Exp. 17 Synthesis of Transition Metal Complexes S20 PDF

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Dr. Petra van Koppen...

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Experiment 17

17 Synthesis of Transition Metal Coordination Complexes

PURPOSE AND LEARNING OBJECTIVES To synthesize, isolate, and analyze the cis and trans isomers of [Co(en) 2Cl 2]Cl. To synthesize and analyze [Co(NH 3) 6]Cl 3 and [Co(NH3)5Cl]Cl2 and see the effect of ligand field strength on the color of a compound. PRINCIPLES An important characteristic of transition metals is their ability to form complexes with small molecules and ions. For example, in the complex Pt(Cl)2(NH3)2, the central metal ion, Pt2+, bonds with two chloride ions and two ammonia molecules in a square planar geometry. The ions and molecules are called ligands that coordinate to the metal. The coordination number is the total number of bonds to the metal. For Pt(Cl)2(NH3)2 the coordination number is four. As a Lewis acid, the Pt2+ accepts electron density from a lone pair of electrons on each of the ligands, the ammonia and chloride. H3N

Cl

Cl

Pt

H3N

NH3 Pt

Cl

H3N

Cl

cis-[Pt(NH3)2Cl2]

trans-[Pt(NH3)2Cl2]

“cis-platin”: anticancer drug

no therapeutic properties

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■ EXPERIMENT 17

The cis-[Pt(NH3)2Cl2] and trans-[Pt(NH3)2Cl2] are geometrical isomers. In isomers, the types of ligands are the same but relative positions of the ligands are different. The properties of geometrical isomers can be substantially different. For example, the cis-[Pt(NH3)2Cl2], also called “cis-platin,” has been used as an anticancer drug whereas the trans isomer has no therapeutic properties. In the synthesis of compounds a racemic mixture of isomers is often produced; that is, a mixture of all the possible isomers is produced. In synthesizing [Pt(NH3)2Cl2], cis-platin is the active isomer which needs to be isolated from the trans isomer before it is used as a drug. In some cases, the inactive isomer can cause undesirable side effects. Synthesizing a single isomer or finding methods to separate the isomers is an active field of research. In this experiment you will synthesize and isolate cis and trans-[Co(en)2Cl2]Cl isomers, where “en” represent ethylene diamine, NH2CH2CH2NH2. The compound [Co(en)2Cl2]Cl is a salt which consists of a coordination complex, the Co(en)2Cl2 + ion, and the chloride ion, Cl−. The chloride outside the square brackets in [Co(en)2Cl2]Cl is a counter ion to balance the charge in the compound (a solid substance is always neutral). Even though there are only four ligands in the Co(en)2Cl2 + complex ion, each ethylenediamine takes up two coordination sites. Ethylenediamine has two amine groups, one at each end of the molecule, NH2CH2CH2NH2. Each nitrogen in ethylenediamine coordinates to the metal. Thus, the coordination number for [Co(en)2Cl2]Cl is six (the counter ion is not included because it is not coordinated to the metal). The geometry is octahedral. CH2 H2N

Cl NH2 H2C

NH2 Co

H2C NH2

NH2

CH2

H2C

CH2 NH2

H2C

CH2 NH2

Co

Cl

NH2 Cl

Cl

cis-Co(en)2Cl2+

trans-Co(en)2Cl2+

The salt of the trans isomer, trans-[Co(en)2Cl2]Cl, is green and the salt of the cis isomer, cis[Co(en)2Cl2]Cl is purple. You will measure the absorption spectrum of these compounds using a spectrophotometer. According to the Beer-Lambert law, the absorbance of light, A, is directly proportional to the concentration of the absorbing sample. A=εbc In this equation, b is the thickness of the absorbing sample in centimeters, ε is the molar absorptivity coefficient in units of L mol–1cm–1, and c is the concentration of the sample in moles/L. Bonding in Coordination Complexes The Lewis model of coordination complexes, where the metal ion accepts electron density from a lone pair of each of the ligands, does not explain the color or magnetism observed for coordination complexes. Another simple but useful model to explain both the color and magnetism

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SYNTHESIS OF TRANSITION METAL COORDINATION COMPLEXES ■

of coordination complexes is crystal field theory. In this theory, the bonds between the metal and ligands are assumed to be ionic. The central metal ion has a positive charge and the ligands have a net negative charge.

M n+

In an isolated metal ion, without any ligands, the energy of an electron is the same in each of the d-orbitals; the d-orbitals are degenerate. However, in coordination complexes, the ligands surrounding the metal ion increase the energy of the d-orbitals. The increase in energy is due to electrostatic repulsion between the negatively charged ligands and the d-electrons. The difference in energy depends on the shape of the d-orbital (look up the shapes of the dz2, dx2 − y2, dxy, dyz, and dxz orbitals in your text). The electron density lobes of the dz2 and dx2 − y2 orbitals point directly at the ligands in an octahedral complex, increasing the energy of the dz2 and dx2 − y2 orbitals relative to the dxy, dyz, and dxz orbitals; the dxy, dyz, and dxz orbitals are positioned between the ligands.

3 5

dz2

dx2-y2

eg

Energy

2 5

dxy

dyz

dxz

t2g

Co(NH3)63+ (low spin)

dz2

dx2-y2

dxy

dyz

dxz

Co3+

The splitting of d-orbitals is called “crystal field splitting.” The difference in energy between the d-orbitals is equal to Δ. The colors of transition metal coordination complexes are due to the excitation of electrons from occupied lower energy d-orbitals, the t2g orbitals, to empty higher energy d-orbitals, the eg orbitals. The frequency of light absorbed is directly proportional to the crystal field splitting, Δ = hν = hc/λ. The larger the crystal field splitting the higher the frequency of light absorbed and the smaller the wavelength. The color of a coordination complex is the complementary color of the light it absorbs. For example, trans-[Co(en)2Cl2]Cl is green because it absorbs in the red region of the visible spectrum. Similarly, cis-[Co(en)2Cl2]Cl is purple (violet) because it absorbs light in the yellow region of the visible spectrum.

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■ EXPERIMENT 17

Light Absorbed Color and Wavelength (nm) Violet Blue Green Yellow Orange Red

Observed Color

Orange

Yellow Orange Red Violet Blue Green

400 450 500 580 600 650

Red

Yellow

Violet

Green Blue

Yellow light has a shorter wavelength and higher energy than red light. Thus, the crystal field splitting, Δ, is greater for cis-[Co(en)2Cl2]Cl than it is for trans-[Co(en)2Cl2]Cl. In the trans isomer, the ethylene-diamine ligands are farther apart than in the cis isomer, resulting in a lower energy complex and a smaller crystal field splitting for the trans isomer. From the magnetism and absorption spectra of coordination complexes, the relative field strength of ligands can be determined. The stronger the ligand field strength the greater the splitting. In the Co(NH3)63+ complex, the Co3+ has a 3d6 electron configuration and according to Hund’s rule one electron will occupy each of the t2g orbitals, all with parallel spin, before a second electron is put into each of these orbitals. Because all the electrons are paired in Co(NH3)63+, this compound is diamagnetic. NH3 is a strong field ligand. For weak field ligands, such as I−, Br−, and Cl, the crystal field splitting is smaller and a single electron will occupy each of the t2g and the eg orbitals before electrons are paired in the t2g orbitals. For example, F− is a weak field ligand in CoF63−. The crystal field splitting is smaller for CoF63− than for Co(NH3)63+. This maximizes the number of unpaired electrons; a high spin state. Because CoF63− has unpaired electrons, it is paramagnetic. When the electrons in the t2g orbitals are paired prior to filling the eg orbitals, the complex is low spin as in Co(NH3)63+.

dz2

dx2-y2

eg large

dxy

dyz

dxz

t2g

Co(NH3)63+ Low spin, diamagnetic

dz2 dxy

dx2-y2 dyz

eg small dxz

t2g

CoF63High spin, paramagnetic

The order of ligand field strength is called the spectrochemical series. The general trend for a selected number of ligands, ranked from weak to strong field ligands is as follows: I− < Br− < Cl− < F−, OH− < H2O < :NCS− < NH3 < en < NO2− < CO, CN− weak field ligands strong field ligands (small Δ, high spin) (large Δ, low spin)

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SYNTHESIS OF TRANSITION METAL COORDINATION COMPLEXES ■

This order does not necessarily hold for every ligand complexing with every possible metal, but the general trend is still useful. For a tetrahedral complex, no d-orbitals point directly at ligands. Thus, the repulsion between the ligands and the d-electrons is less than in octahedral complexes.

dz2

eg

dx2-y2

octahedral

dxy

dyz

dxz

t2g

CoF63Octahedral

dxy dz2

dyz

t2

dxz

dx2-y2

tetrahedral e

CoCl42Tetrahedral

For a given ligand, the crystal field splitting is larger for an octahedral than for a tetrahedral complex, Δtetrahedral ≈ (4/9) Δoctahedral. Note also that the energy of the dz2 and dx2 − y2 orbitals for a tetrahedral complex is lower than the energy of the dxy, dyz, and dxz orbitals. The energy level diagram for square planar complexes is shown in your text. In Parts 1 and 2 of this experiment, you will synthesize [Co(NH3)6]Cl3 and [Co(NH3)5Cl]Cl2. Because NH3 is a strong field ligand and Cl− is a weak field ligand, the color observed for these compounds will be different. Note the difference between the counter ions (the chloride ions outside the square brackets) and the chloride ions coordinated directly to the central metal ion, Co2+. You will measure the absorption spectrum for these compounds. In Parts 3 and 4 of this experiment, you will synthesize and isolate the cis- and trans-[Co(en)2Cl2] Cl isomers; observe the isomerization of the cis to the trans isomer; and obtain the visible spectrum of the two isomers using a spectrophotometer. This procedure was adapted from an article by R. D. Foust and P. C. Ford, published in the Journal of Chemical Education (J. Chem. Ed. 47 (1970) 165). CHEMICALS Activated charcoal (must be fresh) CoCl2∙6 H2O NH4Cl 15 M NH4OH

10% H2O2 (fresh), (density = 1 g/mL) 10% ethylenediamine, (density = 1 g/mL) 12 M HCl (secured in the hood) methanol

SAFETY WEAR SAFETY GLASSES Hydrogen peroxide, concentrated hydrochloric acid, and ethylenediamine will burn your skin. If you come in contact, rinse thoroughly with water. Methanol is flammable. Extreme caution should be taken when using an open flame. Use a hot plate for heating throughout this experiment. The 12 M HCl must be secured in the hood. In case of a spill, use sodium bicarbonate, NaHCO3, to neutralize the HCl.

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■ EXPERIMENT 17

PROCEDURE This is a two-week lab. You will turn in one lab report after you complete the second week of lab. You will work with a partner. It is very important that you read the procedures thoroughly before performing the experiment. You have to work very carefully with your lab partner to ensure that all of the products will be prepared safely and properly. NOTE: If you have time during the experiment, calculate the limiting reagent, and theoretical yield for reactions. Part 3. Week 1. Synthesis of trans-[Co(en)2Cl2]Cl To make this two-week lab efficient, start Part 3 this week before you start on Part 1. You and your partner work together on each part. In step 10 of Part 3, while the solution is left to heat and aerate for 30 minutes, you can start Part 1 of this week’s lab. After you finish step 12 of Part 3 you can store the solution in your locker. You can also stop after you finish step 15 of Part 3. It is better to stop after step 15 (save the crystals and the filtrate in your locker). Part 1. Synthesis of Hexaamminecobalt(III)chloride, [Co(NH3)6]Cl3 1. Weigh 1 gram of CoCl2∙6H2O, to the nearest 0.001 g and record the mass in your notebook. Place the CoCl2∙6H2O in a medium test tube. 2.

Weigh 0.7 grams of ammonium chloride to the nearest 0.001 g and record the mass in your notebook. Add this to the test tube containing the CoCl2∙6 H2O.

3.

Add 1 mL of DI water to dissolve most of the salts. What color is the mixture?

4.

In the hood, carefully add 3 mL of concentrated ammonium hydroxide to the test tube. Record your observations. Using a small spatula, add a pea-sized amount of activated charcoal to the test tube.

5.

Slowly add 3 mL of 10% H2O2 in 1 mL portions to the test tube. It will produce oxygen gas and the test tube will warm up. If the peroxide is added too fast the solution will bubble over the side of the test tube. The hydrogen peroxide is used as an oxidizing agent; it oxidizes Co2+ to Co3+. The activated charcoal acts as a catalyst. Note the color of the solution as the cobalt is oxidized. Record your observations. charcoal

2 CoCl2∙6 H2O + 2 NH4Cl + 10 NH3 + H2O2 ⎯⎯⎯→

2 [Co(NH3)6]Cl3 + 14 H2O

To calculate the limiting reagent for this reaction, you need to know that 10% H2O2 implies 10% by mass. The density of 10% H2O2 is 1 g/mL. Also, the number of moles of NH3 is calculated using 3 mL of 15 M NH4OH. 6.

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Once the bubbling has subsided (approximately 5 minutes), secure the test tube to a ring stand and heat it in a boiling water bath for 5 minutes. Make sure no water gets in the test tube from the hot water bath.

SYNTHESIS OF TRANSITION METAL COORDINATION COMPLEXES ■

7.

Set the solution aside for 30 minutes. At this point you can start Part 2.

8.

Use a 60 mm Büchner funnel to vacuum filter the precipitate. Use the filtrate to wash all of the precipitate from the test tube (at least three washes with the filtrate). Water should not be added because it will cause the product to dissolve. The precipitate on the filter paper is called the filter cake; it contains the product and activated charcoal.

9.

Place the filter cake in a small clean beaker. Add 6–8 mL of water and stir the solution with a clean stirring rod. Add 3–6 drops of concentrated HCl to the mixture and stir.

10. Test the solution for acidity with litmus paper by dipping the stirring rod into the solution and touching the litmus paper with the stirring rod. The litmus paper should turn red. If the solution is not acidic add a few more drops of acid. 11. Set up a filtration system with a 60 mm Büchner funnel and a piece of 55 mm filter paper. Turn on the vacuum and wet the filter paper with some DI water to make a good seal. 12. Fill a 600 mL beaker two-thirds full with hot tap water (the water should be ~50–60ºC). To completely dissolve the product, heat the solution in a water bath until the solution reaches 50–60°C. Filter the hot solution immediately using suction. The charcoal will remain on the filter, your product will be the filtrate (the solution in the flask). Discard the filter paper with the charcoal. 13. Quickly transfer the filtrate (the solution in the flask) to a test tube and, in the hood, add 1.5 mL of concentrated HCl to the filtrate to precipitate the product. Place the test tube in an ice bath for approximately 15 minutes. At this point you can finish Part 2. 14. Filter the precipitate and wash with two 3 mL portions of 95% ethanol. Leave the precipitate on the filter for a few minutes to allow it to dry. If needed, use ethanol to rinse out your test tube. 15. Weigh the product and record the mass in your notebook. 16. Determine the limiting reagent. Calculate the theoretical and percent yield of product. Finish Part 2. Limiting Reagent Theoretical Yield Percent Yield

17. When you finish Parts 1 and 2, dissolve approximately 0.03 g of [Co(NH3)6]Cl3 in 10 mL DI water. Use the spectrophotometer to measure the absorption spectrum. The cuvette should be half-full. Use DI water for your reference sample. Instructions on how to use the spectrophotometer are provided on pages 238–239.

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■ EXPERIMENT 17

Part 2. Synthesis of Pentaamminechlorocobalt(III)chloride, [Co(NH3)5Cl]Cl2 1. Weigh 1 gram of CoCl2∙6 H2O and record the mass to the nearest 0.001 g in your notebook. Place the CoCl2∙6 H2O in a medium test tube. 2.

Weigh 0.7 grams of ammonium chloride and record the exact mass in your notebook. Add this to the test tube containing the metal salt.

3.

Add 1 mL of DI water to the test tube to dissolve some of the salts. What color is the mixture?

4.

In the hood, carefully add 3 mL of concentrated ammonium hydroxide (15 M NH4OH) to the test tube. Record your observations.

5.

Slowly add 3 mL of 10% H2O2 in 1 mL portions to the test tube. The hydrogen peroxide is used as an oxidizing agent. It will produce oxygen gas and the test tube will warm up. If the peroxide is added too fast the solution will bubble over the side of the test tube. Note the color of the solution. Does a precipitate form? Record your observations. You must observe the formation of oxygen gas. If not, the H2O2 needs to be replaced. Report the problem to your TA and start over. 2 CoCl2∙6 H2O + 2 NH4Cl + 8 NH3 + H2O2 →

2 [Co(NH3)5H2O]Cl3 + 12 H2O

6.

Let the effervescence of O2 continue for approximately 15 minutes with occasional swirling. Check on Part 1.

7.

In the hood, add 3 mL of concentrated HCl dropwise to the solution. Record your observations.

8.

In the hood, heat the solution in a 100°C water bath for 15 minutes. Record your observations. HCl [Co(NH3)5H2O]Cl3 ⎯⎯→

9.

[Co(NH3)5Cl]Cl2 + H2O

Let the mixture cool to room temperature and vacuum filter the precipitate. Continue suction for a few minutes to allow the precipitate to dry. Weigh the product and record the mass.

10. Determine the limiting reagent. Calculate the theoretical and the percent yield of product. Finish Part 1. Limiting Reagent Theoretical Yield Percent Yield 11. Dissolve ~0.03 g of [Co(NH3)5Cl]Cl2 in 10mL DI water. Use the spectrophotometer to measure the absorption spectrum. The cuvette should be half-full. 12. Close the program when you are finished. Restart the computer after the last person has finished. ■ 234

SYNTHESIS OF TRANSITION METAL COORDINATION COMPLEXES ■

Part 3. Week 1 and Week 2. Synthesis of trans-[Co(en)2Cl2]Cl NOTE: Clean glassware is very important. Glassware from the stockroom or from your drawer may appear clean but is not necessarily clean enough for this synthesis. 1.

Weigh 1.5 grams of CoCl2∙6 H2O and record the mass to the nearest 0.001 g in your notebook. Put the metal salt in a large test tube. Add 2 mL of DI water to the test tube.

2.

In the hood, measure 5.0 mL of 10% ethylenediamine in a 10 mL graduated cylinder. Carefully add this to the test tube and mix the contents thoroughly with a stirring rod.

3.

Slowly add 5 mL of a fresh 10% H 2O2 solution in 1 mL portions to the test tube. As the hydrogen peroxide is added to the solution, O2 (g) will be produced. Mix the solution with a stirring rod. Record your observations.

4.

Fill a 600 mL beaker two-thirds full with hot tap water. Add a stir bar and set the beaker on a ho...