6: Preparation of Structural Isomers of a Metal Complex (Experiment)

Pre-Lab Questions

1. What is the role of \( \ce {H2O2} \) in this synthesis? Use an equation to help explain the reaction.
2. If you have to prepare the 10% by volume ethylenediamine solution, you are instructed to use 5 mL of deionized water to 0.5 mL of ethylenediamine. What is the actual % by volume ethylenediamine of this solution?
3. What is the limiting reagent for the synthesis of the trans isomer? Show your calculations.
4. What is the limiting reagent for the synthesis of the cis isomer? (HINT: it is not \(\ce{CoCl2 • 6H2O} \))
5. Why would you expect the trans isomer to be more stable?
6. Why do you observe a decrease in the absorbance at 540 nm over time for the

Introduction

In this experiment, you will prepare a coordination compound of Cobalt(III) with the bidentate ligand ethylenediamine ("en"; \( \ce{NH2CH2CH2NH2}\)). Coordination compounds are also called complexes, and if they are ions they are called complex ions. A complex ion contains a central metal to which Lewis bases (ligands) have been attached or coordinated. The purpose of this experiment is to provide an introduction to the field of transition metal coordination chemistry and have you synthesize a coordination compound. This synthesis will allow you to experience some of the common procedures used to prepare and isolate inorganic compounds. The complex you will synthesize is \( \ce{trans-[Co(en)2Cl2]Cl} \), a green colored cobalt transition metal complex, which will be used to study spectrophotometry and kinetics in the experiments which follow.

The principal oxidation states of cobalt are the +2 and +3 states. The aqueous \( \ce{ [Co(H2O)6]^{3+}} \) ion is very unstable because it is a powerful oxidizing agent and is readily reduced by water to \( \ce{[Co(H2O)6]^{2+}} \):

\[ \ce{ 4[Co(H2O)6]^{3+} + 2H2O \rightarrow 4[Co(H2O)6]^{2+} + O2 + 4H^{+}} \]

However, the +3 oxidation state is stabilized by replacing the coordinated water molecules with less labile ligands such as \( \ce{NH3} \), \( \ce{NO} \), \( \ce{CN^{-}} \), and \( \ce{NH2CH2CH2NH2 (en)} \). A labile complex exchanges its ligands rapidly. Lability is associated with the d-electron configuration of the central metal. In general, ligands coordinated to a \( \ce {Co^{3+}}\) ion do not dissociate from the \( \ce {Co^{3+}}\) ion rapidly, and as a consequence, they are not easily replaced by other ligands. Thus, \( \ce {Co^{3+}}\) complexes can endure many laboratory manipulations, and as we shall see, some of them can be prepared in structural forms whose stable existence depends on the durability of the bonds to the cobalt atoms.

In this assignment, you will prepare a complex ion with a net +1 charge in which two molecules of ethylenediamine (en) and two chloride ions are bonded to a central \( \ce {Co^{3+}}\) ion. Each of the two ethylenediamine molecules is attached to the \( \ce {Co^{3+}}\) ion via the lone pairs on the basic nitrogen atoms at each end of ethylenediamine. Thus, six atoms (four nitrogen atoms and two chlorine atoms) are directly bonded to the cobalt atom and form the corners of an imaginary octahedron (eight-faced solid) with the cobalt atom at the center (see Figure 1). Notice that there are two possible structures for the complex. In one structure (the cis- isomer), the two chlorine atoms occupy adjacent corners of the octahedron. In the other structure (the trans-isomer), the two chlorine atoms occupy opposite corners of the octahedron. These structurally different complexes have different physical and chemical properties. For example, the cis complex is dark purple in color, while the trans complex looks green. In this experiment, you will prepare the chloride salt of the trans complex. The trans complex you prepare here will be used later in spectrophotometry and kinetics studies of its acid hydrolysis. Hereafter, we use the symbol “en” for the ethylenediamine molecule/ligand. It is very crucial that you measure all reagents and carry out all procedures exactly as described. Otherwise, you will synthesize an unintended complex or obtain a mixture of products. Please read the appropriate chapter in your textbook, as pre-laboratory preparation for this experiment.

You will need to retain the product for use in the Spectrophotometry and Kinetics experiments that follow.

Part I: Preparation of \( \bf{\ce{trans-[Co(en)2Cl2]Cl}} \)

Procedure

Caution: Use care and wear gloves when handling ethylenediamine, 10% \(\ce{H2O2}\), and conc. \(\ce{HCl}\).

Accurately weigh out 1.5 g of Cobalt(II) chloride hexahydrate, \(\ce{CoCl2 • 6H2O} \), and place it in a casserole dish. Add 4 ml of distilled water and gently swirl the dish until the cobalt chloride has dissolved. Do the next steps in the fume hood. Add 10 mL of 10% (by volume) ethylenediamine. Place a magnetic stir bar in the solution and set the casserole dish on a hotplate. Set the stir control knob so that the stir bar spins but does not splatter solution on the side of the casserole dish. Stir for 5 minutes. Slowly and carefully add drop wise 2.4 mL of 10% \(\ce{H2O2}\) to the solution while it is still stirring. Stir for an additional 10 min. Note any changes.

Next, CAREFULLY add 4.5 mL of concentrated \(\ce{HCl}\). Turn off the stir plate and remove the magnetic stir bar using the magnetic stir bar retriever. Prepare a 600-mL beaker with approximately half full DI water. Add 2 or 3 boiling chips to the beaker. Place your casserole dish on top of the beaker of and set the hotplate to boil the water. Concentrate the solution until there is a crust of dark green crystals on the surface and very little liquid left. Do not allow the solution to evaporate completely. This will take between 30 to 45 min. You may need to add water to the water bath during this time to keep it from boiling dry.

While you are waiting for the crystals to form, fill a 250mL beaker with ice as an ice bath. Obtain ~10 mL of methanol in a small test tube and put tube in your ice bath. You will use the ice cold methanol to rinse your crystals in the final step of this synthesis.

Carefully remove the casserole dish and set it on the bench top to cool. While your solution is cooling, set up a filtration system using an aspirator with a trap. Place a piece of filter paper in the Büchner funnel to collect your crystals. Carefully wet the filter paper with a couple drops of methanol BEFORE you add your crystals.

With the aspirator on, transfer the contents of your casserole dish into the Büchner funnel using a spatula. Add about 2 mL of ice-cold methanol to your casserole dish, swirl and pour this into the Büchner funnel. This will help to remove all of your contents from the dish. Carefully pour ~5 ml of ice cold methanol over your crystals and filter. Repeat if necessary.

Transfer your crystals to a watch glass. It is not necessary to remove the filter paper at this time. Cover the watch glass with a paper towel and place it in your drawer to dry until next period.

Weigh your product and determine the percent yield next week. Please do not forget to do this!!!

PART II: Preparation of \( \bf{\ce{cis-[Co(en)2Cl2]Cl}} \) from \( \bf{ce{trans-[Co(en)2Cl2]Cl}} \)

Procedure: (trans- to cis- isomerization)

Place 0.2 g of \(\ce{trans-[Co(en)2Cl2]Cl}\) in a casserole and add 2 mL of deionized water. Swirl to dissolve the solid, neutralize with 1N \(\ce{NH4OH}\) (4-6 drops total should do, check with pH paper after adding each drop) and evaporate the solution to dryness on the hot plate. Be careful not to decompose the compound by overheating the casserole. What changes do you observe?

Cool the casserole to room temperature. Scrape the solid into a pile with a spatula. Cool the casserole in a salt-ice bath, moisten the solid with 5 drops of ice water from a pipet. Scrub the solid with the ice water for a few seconds using a wooden applicator stick, then immediately pour the contents of the casserole onto the center of a 7 cm circle of filter paper. Cover this with another circle of filter paper and use the wooden applicator like a rolling pin to press the violet solid dry. You may need more than one covering sheet of paper to dry the solid completely.

You should now have a small amount of violet solid in the center of a filter paper that has a large green blot around it. What you have done is to separate the cis and trans isomers of the compound by taking advantage of the fact that the trans isomer is more water soluble. Weigh your product. Determine the % yield and keep the product.

PART III: Kinetics Experiment

Overview of the Aquation Reaction

The reaction studied is the acid hydrolysis of \( \ce{trans-[Co(N−N)2X2]^{+}}\), where \(\ce{N−N}\) is ethylenediamine and \(\ce{X}\) is \(\ce{Cl}\):

\(\ce{[trans-Co(N-N)2X2]^{+}+H2O \rightarrow H^{+}}\)

\(\ce{[trans-Co(N-N)2(H2O)X]^{2+}+X^{-}}\)

This is a classic chemical reaction that has influenced the development of mechanistic inorganic chemistry. Studies on these and related compounds led to the currently understood hydrolysis mechanism for Werner complexes. The green to pink color change on a reasonable time scale upon hydrolysis renders this reaction ideal for study by visible spectroscopy. In this experiment students investigate the kinetics of aquation of the four related compounds, \(\ce{[trans-Co(N-N)2X2]^{+}}\), at different temperatures. They work in pairs for this experiment making measurements on a particular complex at an assigned temperature, with each pair performing a unique kinetic experiment. The reactions proceed by a rate law that under neutral or acidic conditions is first-order in the cobalt complex, consistent with an essentially dissociative mechanism.

PROCEDURE: The Aquation of a Series of Cobalt(III) Complexes

You will work in pairs for this part. Ocean Optics USB2000 Spectrometer will be used to do absorbance measurements at 800nm & 505 nm. Each group is assigned a temperature pairing (40/65\(\ce{^{\circ}}\)C, 45/60\(\ce{^{\circ}}\)C, or 50/55\(\ce{^{\circ}}\)C). Record the temperature of the actual water bath.

Two 400 mL beakers are filled with ~300 mL of water. Set the temperature for one beaker at one of the assigned temperatures and set the other at 70\(\ce{^{\circ}}\)C. Fill a disposable test tube with 3 mL water, mark the water level, discard the water, and dry the test tube. Obtain 10 more disposable test tubes and mark each at the 3 mL mark. Place 30 mL of 0.01 N \(\ce{HNO3}\) in a 25x150 mm test tube and place it in the beaker at the assigned temperature. Fill a small plastic centrifuge tube with 1 mL water and place it in an ice bath for 2 minutes. Meanwhile, weigh out ~0.2g of \(\ce{trans-[Co(en)2Cl2]Cl}\). Add this to the centrifuge tube containing 1 mL cold water. Cap the tube and shake to fully dissolve the solid. Quickly pour the contents into the large test tube containing 0.01 N \(\ce{HNO3}\) and mix thoroughly without removing the test tube from the water bath. Start the timer as soon as the solution is fully mixed.

Remove 3 mL of the reaction mixture immediately after mixing. Add this to one of the small test tubes and place it in the ice bath (do not to let it come to room temperature). Take an absorbance reading and record the absorbance at 505 nm and 800 nm (this is t = 0). The reading at 800 nm should be approximately 0. Repeat the process until 9 samples have been recorded. Reactions at 40\(\ce{^{\circ}}\)C will take 40 min while reactions at 65\(\ce{^{\circ}}\)C will take 5 min. After the 9\(\ce{^{th}}\) sample, place the large test tube in the 70\(\ce{^{\circ}}\)C water bath for no more than 5 min, then cool the test tube in an ice bath and take a reading. This is the t = ∞ point. Repeat for the other assigned temperature.

A quantitative determination of the order and rate constant of the aquation of Cobalt (III) complexes

Your teaching assistant will give you a pair of temperatures within a range of about 40\(\ce{^{\circ}}\)C) – 65\(\ce{^{\circ}}\)C). Every group will have a different temperature if possible in order to find a range of rate constants and temperature. Once you obtain your absorbance versus time data, you will determine the order of the reaction of the cis isomer (as measured by its absorbance) by making a first (\(\ce{ln[A_{∞}-A_{t}]}\) vs. t) and second order (\ \ce{(\frac{1}{(A_{∞}-A_{t})}} \) vs. t) plot of the data. From the correct plot you will calculate the rate constant at the temperature of your study. Compare your rate constant with those of your colleagues, which may have been determined at different temperatures. The activation energy for this process, which has been measured to be 111 kJ/mole will also be determined. You will be required to give the temperature of your study and the rate constant at the given temperature to the TA. All these data will be collected and given to you within a timely fashion. Determine the activation energy and compare to the known value. Explain any deviations. (HINT: See Chapter 18 in Principles of Modern Chemistry 6th edition).

Clean Up

All solutions containing cobalt must be placed in the appropriate waste bottle in the hood. Any remaining solids should be place in a vial labeled with your name, the date, the name of the sample and the formula (ex. \( \ce {trans-[Co(en)2Cl2]Cl} \)) and turn in to your TA.

Data Analysis

In your final report, include copies of all five properly labeled graphs, the calculated rate constant, and activation energy. In your conclusions, include a comparison and discussion about the known activation energy and your value. Again, talk to your TA about any other important calculations and discussion questions that should be included in your final report.

Note

http://pubs.acs.org/doi/pdf/10.1021/ed085p1120