Question

Prelab questions 1,2, and 3 and Part E questions

Also, write a flowchart of the experiments process in each part.

EXPERIMENT 3 DETERMINATION OF Δo

The d orbitals of a metal ion in an octahedral ligand or crystal field are split into a higher energyset (eg) and a lower energy set (t2g) as shown in the following energy level diagram.

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The energy difference between the upper and lower energy levels is Δo, the octahedral crystal field splitting.

The degree of splitting of the d orbitals in octahedral complexes (the magnitude of Δo) depends on several factors including the oxidation state of the metal, the size of the metal, and the nature of the ligand. The situation can be simplified considerably by considering a series of compounds of the same metal in a given oxidation state. The only major variable, in this case, is the nature of the ligands bonded to the metal. From the study of the spectra of such complexes it is possible to arrange the various ligands in a sequence according to their ability to cause d orbital splitting. This series is known as the spectrochemical series.

In this experiment the electronic spectra of several Cr(III) complexes will be measured, from which a portion of the spectrochemical series will be constructed. Chromium(III) chloride hexahydrate and hexaaquochromium(III) nitrate are commercially available. Tris(ethylenediamine) chromium(III) chloride and tris(2,4-pentanedionato)chromium(III) will be synthesized.

PRE-LAB QUESTIONS

1. Why are Cr(III) rather than Cr(II) complexes studied in this experiment?
2. Explain why [Cr(en)3]Cl3 is soluble in water but Cr(acac)3 is not.
3. Outline how you and your lab partner should organize your time in order to complete this experiment in the four hour lab period. Have the TA or technician check your plan at the beginning of the lab.

EXPERIMENTAL PROCEDURE
Part A: Synthesis of tris(2,4-pentanedionato)chromium(III), chromium acac ACACH MUST BE KEPT IN THE HOOD.

In a 25 mL Erlenmeyer flask containing a magnetic stirring bar, place 2.0 mL of distilled water and 130 mg (0.49 mmol) of chromium(III) chloride hexahydrate. When the chromium complex has dissolved, add 500 mg (8.3 mmol) of urea and 0.400 mL (3.84 mmol) of acetylacetone, acac. A large excess of acacH is used as it helps the reaction go to completion. Cover the flask with a watchglass. Clamp the flask in a beaker of boiling water set on a magnetic stirring hot plate. Heat the mixture, with stirring for about 1 hour. As the urea releases ammonia and the solution becomes basic, deep maroon crystals should begin to form as a crust at the surface of the reaction mixture.

Cool the reaction flask to room temperature. Collect the crystalline product by suction filtration using a Hirsch funnel. Wash the crystals with three 0.200 mL portions of distilled water. Dry the product on filter paper. Weigh the product.

Determine the percentage yield. Measure the melting point of the product. See page 14 for the IR spectrum of pure 2,4-pentanedione, acacH.

Part B: Synthesis of tris-(ethylenediamine)chromium(III) bromide

Place 0.5 g (1.88 mmol) of Cr(III) chloride hexahydrate into a 10 mL round-bottomed flask. Add 1.0 mL of dimethylsulfoxide (dmso) and heat the green solution on a sand bath to 190°C with stirring until the dmso starts to boil and white fumes appear at the neck of the flask. Keep the solution between 170-190° C for 5 minutes. Remove from the heat and allow the reaction to cool to 70°C or less. The colour of the solution should be deep violet. Add 0.5 mL of ethylenediamine in 1.0 mL of ethanol. Use another 1 mL of ethanol to rinse the sides of the flask. Slowly heat the stirred solution to 140°C and continue stirring it at that temperature for up to 50 minutes, as necessary. The colour of the solution should change from deep violet to a red-brown and a red-brown solid should be formed. (The red-brown solid should begin to form at about 100°C.) Cool and then separate the solid by suction filtration using a Hirsch funnel, wash the solid with 2 mL of ethanol and leave the solid on the filter. Discard the filtrate and replace the filter flask with a new, clean one. Now dissolve the solid on the filter frit in 5 mL of water. Filter and transfer the resulting filtrate to a 50 mL beaker. Add 1-2 mL of a saturated solution of NaBr and cool the solution on ice. A yellow solid, [Cr(en)3]Br3 , should separate out upon standing for about 10 minutes. Separate the solid by vacuum filtration and wash with 2 mL of a 1:1 mixture of ethanol:ether. Air dry. Weigh the product.

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Part C: Characterization of Complexes

1. Prepare a solution of Cr(acac)3 in toluene by dissolving 30 mg of your product in a 10 mL volumetric flask. The solution of [Cr(en)3]Br3 will be prepared in water by dissolving 20 mg in the volumetric flask. Transfer to a cuvette and measure the visible spectrum (400 - 700 nm) of each of the solutions.

2. Measure the visible spectrum of an aqueous CrCl3 solution and repeat the measurement every hour throughout the lab period. A spectrum of CrCl3 taken after

24 hours in solution is provided on page 15. A spectrum of Cr(NO3)3 is provided on page16.

Part D: Calculations

Determine the molar extinction coefficient for each of the complexes at the longest wavelength absorption maximum using Beer‟s law, A = εlc.

Convert the wavelengths into frequency units of reciprocal centimeters using the relationship Δo = ν= (1/λin nm)(107 nm/cm)
and into units of kJ mol-1 using the relationship
1 cm-1 = 0.01196 kJ mol-1.

Arrange the various ligands in order of increasing Δo . Compare this with the spectrochemical series and comment.

Part E: Questions

The visible spectrum of the acac complex is significantly different from the others. Why?

Account for the changes in the spectrum of the CrCl3 solution with time. What reaction is occurring in solution?

Answer 1

Cr(III) has 3 electrons in 3d orbtial which will be in t2g level
Cr(II) has 4 electrons in 3d orbital. There are two possibilities depending on the nature of the ligand.
(a) 3 electrons will be in t2g level and 1 electron will be in eg level.
(b) All the four electrons will be in eg level.
Since there are two different possibilities, the complexity is higher in determining the spectrochemical series. Hence, Cr(II) complexes not studied.
2) [Cr(en)3]Cl3 is an ionic complex. Cation is [Cr(en)3]+ and the anion is Cl-. Ionic complexes dissolve in water as water is polar solvent. The cation and anion are hydrated with water molecules.
The complex [Cr(en)3]Cl3 is covalent compound, hence, insoluble in water.
3) Part A and Part B should be done simultaneous in the first 2 hours of the lab.
Part C and D should be done in third and fourth hour and since spectrum is to be recorded after every hout, the efforts shoud be made to get the maximum number of runs.