Synthesis and Analysis of Cobalt Coordination Complexes Essay

Introduction

Complex salts are characterized by unique optical and configurational properties capable of relatively simple rearrangement of atoms, which allows the creation of new substances. As a rule, complex compounds readily change into new forms when interacting with simple inorganics (LT, 2023). The present work investigates an example of cobalt complex salts whose inner sphere and, specifically, ligands are different. The paper describes the milestones of the three-step analysis, the calculation of product yields, and the comparison of empirically measured wavelengths of maximum absorption with literature data. Thus, the aim of the present work is to experimentally synthesize [Co(NH₃)₅Cl]Cl₂, [Co(NH₃)₅ONO] 2, and [Co(NH₃)₅H₂O]Cl₃, and to study the purity of these products.

Materials

The following substances, shown in Table 1 below, were used in the present laboratory work. The table contains the involvement of the substance in a particular phase of synthesis and the amounts that were used.

Table 1. Materials used.

Procedure

The present laboratory work was based on the performance of experiments and included four parts. The first part of the laboratory work was aimed at synthesizing [Co(NH₃)₅Cl]Cl₂: 10.1650 g of ammonium chloride was mixed with 20 g of COCl₂•6H₂O under vigorous stirring. To the resulting substance, 16 mL of 30% hydrogen peroxide solution was added, and after cooling the flask, another 60 mL of concentrated hydrochloric acid was added. After heating the mixture in an ice bath at 55°C to 65°C for 15 minutes, another 50 deionized water was added. The solid product filtered on a Buechner funnel was washed three times with 95% ethanol (15 mL each portion), after which the product was weighed. In the second part of the work, 5.0084 g of [Co(NH₃)₅Cl]Cl₂ obtained in the first step was mixed with 75 mL of 5% aqueous ammonium in a 250-mL flask: the mixture was heated until dissolved, after which it was cooled on a water bath. To the solution, then, is was added 2.0M HCl solution, 5.0580 g of sodium nitrite, and 5 mL of 6M HCl under continuous cooling conditions for about one hour.

The solid product [Co(NH₃)₅ONO]Cl₂ filtered on a Buechner funnel was washed twice with 95% ethanol (25 mL each portion), after which the product was weighed. In the third part of the experiment, it was necessary to synthesize [Co(NH₃)₅H₂O]Cl₃: 5.0062 g of [Co(NH₃)₅Cl]Cl₂ obtained in the first step were mixed with 75 mL of 5% aqueous, heated until dissolved, and cooled to 10°C on an ice bath. Under a fume hood, 6M hydrochloric acid was added to the mixture in such an amount as to observe the formation of a red precipitate and a thick mist. The solution was cooled and then filtered on a Buechner funnel, the solid product was washed twice with 95% ethanol (25 mL each portion), after which the product was weighed. Finally, in the fourth part of the experiment, a spectrometric study was performed for a small amount of each of the three substances, the results of which included measuring the color and the wavelength at which maximum absorbance was observed.

Results

The present work was aimed at the synthesis and subsequent analysis of the three complex salts. The results of the work carried out are summarized in Table 2: the second column shows the mass information of each substance as a consequence of the methodological procedures performed previously. The table also contains data on the color of each of the complexes, and it can be seen that they are in the red zone with different tonalities. The determined wavelengths of maximum absorption for each of the substances are also found in Table 2 in the last column. It can be seen that as new complex salts were created (in the order shown in the Procedure), the wavelengths decreased.

Table 2. Results of mass measurement, visual color analysis, and spectroscopic study of maximum absorption wavelength.

The results obtained from the spectroscopic study can be used to determine the purity of the products. For [Co(H₂O)]Cl₂, the empirically determined λ_MAX was 17.4% below the reference value, for [Co(NH₃)₅Cl]Cl₂, the value was 6.2% below the reference value, for [Co(NH₃)₅ONO]Cl₂, the measured value was 3.1% above the reference value, and finally, for [Co(NH₃)₅H₂O]Cl₃, the empirical value was 10.7% below the reference value (RUK, 2020). Thus, the average level of error made was 7.8%. This result may indicate the comparatively high purity of the synthesized products and the low content of impurity compositions.

Another direction to explore the results of the work is to calculate the yield of substances for each of the stages of the three-step process. The first reaction was carried out under the following equation (SL, 2021):

• 2CoCL₂ * 6 H₂O + 2 NH₄Cl + 8 NH₃ + H₂O₂ → 2 [Co (NH₃)₅ CL] CL₂ + 14 H₂O

Calculation of product moles:

moles _{[Co (NH3})₅ CL] CL₂= moles _CoCL2 * 6 H₂O = 20.0152 g / 237.93 g.mol^-1 = 0.084 moles

Given this, the mass of the product is:

mass _{[Co (NH3)5 CL] CL2} = 0.084 moles * 250.44 g.mol^-1 = 21.068 g

Then, the yield of the product was:

η(%) = (17.600g / 21.068g) * 100% = 83.54 %

The second reaction was based on the equation (SL, 2021):

• [Co (NH₃)₅ CL] CL₂ + NANO₂ – ^H+ → [Co (NH₃)₅ NO₂] CL₂ + NaCL

Calculate the moles of the product, given the mass of reactant taken:

moles [Co (NH₃)₅ NO₂] CL₂ = 5.0084g / 250.44 g.mol^-1 =0.020 moles

Given this, the mass of the product is:

mass [Co (NH₃)₅ NO₂] CL₂ = 0.020 moles * 261.00 g.mol^-1 = 5.2196 g

Then, the yield of the product was:

η(%) = (3.8224 g / 5.2196 g) * 100% = 73.23%

Finally, the third reaction was based on the equation:

• [Co (NH₃)₅ NO₂] CL₂ + H₂O → [Co (NH₃)₅ H₂O] CL₃

Calculate the moles of the product, given the mass of reactant taken:

moles [Co (NH₃)₅ H₂O] CL₃ = moles [Co (NH₃)₅ NO₂] CL₂ = 5.0062 g / 261.00 g.mol^-1 = 0.019 moles

Given this, the mass of the product is:

mass [Co (NH₃)₅ H₂O] CL₃ = 0.019 moles * 268.46 g.mol^-1 = 5.1493 g

Then, the yield of the product was:

η(%) = (2.3309g / 5.1493g) * 100% = 45.27%

Thus, the product yields in each stage were 83.54%, 73.23%, and 45.27% for the first, second, and third phases, respectively.

Discussion

The results of the work demonstrated an overall moderate level of accuracy of the obtained results, with product yields ranging from 45.27% to 83.54% and an average spectrometric measurement error equal to 7.8%. During the execution of the experiments some changes from the methodological plan were made, namely the increase in the duration of the execution (two sessions instead of one), insufficient time for drying the product in the first stage, and the choice of laboratory produced products for spectrometry. Limited time resources could be a source of errors that affected the accuracy of the results of quantitative product identification.

Conclusion

The present study was aimed at the synthesis of cobalt complex salts and analyzing the purity of the obtained products using both yield calculations and spectrometric analysis. The results showed that the yield of the products ranged from 45.27% to 83.54% depending on the phase of the experiment: the yield could be reduced due to the loss of substance during transport. In addition, the average error of the maximum absorption wavelength measurement was 7.8%. It follows from this that the laboratory work was, overall, successfully performed.

References

LT. (2023). The formation of complex ions. LibreTexts Chemistry. Web.

RUK. (2020). Absorption spectra – empirical aspects[PDF document]. Web.

SL. (2021). Synthesis and characterization of [Co(NH3)5Cl]Cl2 [PDF document]. Web.