Chem 18.1 52 L | Group 3 1 Exercise No 09 | Semester 1, 2018-2019 Name: FERRER, Ivy Geraldine D. Groupmates: Dela Torre, Athena Janssen Fernandico, Kristine Psyche Dela Rosa, Sean Vincent Date Performed: October 18, 2018 Date Submitted: October 31, 2018 Exercise 9 METAL-COMPLEX EQUILIBRIA I. Introduction Metal complexes (ML) are the entities formed due to the combination of a metal cation with a Lewis base. It is an ion containing a central metal ion (M), bonded with one or more ions or molecules or ligands (L). The central metal ion, or the transition metal acts as the electron acceptor that is attached to the ligands, which is a Lewis base, having a lone electron pair which can be donated for the formation of a covalent bond. The formation reaction is shown below: M + L ο ML equation 9.1 Metal ions add ligands one at a time characterized by equilibrium constants called stability constants or formation constants (Kf) (Zumdahl, 2013). The corresponding pK and pH values and stability constant of the complex and free concentration of the ligand were used as variables in depending the concentration of metal ion. On the opposite, the larger Kf is, the more stable the complex ion is (Chang, 2010). Transition complexes can also establish different colors, depending on the metal center and the number of ligands. The strength of the interactions with the metal ions can be identified by the arrangement of ligands. The field strength of ligands is arranged in a list called the spectrochemical series. Once the metal complex reached its equilibrium, the stability constant is attained using the formula: [ππΏ] Kf = [π][πΏ] equation 9.2 The color formed in the solution from the metal complex corresponds to its degenerate orbitals. The color observed is complementary to the wavelength of light absorbed in the color wheel. This is due to the classification of weak field ligands with high spin and strong field ligands with low spin. Upon the experimentation of the color observed, its complementary color exhibits the range of wavelength of the ligand. Strong field ligands tend to absorb short wavelength with high frequency while weak field ligands tend to absorb long wavelengths with low frequency. At the end of the exercise, preparation of different cobalt (III) complexes using various ligands, writing complexation reactions and formation constant expressions, and construction of spectrochemical series based on the colors of the solutions formed were achieved. Chem 18.1 52 L | Group 3 2 Exercise No 09 | Semester 1, 2018-2019 II. Materials and Methods Reagents: • 0.05 M Cobalt Chloride solution (CoCl2) • 15 M Ammonia (NH3) • 6% Hydrogen Peroxide solution (H2O2) • 0.05 M Sodium Nitrite solution (NaNO3) • 1.5 mL of Glacial Acetic Acid (CH3COOH) • 1.0 mL of saturated Sodium Bicarbonate (NaHCO3) • 1.0 mL of 6 M Nitric Acid (HNO3) Apparatus and Equipment: • Pasteur Pipette • Test tube • Test tube rack • Pipette • Aspirator A. Preparation of different Cobalt (III) metal complexes 1 mL of 0.05 M cobalt chloride solution was placed in a three clean, dry test tubes. Pink Co(H2O)62+ complex with water as ligand results to the color of the solution. The formation reaction is written as: [Co(H2O)62+ (aq) + 4Cl- ο [CoCl4]2- (aq) + 6 H2O equation 9.3 The three test tubes were labelled as A, B, and C, respectively. Few drops of 6 M ammonia were added to the test tube labelled A and mixed well. 1.0 mL of 6% hydrogen peroxide solution was added afterwards. The complexation of Cobalt (III) with ammonia resulted to the change in color observed in the solution. Test tube B was contained with a few drops of 0.05 M sodium nitrite solution, followed by 1.5 mL of glacial acetic acid. 1.0 mL of saturated bicarbonate solution was added and 1.0 mL of 6% hydrogen peroxide solution were then added to test tube C. The measured 1.0 mL solution from test tube C was transferred to a clean, dry test tube labelled D. 1.0 mL of concentrated 6 M nitric acid was slowly added to produce [Co(H2O)6]3+. Chem 18.1 52 L | Group 3 3 Exercise No 09 | Semester 1, 2018-2019 Observations for color changes, precipitate formation, and evolution of gas were evaluated and results were recorded on Table 9.2 B. Predicting the arrangement of the ligands in the spectrochemical series The complexes were arranged in terms of the amount of light absorbed using Table 9.1. After which, the ligands were arranged in terms of field strength. Table 9.1. Perception of color based on the wavelength of light absorbed Wavelength absorbed (nm) Color absorbed Color observed 400-435 Violet Greenish yellow 435-480 Blue Yellow 480-490 Greenish blue Orange 490-500 Bluish green Red 500-560 Green Reddish violet 560-580 Yellowish green Violet 580-595 Yellow Blue 595-605 Orange Greenish blue 607-750 Red Bluish green C. Waste Management All mixtures were disposed to a waste bottle designated for Cobalt containing waste. III. Results and Discussion Table 9.2. Observations on the upon addition of different ligands to Cobalt Chloride Test Tube A B C D Ligand Added Ammonia Nitrate Carbonate Water Observations Dark green Faint pink Moss green; bubbling upon addition of bicarbonate solution Colorless Table 9.2 shows the data observed upon the addition of different ligands to Cobalt Chloride. Addition of few drops of 6 M ammonia and 1.0 mL of 6% hydrogen peroxide solution in test tube A turned into a dark green solution with the presence of precipitate. The solution in test Chem 18.1 52 L | Group 3 4 Exercise No 09 | Semester 1, 2018-2019 tube B turned faint pink in color when added with few drops of 0.05 M sodium nitrite (NaNO3) solution, followed by 1.5 mL of glacial acetic acid. Test tube C turned moss green with formation of bubbling upon addition of 1 mL of saturated bicarbonate (NaHCO3) solution and 1.0 mL of 6% hydrogen peroxide solution. Addition of 1.0 mL of concentrated 6 M nitric acid (HNO3) in test tube D leads to a colorless solution. Table 9.3. Net ionic equations involved in the formation of Cobalt (III) complexes Test Tube A B Net Ionic Equations 2Co(NH3)63++2OH− Co + 2NO2 Co(NO2)2 NO2- + CH3COOH CH3COO- + HNO2 Co(NO2)2 + 2 HNO2 Co(NO2)3 + H2O + NO Co(NO2)3 + 3NO2[Co(NO2)6]3[Co(H2O)6]2++6HCO3-→[Co(CO3)3]3-+3H2O + 3CO2 [Co(CO3)3]3-+6H3O+→[Co(H2O)6]3+ 3H2O + 3CO2 2Co(NH3)62++H2O2 2+ C D Theoretical Color Red orange Yellow Green Blue Table 9.3. shows the net ionic equations involved in the formation of cobalt (III) complexes. Test tube A shows the net ionic equation of 2Co(NH3)62++H2O2 2Co(NH3)63+ + 2OH- with the corresponding theoretical color of red orange. Test tube B shows the net ionic equations of Co2+ + 2NO2Co(NO2)2 + 2 HNO2 Co(NO2)2, NO2- + CH3COOH CH3COO- + HNO2, Co(NO2)3 + H2O + NO and Co(NO2)3 + 3NO2- [Co(NO2)6]3-, exhibiting a yellow color. Test tube C involves the net ionic equation of [Co(H2O)6]2++6HCO3-→ [Co(CO3)3]3-+3H2O + 3CO2 wherein green can be seen as the theoretical color. Lastly, test tube D perceives the color blue with the corresponding net ionic equation of [Co(CO3)3]3+6H3O+→[Co(H2O)6]3+ 3H2O + 3CO2. This shows that upon addition of the different ligands to the cobalt chloride, its net ionic equation and perceived color changes. Table 9.4. Stability constant expression (Kf expression) of the net ionic equation Test Tube A B C D Stability constant expression (Kf expression) [Co(Nπ»3 ]3+ ] [Co3+ ][ππ»3 ]6 [Co (ππ2 )6 ]3− [Co3+ ][Nπ2 ]6 [Co (Cπ3 )3 ]3− [Co3+ ][πΆπ3 ]3 [Co (ππ»2 )6 ]3+ [Co3+ ][ππ»2 ]6 Chem 18.1 52 L | Group 3 5 Exercise No 09 | Semester 1, 2018-2019 Table 9.4. shows the stability constant expression (Kf expression) of the net ionic equations involved in the experiment to solve for the formation constant. The Kf expression consists of ligands and its central atom. Based on the table shown, the product corresponds to the numerator and the denominator were the reactants in the net ionic equation. Table 9.5. Perception of color based on the wavelength of light absorbed (Experimental) Test Tube A B C D Ligand Ammonia Nitrite Carbonate Water Wavelength absorbed (nm) 605-750 500-560 400-435 0 Color absorbed Red Green Violet Colorless Color observed Bluish green Reddish violet Greenish yellow Colorless Table 9.5 shows the experimental perception of color based on the wavelength of light absorbed. The color observed in test tube A, containing ammonia as the ligand, was bluish green (dark green), corresponding to its complimentary color of red as the color absorbed in the spectrochemical series, absorbing a wavelength of 605-750 nm. Test tube B, containing nitrite as the ligand, shows 500-560 nm as its wavelength, thus absorbing the color green, with reddish violet (faint pink) as the color observed. Carbonate ligand contained in test tube C shows the color greenish yellow (moss green), complementary with violet as the color absorbed, therefore, having a range of wavelength from 400-435 nm. Test tube D, containing water as the ligand, exhibited a colorless solution, resulting to colorless absorption without corresponding wavelength. Table 9.6. Perception of color based on the wavelength of light absorbed (Theoretical) Test Tube A B C D Ligand Ammonia Nitrite Carbonate Water Wavelength absorbed (nm) 480-500 435-480 605-750 580-595 Color absorbed Bluish green Blue Red Yellow Color observed Red orange Yellow Green Blue Table 9.6 shows the theoretical perception of color based on the wavelength of light absorbed. The color observed in test tube A, containing ammonia as the ligand, was red orange, corresponding to its complimentary color of bluish green as the color absorbed in the spectrochemical series, absorbing a wavelength of 480-500 nm. Test tube B, containing nitrite as the ligand, shows 435-480 nm as its wavelength, thus absorbing the color blue, with yellow as the color observed. Carbonate ligand contained in test tube C shows the color green, complementary with red as the color absorbed, therefore, having a range of wavelength from 605-750 nm. Test tube D, containing water as the ligand, exhibited a yellow absorption with 580-595 nm, resulting to the observation of blue in color. Chem 18.1 52 L | Group 3 6 Exercise No 09 | Semester 1, 2018-2019 Figure 1. Test tubes containing the different cobalt (III) complexes formed in the experiment The results were based on four prepared set-ups that contained reagents. These set-ups were used to observe the changes in color, formation of precipitation, and evolution of gas that will arise upon addition of different ligands such as the ammonia, nitrite, carbonate and nitric acid. The arrangement of ligands in terms of field strength was identified. Data and information were recorded after observing the set-ups. Changes in color, formation of precipitation and evolution of gas were seen in the test tubes labeled A, B, C, and D as shown in Table 9.2 upon the addition of ammonia, nitrite, carbonate, and water to Cobalt chloride, respectively. Ligands express a certain color once it attaches to the central metal ion through ligand exchange reaction where substitution of ligands occurs such that a metal ion is replaced by another. Formation of precipitation was observed since the concentrations were dropped when some ligands are placed in the solution, thus, the precipitate disappeared and further dissolved after continuous addition of ligand. Based from the experiment, test tube A was perceived as dark green in color upon the addition of the ligand ammonia. Test tube B presented a faint pink color when ligand nitrite was added while test tube C became moss green after adding the ligand carbonate and test tube D was observed as colorless after adding the ligand water. In the theoretical observation, changes in color were different from the experimental changes. Test tube A must perceive a change in color red-orange with precipitate when added with ammonia, while test tube B must exhibit a yellow color upon addition of nitrite. In test tube Chem 18.1 52 L | Group 3 7 Exercise No 09 | Semester 1, 2018-2019 C, the carbonate ligand combined with the cobalt chloride must present a green color with precipitate and test tube D must have change in color blue with gas when added with water. The possible source of error in the experiment is due to the impurities of the solution and inaccurate measurements the reagents or the ligands used. As shown in Table 9.4., the Kf expression of the net ionic equation included was also recorded. This expression was used to solve for the formation or stability constant in order to determine the strength of interactions of the metal complexes. The theoretical and experimental perception of color based on the wavelength of light absorbed were arranged using the spectrochemical series. The use of the colors observed corresponds to its complementary color to determine the absorbed wavelength. Upon observation of Table 9.5 and Table 9.6, the difference between the theoretical color and experimental color is clearly seen, therefore, having different ranges of wavelength and color absorption. IV. Summary and Conclusion Transition metal complex is a formation of a Lewis acid which accepts electrons with a Lewis base which donates electrons. These metal complexes exhibit different colored solutions depending on the given central metal ion and the corresponding ligands. Ligands, when attached to the metal center, can be arranged according to the strength of interactions. Since the perceived color is complementary to that of the color absorbed and to the wavelength of light absorbed, strong fields tend to absorb radiation with shorter wavelength and high frequency while weak field ligands tend to absorb radiation with longer wavelength and low frequency. Based on the given data in Table 9.5, it can be concluded that the arrangement of ligands in terms of increasing field strength can be evaluated by the comparison of the colors shown. The strongest ligand was nitrite with a wavelength ranging from 435-480 nm, followed by ammonia with a wavelength ranging from 480-500 nm. The second to the weakest ligand is the water with a wavelength ranging from 580-595 nm while carbonate is shown to be the weakest ligand out of the four with a wavelength ranging from 605-750 nm. The arrangement of ligands based on its increasing field strength would be: Carbonate < Water < Ammonia < Nitrite The addition of four different ligands to the metal center form cobalt (III) complexes. Upon the addition of the ligands to each Cobalt Chloride, it can be concluded that there is a change in their color of the complex, presence of precipitate formation and evolution of gas. Chem 18.1 52 L | Group 3 8 Exercise No 09 | Semester 1, 2018-2019 V. References Book: Chang, R. (2010). Chemistry (10th ed.). Boston: McGraw-Hill, pp. 531-533. Zumdahl, S., ZUMDAHL, S.A. (2013). Chemistry: An atom’s first approach. Lorong Chuan, Singapore: Cengage Learning Asia Pte Ltd.
