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4.4 Isomerism in Transition Metal Complexes: Preparation and Characterization of cis and trans- Dichlorobis(ethylenediamine)cobalt(III) Chloride Isomerism is one of the interesting and important features of transition metal complexes which have been the subject of numerous investigations since the beginning of the field of coordination chemistry. Indeed one of the reasons coordination chemistry may become quite complicated is that there are many ways in which isomers can arise. There are different types of isomerism such as: geometric, optical, linkage, ligand, ionization and coordination isomerism. Two simple geometric isomers of cis and trans are possible for octahedral complexes of the type MA2B4, in which the A ligands may be either next to each other (cis) or on opposite apexes (trans) of the octahedron: 00 B 09 M M B B B 00 B cis trans Often the cis and trans isomers of a complex can have very different physical properties. Complexes of this type were first studied by Wemer, who showed that the praseo (green) and violeo (violet) complexes of cobalt(III) can exist with the formula CoC13.4NH3. Recently there has been renewed interest in them as indicators of the effects of lowered symmetry on electronic transition spectra, as color changes from an isomer to another one. In this study, you use efficient method for the synthesis of cis and trans-dichlorobis (ethylenediamine)cobalt(III) chloride. Characterization of the complexes is done by UV-Visible and IR spectroscopy. In this experiment, you will prepare both cis- and trans-[Co(en)2Cl2]CI (en = ethylenediamine) and characterize the products by UV-vis and IR spectroscopy.
Materials: Cobalt(II) chloride hexahydrate, CoCl2.6H30 Ethylenediamine, HANCHCH2NH2 (20% in water) Hydrogen peroxide, H2O2 (30%) Methanol, CH OH Ethanol, CH OH HCI (35-37% in water) Ammonia, 1 M NH3(aq) Preparation of trans-(Coſen)2C17C1 Dissolve 0.4 g of CoCl2.6H20 (accurately weighed) in 0.5 mL of distilled water in a small beaker. To this, add 0.7 mL of 20% aqueous ethylenediamine drop wise with stirring. Cool the solution in an ice-water bath (4 °C) and slowly add 0.4 mL of 30% H2O2. Then the solution is gently warmed to about 60-70 °C for 15-20 min. Add 1 mL concentrated HCI (35-37%) drop wise to this solution. The solution is then evaporated with stirring to about half of its volume on a hot plate or steam bath. Cool the solution to 5 °C, add 3 mL of ethanol, and keep cooling the solution for 10 more minutes. Vacuum filter the resulting green solid wash with cold ethanol (2 mL), and let the solid product dried by suction filteration. Purification of trans-(Co(en) C12]CI It is necessary to drive off HCI molecules of crystallization. Transfer your Co complex product to a small beaker, add 1 mL of methanol, and vigorously stir with a stirring rod. Transfer the resulting slurry to a large test tube. Place this in a beaker of water on a hot plate or the steam bath in the hood. Slowly heat the water to a boil. After the methanol has evaporate, maintain heating for an additional 15 minutes to drive off HCI(g). You can test for the presence of HCl(g) by holding a wet piece of blue litmus paper over the test tube. The light green solid remaining in the test tube is purified trans-[Co(en) Cl]CI.
Preparation of cis-[Co(en)2Cl2]CI On a watch glass or in a small beaker, combine 20-50 mg of trans-[Co(en)2Cl2]CI (accurately weighed) with the minimum required amount of distilled water for the complex to dissolve. Add 1 drop of ammonia solution (1 M) and warm it on a hot plate or steam bath until a dry residue of cis-[Co(en)2Cl2]Cl remains. If some of the green trans isomer remains, repeat the dissolving and add one drop of 1 M NH3(aq), and heat the solution on a steam bath until a dry residue of purple cis-[Co(en)2Cl2]Cl remains. Cool the product and carefully scrape i from the glass surface. Weigh the material and calculate the percentage yield. A quantitative yield is usually obtained. Characterization of the product Record UV-vis spectra of cis and trans isomers in distilled water. To the trans isomer sample, add 1 drop of 6 M HNO3, and record the spectrum. Record IR spectra of the cis and trans isomers of [Co(en)2Cl2]CI.