Experiment to Determine Delta O in Octahedral Ligand Field

Experiment # 4: Delta O

Introduction:

 The purpose of the experiment was to be able to experimentally determine the $∆O$

(delta O) of a complex when in an octahedral ligand field. The $∆O$

was determined through analysis of their UV-Vis spectra. Specifically, tris(ethylenediamine) chromium (III) chloride and tris(2,4-pentanedionato) chromium(III) complexes were synthesized.

 When the energy of the d orbitals in a metal ion are changed due to the bonding of octahedral ligands, they cause the splitting of the energy levels of the d orbitals into two sets, the higher eg and the lower t2g due to the influence of the ligands bound to the metal ion. This is called crystal field splitting.¹ This effectively breaks the degeneracy of the d orbitals that was previously established for stability because of the bonded ligands.² The

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(delta O) is the difference in energy between the two sets.¹ The amount of the splitting of the energies are dependent on oxidation state of the central metal atom, the size of the metal and the type of ligand.¹ The ligands that bond to the metal atom can be either weak or strong depending on its position on the spectrochemical series.³ 

Pre-lab Questions:

1. Cr (III) was used instead of Cr(II) complexes because they allow for higher $∆O$

   . This allows for easier measurements; the greater splitting of energy occurs between the d orbital energy levels, the easier it is to observe.

2. $\left[\mathit{Cr}\right(\mathit{en}{)}_3]{\mathit{Cl}}_3$

   is soluble in water while ${\mathit{Cr}\left(\mathit{acac}\right)}_3$

   is not because once in water, the Cl^– can dissociate and be surrounded by water molecules, while for ${\mathit{Cr}\left(\mathit{acac}\right)}_3$

   this is unable to occur and therefore does not allow for dissociation.

3. Parts A and B were split between partners, with the rigor between both parts being approximately the same. The equipment and other instruments were evenly distributed.

Materials:

Experimental Procedure:

Part A:

1. A magnetic stir bar, 2 mL of distilled water and 130 mg of $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

   were added to a 25 mL Erlenmeyer flask.

2. After the $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

   was dissolved, 500 mg of urea and 0.400 mL of acetylacetone(acac) was added to solution.

3. The flask was covered with a watch glass, and the flask was clamped and set in a boiling water set over top of a magnetic stirring plate to allow for heating and stirring for approximately an hour.
4. The solution overtime became basic and deep maroon coloured crystals formed. The solution was cooled to room temperature and the crystals were collected via suction filtration using a Hirsch funnel. After filtering, the crystals were washed with 0.200 mL of distilled water three times and allowed to dry. The crystals were then weighed.
5. The UV Spectra was then found by creating a solution out of the crystals and transferring the solution to a quartz cuvette and its UV-Vis spectra was recorded.

Part B:

1. 0.5 g of $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

   and 1 mL of DMSO, in a 10 mL round-bottomed flask. The green solution was then placed on a sand bath and heated to 190°C while stirring until the DMSO boils. The solution was kept at 170 – 190°C for about 5 minutes.

2. The solution was cooled to 70° C and its colour turned deep violet. 0.75 mL of a 2:1 DMSO and ethylenediamine solution and 2 mL of ethanol.
3. The solution was then heated and stirred to 140°C for approximately 50 minutes.
4. The solution was ten cooled down and the solid formed was separated with the use of a Hirsch funnel and washed with 2 mL of ethanol. The filtrate was then discarded, and the filter was replaced and the solid was dissolved in 5 mL of water.
5. The solution was then filtered, and the filtrate transferred to a 50 mL beaker. Approximately 1 – 2 mL of sodium bromide was added to the solution and was cooled in an ice bath.
6. After about 10 minutes, yellow solid precipitated out of solution, which was $\left[\mathit{Cr}{\left(\mathit{en}\right)}_3\right]{\mathit{Br}}_3$

   . The solid was collected via vacuum filter and washed with 2 mL of a 1:1 mixture of ethanol:ether. The solid was allowed to dry and then weighed. A solution was made with the solid and its UV-Vis spectrum was recorded.

Observations/Results:

Part A: tris(2,4-pentanedionato) chromium (III)

 When $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

was added with distilled water, it produced a light green solution. As urea and acac was added, the colour/shade of the solution turned to a very dark green. When the solution was heated in boiling water, the colour gradually changed to a lighter purple and started to create a film of solid on the glass. After cooling and filtering, the solid produced was of a purple, taro-like colour and was slightly sparkly in colour. The solution created when dissolving part of the solid in toluene was of a purple colour. Approximately 0.0847 g of tris(2,4-pentanedionato) chromium (III) was produced. 

Part B: tris(ethylenediamine) chromium (III) chloride

 The synthesis of the tris(ethylenediamine) chromium (III) chloride complex was attempted, but unfortunately was unable to be produced. The expected observations have been referenced from the lab manual. The data for part B including the amount produced and its UV-Vis spectra were provided by. The following observations were recorded up until the expected mistake occurred.

 When $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

was added with dmso, a green solution was created. As the solution was being heated in a sand bath, the colour changed to a deep violet. After adding ethylenediamine and ethanol, and heating and stirring the solution, the colour should have changed to red-brown. A red-brown solid should have formed and after filtering twice and adding sodium bromide, a yellow solid should have been produced. The yellow solid was the tris(ethylenediamine) chromium (III) chloride.

Calculations:

Part A:

Theoretical Yield of tris(2,4-pentanedionato) chromium (III):

$0.13\mathit{ g\; Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }\times \frac{1\mathit{ mol\; Cr}{\mathit{Cl}}_3\bullet 6H_{2}O}{266.45\mathit{ g }1\mathit{ mol\; Cr}{\mathit{Cl}}_3\bullet 6H_{2}O} \times \frac{349.32\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3}{1\mathit{ mol\; Cr}{(C}_5{H}_7{O}_2{)}_3} = 0.1704\mathit{ g}$

Percent yield:

$\frac{0.0847\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3 }{0.1704\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3} \times 100 \% =$

49.7 %

Percent Error:

$\frac{0.1704\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3 – 0.0847\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3}{0.1704\mathit{ g\; Cr}{(C}_5{H}_7{O}_2{)}_3} \times 100\% = 50.3 \%$

The theoretical yield, percent yield and percent error calculations should also be done the same way for part B: tris(ethylenediamine) chromium (III) chloride as was for part A.

Molar Extinction Coefficient:

Beer-lambert Law:

$\epsilon = \frac{A}{\mathit{lc}}$

Beer-lambert law comprises of A for absorbance, l for path length and c for concentration of the solution in mol/L. 4.9 x 10^-3 mol of tris(2,4-pentanedionato) chromium (III) was used in 0.1 L, the absorbance of 0.5 and path length of the quartz cuvette of 1 cm. The molar extinction constant for tris(2,4-pentanedionato) chromium (III) was 10.20 $L/\mathit{mol}\bullet \mathit{cm}$

.

Calculation of Delta O using UV-Vis Spectra Data:

$∆O=\frac{{10}^7\mathit{ nm}/\mathit{cm}}{\mathrm{\lambda }\left(\mathrm{nm}\right)} \times 0.01196\mathit{ kJ}/\mathit{mol}$

To calculate the delta O for each of the complexes, the UV-Vis spectra of each were analyzed and by taking the highest wavelength peak and calculating it using the equation above, the delta O can be determined.

The highest peak for tris(2,4-pentanedionato) chromium (III) was at 560 nm and its delta O was 213.57 kJ/mol. For tris(ethylenediamine) chromium (III) chloride, the peak was at 460 nm and its delta O was 260 kJ/mol. For chromium (III) chloride, its delta O was 192.90 kJ/mol. For chromium (III) nitrate, its delta O was 207.10 kJ/mol. From the ligands used in the lab, the following spectrochemical series was constructed below from lowest to highest delta O.

  $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }(t=0 \mathit{hr}) <\mathit{ Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }(t=1 \mathit{hr}) <\mathit{ Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }(t=2 \mathit{hrs}) <\mathit{ Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }(t=3 \mathit{hrs}) <\mathit{ Cr}(N{O}_3{)}_3<\mathit{ Cr}{\mathit{Cl}}_3\bullet 6H_2\mathit{O }(t=24 \mathit{hr}) <\mathit{ Cr}(\mathit{acac}{)}_3 < \left[\mathit{Cr}\right(\mathit{en}{)}_3]{\mathit{Br}}_3\mathit{ }$

Discussion:

For the $\mathit{Cr}{\mathit{Cl}}_3\bullet 6H_{2}O$

with four graphs, they all had two peaks representing the two different transitions between eg and t2g. All the different complexes had different wavelength absorbance peaks due to the strength of the ligand field depending on the type of ligand bound (depends on the spectrochemical series).

Questions:

1. Compared to the other chromium complex (tris(ethylenediamine) chromium (III) chloride), the acac complex is significantly different in the other due to having two distinct peaks versus the complex produced in part B which only had one. Since both complexes synthesised from part A and B of this lab had the same central metal atom of chromium, the difference in their UV-Vis spectra must be due to their different ligands. As shown in the spectrochemical series, the ligand field strength affects the peaks that appear in the spectra.
2. The ${\mathit{CrCl}}_3$

   spectra changed overtime, with the longest peak decreasing from 640 nm at t = 0 hrs to 600 nm at t = 3 hrs. This is due to the complex decomposing overtime and thus the delta O (ligand field splitting) increased overtime.

Summary:

 The main purpose of the experiment was to be able to analyze the $∆O$

of complexes through the use of their UV-Vis spectra. From that information, a spectrochemical series was able to be established for the ligands involved in the complexes. In conclusion, by analysing where the peaks are in the UV-Vis spectra for a complex and knowing its wavelength, the delta O splitting can be determined quantitatively.

Learning Objectives:

1. Use of the Hirsch funnel was learned through the process of this lab, allowing for the suction filtration of crystalline product.¹
2. The use of quartz cuvette compared to the normally used plastic cuvette was novel as it is needed to be used instead of the plastic cuvettes as the wavelength of the complexes were in the UV range.
3. The ability to relate the wavelength absorption to the complexes using Beer-Lambert’s law, allowing for a quantitative method to understand wavelengths.
4. The ability to measure the $∆O$

   of complexes, allowing to understand how the d orbitals energies split in an octahedral field.

5. The ability to analyze the UV-Vis spectrum of a solution over a period of time and comparing how it changed was learned.

References:

1. Fekl, U., CHM231H5F Laboratory Manual, University of Toronto Mississauga, 2018, page 17, 18, 19, 20, 21, 22, and 23.
2. Lancashire, R. J. Crystal Field Theory. https://chem.libretexts.org/Textbook_Maps/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Crystal_Field_Theory/Crystal_Field_Theory (accessed Nov 20, 2018).
3. Transition Metals – Crystal Field Theory. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch12/crystal.php (accessed Nov 20, 2018).
4. Clark, J. THE BEER-LAMBERT LAW – Reactions of the Group 2 elements with water. https://www.chemguide.co.uk/analysis/uvvisible/beerlambert.html (accessed Nov 20, 2018).