3

configuration t2g4eg2

17

[Co(NH3)5(H2O)]Cl2

+3

t2g4eg2

18

[Co(NH3)5ONO]Cl2

+2

t2g5eg2

18

[Co(NH3)5NO2]Cl2

+2

t2g5eg2

18

[Co(NH3)6]Cl2

+3

t2g4eg2

18

Complexes where the metal has a higher oxidation state will have stronger ligand fields. Because the cobalt ion in the nitro and nitrito complexes had a lower oxidation state, the ligands were lower on the spectrochemical series than they would have been if the cobalt had a +3 oxidation state.1 The product was obtained at 122% yield. This value is erroneous because it is not possible to obtain more product than the limiting reagent is capable of creating. A likely reason for the inflated yield value is the presence of water or ethanol from the washes. Because the product was not dried prior to weighing, the residual liquid increased the mass of the sample. A way to remedy this would be to dry the product by blowing air across the top. This would accelerate the evaporation of the water and ethanol.

Conclusion In this experiment, the cobalt complex of nitritopentaamminecobalt (III) chloride, or [Co(NH3)5ONO]Cl2 was synthesized from chloropentamminecobalt chloride, [Co(NH3)5Cl]Cl2, and sodium nitrite. The product was obtained at 122% yield, likely due to the presence of water and ethanol used to perform washings of the product after vacuum filtration. A way to improve the accuracy of the mass recorded would be drying the product with air prior to weighing. A UV/vis spectrum of the product showed a λmax value of approximately 480 nm. This was compared with that of [Co(NH3)5Cl]Cl2, [Co(NH3)5(H2O)]Cl2, [Co(NH3)5NO2]Cl2, and [Co(NH3)6]Cl2. The approximate λmax values were 530 nm, 500 nm, 475 nm, and 470 nm, respectively. From these results the following spectrochemical series was created: Cl- < H2O < ONO- < NO2- < NH3.

References 1. House, J.E.; Descriptive Inorganic Chemistry; Academic Press, Elsevier Inc.: Burlington, MA, 2010; pp. 464 – 471. 2. Doulepov, V.; Boldyreva, E. A comparative study of the nitro-nitrito thermalisomerization kinetics of solid [Co(NH3)5ONO]Br2 and [Co(NH3)5ONO]Cl2. Sibir. Khim. Zhur., 1992, 5, 109 – 117. 3. Cox, T; Inorganic Chemistry; Garland Science, BIOS Scientific Publishers: New York, NY, 2005; pp. 98 – 112....