Inorganic Laboratory Experiments

Aims and Objectives

These experiments were designed to be carried for CHEM2101 and some are included in the CHEM2111 Laboratory course of 4 hour sessions.
In the old course:
In the first week, several simple coordination complexes are prepared.
For weeks 2,3 and 4 a number of spectroscopic investigations are performed, including: IR, UV/Vis and a determination of a magnetic moment. This is done using a rotation scheme to accommodate the time required at each instrument and to give each student the opportunity to record their own spectra. A part of the session will be spent in the Chemistry Resource Centre using the Tanabe-Sugano programs to interpret the Visible spectra recorded.
During this period as well, some qualitative tests are performed on first row transition metal ions and the analysis of the ferrioxalate complex is performed.
In the fifth week, a kinetics experiment on the acid hydrolysis of a metal complex is carried out as a group exercise. Each student is assigned a variation such as initial concentration of complex, temperature, ionic strength or [H+] concentration.
At the end of the session, all rate constants are pooled so that more extensive analysis can be done.

In the final week, any outstanding experiments are completed.
As you do these experiments you should work toward the following objectives:

It is intended that the laboratory experiments will reinforce the lecture material and give students practice at assigning spectral bands and interpreting magnetic properties as well as giving an introduction to the study of reaction kinetics.


Experiment 2016-1 Perborate synthesis
Experiment 2016-2 Computer Laboratory on Symmetry elements and operations

Experiment 1:
Preparation of some chromium(III) complexes, either:
  1)   [Cr(en)3]Cl3
   and K[CrEDTA(H2O)]  
  
  2)   [Cr(urea)6]Cl3 
   and K3[Cr(NCS)6]  
   
or  3) cis-[CrCl2(en)2]Cl.1.5 H2O
  and cis-K[Cr(ox)2(H2O)2].2H2O
The final reports should include the Vis and IR spectroscopic analysis and Gouy or Evans Method determination of the magnetic moment. It is expected that the discussions will be based on the appropriate Tanabe-Sugano diagrams and include an assessment of how the measured values compared with what was expected.
Experiment 2:
Preparation and analysis of potassium trisoxalatoiron(III) trihydrate
Experiment 3a:
Preparation and aquation of trans-dichlorobis(1,2-diaminoethane)cobalt(III) chloride
Experiment 3b:
Preparation of pentaamminechlorocobalt(III) chloride and the kinetics of its aquation
Experiment 4:
Preparation of some Werner complexes.
Experiment 5:
Preparation and reactions of ferrocene.
Experiment 6:
Qualitative Tests: Each student should carry out the qualitative tests on 2 transition metal ions.

All samples should be submitted in properly labelled (name, sample, weight etc.) containers for marking, together with your reports.

Experiment 1.
The preparation of some chromium(III) complexes

1 a) Preparation of tris-(1,2-diaminoethane)chromium(III) chloride,
[Cr(en)3]Cl3

1 g granular zinc, 2.66 g CrCl3.6H2O, 10 cm3 of 1,2-diaminoethane (ethylenediamine), and 10 cm3 of methanol are refluxed on a steam bath for one hour.
The solution is then cooled to room temperature and the yellow product collected on a sintered glass. The filtered product is then washed with acetone/methanol mixture until the washings are colourless. Unreacted zinc is separated by dissolving the product using a minimal amount of distilled water. The yellow solid is precipitated from the filtrate with acetone. Filter and allow the product to dry, determine the percentage yield and transfer to a labeled vial.

The X-ray structure of this complex has been determined and can be viewed with Jmol.

1 b) Preparation of K[Cr(EDTA)(H2O)]

A suspension of H4EDTA (3 g) and KOH (0.8 g) in 25 cm3 of distilled water is transferred to a Teflon reactor. To this suspension is added Cr2(SO4)3 (3.3 g). The reactor is securely sealed and heated in a microwave oven for 5 minutes. The resulting red solution is transferred to an evaporating dish and placed on a steam bath. Evaporate until dryness, and collect the purple solid using methanol as washing solution. Dry at the pump and determine the yield.

The X-ray structure of this complex has been determined and can be viewed with Jmol. Note that the EDTA has one "arm dangling".

2 a) Preparation of hexakis-(urea)chromium(III) chloride

Chromium chloride hydrate (2.7 g) and urea (3.6 g) are dissolved in 10 cm3 of distilled water and a few drops of 3M HCl is added. The solution is heated on a steam bath until a crystalline crust forms. The slurry obtained is dissolved in the minimum of water at 50-60 C and rapidly filtered. The salt crystallizes as light green needles. Dry at the pump and determine the yield.

The X-ray structure of this complex has been determined and can be viewed with Jmol.
The FTIR of this complex has been determined and can be viewed with JSpecView.

2 b) Preparation of Potassium hexathiocyanatochromate(III), K3[Cr(NCS)6]

Make an aqueous solution of potassium thiocyanate, KSCN (2.5 g), chrome alum ( {KCr(SO4)2.12H2O} 3.0 g) using distilled water (10 cm3). Pour the solution into an evaporating dish and place on a steam bath. Evaporate to dryness, to obtain a mass of red crystals. Extract the solid, via suction filtration, using alcohol. The desired K3[Cr(NCS)6], should dissolve very readily while K2SO4 remains as a residue. After evaporation of the filtered alcohol extract, collect the dark red-violet crystals. Dry at the pump and determine the yield.

The X-ray structure of this complex has been determined and can be viewed with Jmol.

3 a) Preparation of cis-dichlorobis-(1,2-diaminoethane)chromium(III) chloride,
cis-[CrCl2(en)2]Cl.1.5H2O

In a 250 cm3 beaker, dissolve chromium chloride (10 g) in 30 cm3 dimethylformamide. Heat the solution on a hot plate and reduce the volume to half. Add 5 cm3 1,2-diaminoethane slowly using a dropper. Precipitate the purple solid using methanol and collect using a sintered glass filter funnel. Dry at the pump and determine the yield.

The X-ray structure of this complex has been determined and can be viewed with Jmol.

3 b) Preparation of cis-K[Cr(ox)2(H2O)2].2H2O.

Prepare an intimate mixture of finely ground potassium dichromate (2 g) and oxalic acid dihydrate (6 g) and heap the powder in a 15 cm evaporating dish. Place one drop of water in a small depression in the mixture and cover the dish with a watch glass. After a short induction period the reaction commences and soon becomes vigorous with the evolution of steam and carbon dioxide.
The product of this reaction is a purple viscous liquid over which is poured 20 cm3 of ethanol and the mixture stirred and ground with a glass rod until the product solidifies. If solidification is slow, decant the liquid and repeat the process with a second portion of ethanol. Filter, dry at the pump, and record the yield.

The X-ray structure of this complex has been determined and can be viewed with Jmol.

Experiment 2.
The preparation of Potassium tris(oxalato)ferrate(III) trihydrate

K3[Fe(C2O4)3]
Mark the level of 45 cm3 water in a 250 cm3 beaker. To a well-stirred solution of 5 g of ferrous ammonium sulfate in 20 cm3 of warm water containing 1 cm3 of dilute sulfuric acid in the beaker, add a solution of 2.5 g of oxalic acid dihydrate in 25 cm3 of water. Slowly heat the mixture to boiling (beware of bumping) then allow thc yellow precipitate to settle. Decant the supernatant through a Buchner funnel making sure it has a properly fitted filter paper. Add 15 cm3 of hot water to the solid, stir and filter. Drain well and then transfer all the precipitate from the paper back into the beaker with 10 cm3 hot water.
Add 3.5 g solid potassium oxalate monohydrate and heat to approximately 40 C. Add slowly, using a dropper, 9 cm3 of "20 vol" hydrogen peroxide. (If the precipitate looks yellowish, not brown and settles readily, decant the supernatant, add a solution of 0.2 - 0.4 g potassium oxalate monohydrate in 1 - 2 cm3 water and then hydrogen peroxide dropwise until the precipitate dissolves. Then add the previously decanted supernatant). Heat to boiling, and add a solution of 2 g of oxalic acid dihydrate in 30 cm3 of water in portions, add 20 cm3 initially, then if the brown precipitate still remains, add more solution little by little until it all dissolves. Boil the clear solution down to a volume of 40 - to 50 cm3, filter through a Buchner funnel with well fitting paper and add 95% ethanol slowly until a precipitate starts to form (~30 cm3). Redissolve any crystals by heating (beware of fire) and leave to crystallise.
Filter and wash the crystals on the Buchner with a 1:1 ethanol / water mixture and finally with acetone, (beware fire again). Dry in the air and weigh. The complex is photosensitive and should not be exposed to light unnecessarily. Store in a sample bottle wrapped in foil.
An IR spectrum is available.

Determination of the oxalate content of Potassium trisoxalatoferrate(III) trihydrate.

The iron(III) complex is first decomposed in hot acid solution and the free oxalic acid is titrated against standard (0.02 M) potassium permanganate solution. No indicator is required.
In duplicate, weigh accurately about 0.2 g of the potassium trisoxalatoferrate(III) complex previously prepared. Boil the sample with 50 cm3 of 1 M sulfuric acid in a conical flask. Allow the solution to cool to about 60°C and titrate slowly with the potassium permanganate solution provided (which you will need to standardise). Continue until the warm solution retains a slight pink colouration after standing for about 30 sec.
Calculate the percentage by weight of oxalate in the complex, compare this with the theoretical value and thus obtain the percentage purity of the complex.

MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
C2O42- → 2CO2 + 2e-

Photochemical reactions of Potassium trisoxalatoferrate(III) trihydrate.

Prepare duplicate solutions containing 0.2 g accurately weighed of your sample in 15 cm3 of dilute sulfuric acid. Dilute the solutions to 50 cm3 with distilled water and expose them to sunlight for one hour (note carefully what happens). Titrate with your standardised permanganate to determine the amount of reducing agent present.
Expose a small portion of your product to sunlight for several hours. Make sure that the crystals have been ground to a fine powder and that you periodically stir the crystals so that all the sample gets exposed equally to the sunlight. Perform the following tests on samples of both irradiated and unirradiated complex:
Dissolve your sample in dilute sulfuric acid and divide the solution into three.
  1) treat with a freshly prepared solution of potassium ferrocyanide.
  2) treat with a freshly prepared solution of potassium ferricyanide.
  3) treat with a solution of potassium thiocyanate.
Record carefully all observations.

Experiment 3:
The Mechanism of Aquation of trans-dichlorobis(1,2-diaminoethane)cobalt(III) chloride

Introduction

Coordination complexes of cobalt(III) undergo ligand exchange or substitutions slowly as compared to many other transition metal compounds. Their slow reactions have made them suitable for kinetic investigations of their reaction mechanisms. The present experiment involves a kinetic study of the acid hydrolysis of trans-[CoCl2(en)2]Cl, whereby the probable mechanism of the octahedral cobalt(III) substitution can be determined. The reaction will be conducted such that each student will investigate one of the following variations: pH, temperature, concentration and ionic strength. The entire class will collate these results which can be used to show the effect of these variations on the aquation.
Complexes of this type undergo aquation in a step-wise fashion according to the equations:

trans-[CoCl2(en)2]+ + H2O → trans-[CoCl(en)2(H2O)]2+ + Cl-
trans-[CoCl(en)2(H2O)]2+ + H2O → [Co(en)2(H2O)2]3+ + Cl-

where the first step is the one to be measured quantitatively in this experiment.
In principle, two fundamentally different mechanisms are possible for these reactions; a dissociative or associative mechanism. The kinetic rate laws expeected for these two types of mechanism are:

dissociative: Rate = k1[complex]
associative: Rate = k2[complex][H2O]
                           = kobs[complex]

that is they are dependent only on the concentration of the complex and are first order. This observation, however, furnishes no information as to the role played by the water and does not give any information about the molecularity of these reactions.
Nevertheless the way in which the rate constant is affected by various changes in the nature of the complex ion is expected to give us information about the mechanism. It has been found that increasing chelation such as replacing two NH3 ligands by one ethylenediamine slows down the rate of acid hydrolysis. Allowing for the chelation effect the divalent monochloro complexes react about 100 times slower than the univalent dichloro complexes.
trans-[CoCl2(en)2]Cl

Preparation of trans-[CoCl2(en)2]Cl

CoCl2,6H2O (2 g) is dissolved in 2 cm3 of water in a beaker and 1 cm3 of 1,2-diaminoethane (en) in 5 cm3 of water is slowly added cautiously and with stirring. The solution is cooled in an ice bath to 5°C and 2 cm3 of H2O2 (30%) is slowly added while maintaining the temperature at 5°C. [ CAUTION: Keep H2O2 off the skin and eyes!]. Then the solution is gently warmed to about 60-70°C for 15-20 minutes.
Concentrated HCl (4 mL) is then added, and the solution evaporated on a steam bath with occasional stirring to about 10 cm3. After cooling the solution in an ice-bath, 3 mL of ethanol is added and the mixture cooled for a further 10 minutes. The resultant green crystals of trans-[CoCl2(en)2]Cl.HCl.2H2O are filtered onto a sintered glass Buchner funnel, washed with ethanol and sucked dry.
An IR spectrum is available.
To drive off the HCl of crystallization, place the dark green crystals in a small beaker containing 5 cm3 of methanol and vigorously stir these with a glass stirring rod. Transfer the resultant slurry to a large test tube. Place the test tube in a beaker of water, then heat the water (gently at first) until the methanol has evaporated and no more HCl gas is driven off. (The presence of HCl can be tested by holding a piece of moist litmus paper at the mouth of the test tube).
Boiling for 15 minutes after the methanol has evaporated is usually sufficient. Trans-[CoCl2(en)2]Cl thus obtained is a light green powder.

Spectral Studies

The visible spectrum of the complex ion, trans-[CoCl2(en)2]Cl, has been investigated thoroughly. The peak at 625 nm is clearly defined with an extinction coefficient of 3.33 m2mol-1. The trans configuration is well characterized by its shoulder at 440 nm. In acidic solution the trans-dichloro complex gradually converts to a mixture of 35% cis- and 65% trans-[CoCl(en)2(H2O)]2+. By repetitively scanning the spectrum, this conversion is revealed by the presence of three poorly defined isosbestic points (588 nm, 448 nm and 408 nm).

Kinetic Studies

Vis Spec aquation study

Kinetic runs for the acid hydrolysis of the trans-[CoCl2(en)2]Cl complex are to be recorded (for single beam use 515 nm where the increase in absorbance shows a maximum change to occur between the reactant and product). In addition to the above CSV file of the full spectrum run over several hours, the first and last scan from a typical student run are available as JCAMP-DX files for comparison.
Each student will be assigned to study the effect of one variation on the observed rate constants, from the following parameters:

  (a)   Complex concentration (3 mM to 15 mM).
  (b)   Hydrogen ion concentration (0.1 M to 0.5 M).
  (c)   Ionic strength (0.1 to 0.5 M).
  (d)   Temperature (25 to 45°C).

A typical run should be done as follows.

Add required volumes of stock HNO3 and NaNO3 solutions to a 50 cm3 volumetric flask and add distilled water to make it about 80% full. The flask is then immersed in a thermostatic water bath for temperature equilibration. Weigh the required amount of the complex on an analytical balance and dissolve the complex with a small volume of distilled water in a beaker. Transfer the solution quantitatively to the thermostated 50 cm3 flask and make up to the mark with distilled water. Transfer immediately about 3 cm3 of the solution from the flask to a 1 cm glass or plastic cell, start your stop watch and place the cell in the cell holder of the spectrophotometer. Read and record the absorbance at 10 minute intervals for at least 2 hours. The absorbance reading at infinite time can be taken after 5 hours. Alternatively the reading at infinite time can be obtained by first warming a sample of the mixture on a water-bath for 5 minutes and then placing in the spectrometer.

For first order kinetics the following rate expression is expected:

ln( (Ainf - At) / (Ainf - A0) ) = kt

where k is the rate constant
A0 is the initial Absorbance
Ainf is the Absorbance at infinite time
and At is the Absorbance at any time, t.
Calculate the pseudo first order rate constants by plotting ln(Ainf - At)vs time, t, in sec.

Questions

1. Draw the two structures of the cis and trans isomers of [CoCl2(en)2]Cl. Which, if any, of these geometric isomers is potentially resolvable into optically active isomers?
2. The green product first isolated, is best represented as trans-[CoCl2(en)2]+[H5O 2]+2Cl-. This contains an example of a hydrated proton, where the O-H-O moiety is linear and the O-O separation is 200 pm. Draw the likely structure of the cation. Give examples of other known hydrated proton structures.
For a clue see here.
3. In the complex [CoCl(NH3)5]2+ increasing chelation such as replacing the two NH3 ligands by one ethylenediamine slows down the rate of acid hydrolysis. Why?
4. Why should the rate of hydrolysis of [CoCl2(en)2]+ be 100 times faster than that of [CoCl(en)2(H2O)]2+ ?
5 Tabulate all the experimental data collected during the class and comment on the effect of (a) pH, (b) temperature and (c) ionic strength on the rate of hydrolysis.
6. Calculate ΔH# and ΔS# values from your temperature dependence data and compare these values with the values obtained for other chloroamine complexes of cobalt(III).

Complex k ΔH# (kJ/mol) ΔS# (J/deg mol)
trans-[CoCl2([13]aneN)4]+ 6.76 x 10-4 107.1 52.7
trans-[CoCl2([14]aneN4)]+ 1.1 x 10-6 102.9 -12.6
trans-[CoCl2(trien)]+ 3.5 x 10-3 108.8 64.9
trans-[CoCl2(en)2]+ 3.2 x 10-5 109.6 58.6

References

F. Basolo and R.G. Pearson, Mechanism of Inorganic Reactions, 2ed., John Wiley and Sons, New York, 1967, p162.
B. Douglas, D.H. Daniels and J.J. Alexander, Concepts and Models of Inorganic Chemistry, 2nd Edition, John Wiley and Sons Inc., New York, 1983, pps 361-363.

Experiment 5 :
Preparation of Ferrocene

Introduction

The first step involves the preparation of cyclopentadiene. This is obtained by cracking the dimer, dicyclopentadiene, by distillation (BP ~42C). This corresponds to a retrograde Diels-Alder reaction and the cyclopentadiene monomer thus obtained dimerises slowly (t ½ ~12 hours at room temperature) and should be used without delay. It is also highly flammable and should be stored in ice.
The next step makes use of the fact that alkali metal hydroxides are able to deprotonate the cyclopentadiene when used in a non-hydroxylic solvent in which they are essentially insoluble, in this case DMSO.
The final step is the reaction with the ferrous chloride which should be done quickly to avoid too much air affecting the reaction.
It is essential to efficiently coordinate these steps of the procedure if good yields are to be obtained. The experiment MUST be carried out in a fume hood.

Procedure

The equipment for cracking the dicyclopentadiene will be set up by a demonstrator. Each student requires 4 cm3 of the cyclopentadiene. Take two boiling tubes each fitted with a cork. Into the first place 6 cm3 of dimethyl sulfoxide and 3g of KOH pellets which have been finely ground in a mortar and pestle. (Caution - If KOH is spilt on the bench, it needs to be cleaned up immediately).
Into the second boiling tube, place 10 cm3 of dimethyl sulfoxide and 2 g of anhydrous ferrous chloride.
The air in the boiling tubes must now be replaced by dinitrogen and the tubes shaken. Consult the demonstrator for the use of the dinitrogen gas cylinder.
The next two operations must be carried out quickly so as to prevent too much air getting into the boiling tubes. Add the 4 cm3 of cyclopentadiene to the KOH suspension, stopper the tube and shake well. When a dark red brown colour develops in the tube (about 3 minutes) add the ferrous chloride solution, stopper and shake. Cool in ice if it gets too hot. Allow the mixture to stand for 5 minutes then pour it into 200 cm3 of cold water and filter the resulting mixture using a Buchner Funnel. After drying at the pump for 20 minutes, transfer the solid and filter paper to a dry 200 cm3 beaker. Add 50 cm3 of 60-80 petroleum ether and heat to boiling on a water-bath. Filter into another dry 200 cm3 beaker using a filter funnel fitted with fluted filter paper. Concentrate the resulting orange solution on a water-bath until crystallisation begins (reduce to 5-10 cm3). Cool in ice to complete crystallisation.
After standing for 30 minutes, filter off the crystalline product and dry at the pump. This last filtration may be carried out in the open laboratory. Record your yield as a percentage based on FeCl2 and determine the M.P (this should be compared to the literature value).

Reactions of Ferrocene

a) Oxidation

A cyclic voltammogram of ferrocene (0.1 mM)
cv of ferrocene

recorded in DMF using [n-Bu4N]PF6 (0.1M) as supporting electrolyte shows a reversible oxidation wave at E1/2 ~0.5 Volt which indicates that at a potential slightly higher than this (eg 0.6 V) a bulk electrolysis experiment should produce the ferrocinium cation.
Alternatively an oxidant such as H2SO4 can be used. The ferricinium ion [Fe(C5H5)2]+ produced is soluble in water and may be precipitated using a large counter anion such as picrate, 12-tungstosilicate, reineckate- or even perchlorate. In this exercise we will use 12-tungstosilicate.
Dissolve 0.5 g of ferrocene in 10 cm3 concentrated sulfuric acid; allow the solution to stand for at least half an hour, then pour it into 150 cm3 distilled water. Stir the solution for a few minutes and filter off any precipitate. To the filtrate add a solution of 2.5 g 12-tungstosilicic acid in 20 cm3 water slowly with stirring. Collect the pale blue precipitate, wash with water and dry.

b) Acetylation

Ferrocene has an extensive aromatic-type reaction chemistry, which is reflected in its name and undergoes substitutions more readily than does benzene. One example is acetylation of both rings in the presence of a Friedal-Craft catalyst.
Alternatively, the acetylation of ferrocene can be carried out under milder conditions using acetic anhydride in phosphoric acid to yield the mono-acetylated product.

References

G.A. Perkins and A.O. Cruz, J. Amer. Chem. Soc., 1927, 49, 517.

Questions

1. Propose a reaction scheme for the acetylations. The 1H nmr spectra for ferrocene and a crude mixture obtained from the mono-acetylation using acetic anhydride are provided. Record the chemical shifts (NB. TMS=0) and interpret each spectrum.

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