Download Halide ions as ligands: preparation and characterization of

Part III - Groups 17 and 18 elements
Experiment 9
Halide ions as ligands: preparation and characterization of
hexahalogenotellurates (IV)
Introduction
All the halide ions have the ability to function as ligands and they are by far the most common of all
ligand types, forming perhalogeno complexes with all metal and many metalloid ions, e.g. FeCl4- , IrCl6- ,
SiF 62- , as well as many mixed complexes in conjunction with other ligands, e.g. [Co(NH3)5Br]2+. In
this course we are focusing on the main group elements, and in this experiment we will prepare and
study perhalogeno complexes of the metalloid tellurium. Tellurium is a member of Group 16 of the
periodic table, so that this experiment is a study of both Group 16 and Group 17 elements.
One of the important general questions that arises concerns the relative affinities of the several halide
ions for a given metal or metalloid ion. There is no simple answer to this, however. For crystalline
materials it is obvious that lattice energies play an important role, and there are cases, such as BF 4- ,
BBr4- , in which the heavier halogens give less stable complexes, and are in fact known only in BCl4- ,
the form of crystalline solids with large cations, where lattice energies are governing. In considering the
stability of the complex ions in solution, it is important to recognize that (a) the stability of a complex
involves, not only the absolute stability of the M-X bond, but also its stability relative to the stability of
ion-solvent bonds, and (b) in general an entire series of complexes will exist, Mn+(aq), MX(n-1)+(aq),
MX2(n-2)+(aq), --- MXx(n-x)+(aq), where x is the maximum coordination number of the metal ion. Of
course these two points are of importance in all types of complex ions in solution, but they have
probably been best studied for halide complexes.
A survey of the available data on the stability of halide complexes shows that generally the stability
decreases in the series F . Cl . Br . I, but with some metal ions the order is the opposite, namely F , Cl ,
Br , I. We may use the Hard and Soft Acids and Bases theory to rationalize such trends. The
underlying theory behind the applicability of the HSAB rules include the charge-radius ratio,
polarizability, and ability to use empty outer d orbitals for back-bonding are significant factors. From
the available results it appears that for the complexes where the replacement stability order is Cl , Br , I,
the actual order of M-X bond strength is Cl . Br . I, so that ionic size and polarizability appear to be the
critical factors.
For hexahalogenotellurates(IV), the fluorides are by far the least stable, and have not been isolated to
this date. The stability of the remaining halogenates (Figure 9-1: Cl, 1, , Br, 2, and I, 3) is found to be
Cl > Br > I.
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Experiment 9
Part III - Groups 17 and 18 elements
Br
Cl
Cl
Cl
Br
Cl
Cl
Br
Te
Br
Br
I
I
Br
Te
Te
Cl
I
I
I
I
3
2
1
These six-coordinate anions are isoelectronic with IF6- and XeF6. There has therefore been persistent
interest in the physical structures of these compounds, since they should possess a lone pair. This
therefore poses a challenge to the VSEPR theory of structure prediction. In fact, it has been speculated
that the lone pair might be stereochemically inactive. Since our laboratory is not equipped to prepare
the noble gas fluorides, this experiment is also meant as a "model system" for this aspect of the chemistry
of Xe.
Instructional goals:
Properties of the following elements are highlighted: Cl, Br, I, Te, {Xe}
(1) The preparation of salts of non-metal halogen complex ions.
(2) Experience in the use of vibrational spectroscopy in the study of high-symmetry complex
ions.
(3) Experience in the use of electronic absorption spectroscopy for the study of the electronic
structure of inorganic compounds.
(4) To gain an understanding of Mössbauer spectroscopy and its insights into chemical
structure.
(5) To acquaint the student with the concept of "stereochemically inactive lone pairs", and the
resulting controversies.
Pre-lab exercise
1.
Write balanced equations for Part 1 and Part 2 of the preparation you are following (i.e. either
A, B or C).
2.
What is the structure of [TeCl6]2- as predicted by the VSEPR theory of chemical structures?
Sketch the structure and assign the point group. Now prepare a model using HyperChem. For
this function, draw Te with six Cl atoms attached. Then add a lone pair from the HC periodic
table. Invoke model build. No further optimization is possible.
3.
What is the actual structure of [TeCl6]2- in its salts (as determined by crystallography, Ref. 9)?
Sketch this structure and assign its point group. To build this model, take the previous example
and optimize it using the PM3 method. Be sure to set the charge correctly.
4.
What range of IR frequencies do you need to record to get the data required for this
experiment?
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Part III - Groups 17 and 18 elements
5.
Experiment 9
Map out the timing of your afternoon's work. Use free gaps of time to do other operations. Be
realistic in time allotted for each operation!
SAFETY NOTES
1
2.
3.
4.
Tellurium and tellurium compounds are toxic. They are converted in the body to
dimethyl telluride which imparts a garlic-like odour to the breath and sweat. Heavy
exposures may, in addition, result in headache, drowsiness or respiratory arrest.
Precautions: handle in a fume hood at all times - wear polyethylen disposable gloves place ALL the solid wastes, including the gloves, in the special waste container!
Concentrated HCl, HBr and HI are caustic and release vapours of the acid. Handle
only in the fume hood.
Aqueous wastes from this experiment go into their own separate waste container, and
NOT down the drain!
Procedure 9A - Preparation of K2[TeCl6] and [NBu4]2[TeCl6]
Part 1 - Preparation of K2[TeCl6]
2.0 g of tellurium(IV) oxide, TeO2, is dissolved in the minimum quantity, up to 30 mL, of concentrated
hydrochloric acid. A saturated solution of two molar equivalents of KCl in water is added with stirring.
The solution is evaporated on a steam bath (FUME HOOD!) and stirred until the yellow crystals settle
well. The salt is recrystallized from 4 M HCl and is dried in a vacuum desiccator over solid NaOH.
Part 2 - Preparation of [NBu4]2[TeCl6]
1.0 g of K2TeCl6 is dissolved in 4 M HCl and two molar equivalents of tetra-n-butylamonium chloride
in 1 M HCl is added. If the solution of NBu4Cl is cloudy due to organic impurities, it must be filtered by
gravity through a fine filter-paper before it is added to the tellurate. The yellow precipitate which is
formed immediately is filtered off on a Büchner funnel, washed twice with a small amount of 1 M HCl
and dried in a vacuum dessicator.
Procedure 9B - Preparation of K2[TeBr6] and [NBu4]2[TeBr6]
Part 1 - Preparation of K2[TeBr6]
2.0 g of tellurium(IV) oxide, TeO2 is dissolved in the minimum quantity, up to 30 mL, of concentrated
hydrobromic acid. A saturated solution of two molar equivalents of KBr in water is added with stirring.
The solution is evaporated on a steam bath (FUME HOOD!) and stirred until the orange crystals settle
well. The salt is recrystallized from 4 M HBr and is dried in a vacuum desiccator over solid NaOH.
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Experiment 9
Part III - Groups 17 and 18 elements
Part 2 - Preparation of [NBu4]2[TeBr6]
1.0 g of K2TeBr6 is dissolved in 4 M HBr and two molar equivalents of tetra-n-butylamonium bromide
in 1 M HBr is added. If the solution of NBu4Br is cloudy due to organic impurities, it must be filtered
by gravity through a fine filter-paper before it is added to the tellurate. The orange precipitate which is
formed immediately is filtered off on a Büchner funnel, washed twice with a small amount of 1 M HBr
and dried in a vacuum dessicator.
Procedure 9C - Preparation of K2[TeI6] and [NBu4]2[TeI6]
Part 1 - Preparation of K2[TeI6]
2.0 g of tellurium(IV) oxide, TeO2, is dissolved in the minimum quantity, up to 30 mL, of concentrated
hydroiodic acid. A saturated solution of two molar equivalents of KI in water is added with stirring.
The solution is evaporated on a steam bath (FUME HOOD!) and stirred until the black crystals settle
well. The salt is recrystallized from 4 M HI and is dried in a vacuum desiccator over solid NaOH.
Part 2 - Preparation of [NBu4]2[TeI6]
0.5 g of K2TeI6 is dissolved in 4 M HI and two molar equivalents of tetra-n-butylamonium iodide in 1
M HI is added. If the solution of NBu4I is cloudy due to organic impurities, it must be filtered by gravity
through a fine filter-paper before it is added to the tellurate. The black precipitate which is formed
immediately is filtered off on a Büchner funnel, washed twice with a small amount of 1 M HI and dried
in a vacuum dessicator.
Characterization
Make sure that the samples are completely dried before attempting to characterize them! Obtain the
melting point and IR spectra of both compounds prepared. Record the spectra as KBr pellets. Be
very careful to keep the samples dry during the required manipulation. Also collect an IR spectrum of
the corresponding [NBu4]X salt to aid in the interpretation of your IR spectra. Record the UV-visible
absorption spectra of the [NBu4]2[TeX6] salt as a dilute solution (0.1 mM) in CH2Cl2 (quartz cells, not
plastic!) Prepare solutions of accurately known concentration, and use 1 cm quartz cuvettes to measure
the spectrum. It may be necessary to use solutions of different concentration in differing regions of the
spectrum. Use either the Cary-1E or the Varian DMS spectrophotometers for this study.
Molecular Modeling
1.
2.
Use the PM3 optimized octahedral structure of TeCl62– for all parts of this lab. Bromide and
iodide complexes are difficult to model by quantum mechanics, but we can asume that the same
principles apply to all three halide complexes, and indeed to that of the fluoride as well.
Calculate a geometry optimization with PM3 on the dianion (set charg and multiplicity correctly).
Then calculate the orbitals. Note that you may have to adjust the Orbital Exponent term in the
Plot Orbital menu box in order to “see” what is going on in this calculation. Usually this means
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Chemistry 3810 Laboratory Manual
Part III - Groups 17 and 18 elements
3.
4.
Experiment 9
that you will have to increase the exponent value, which has the effect of shrinking the outside
surface of the orbital. Once you do this correctly, you should have no difficulty in identifying the
consituent atomic orbitals in the MO’s. Also, be sure to use Align Molecule from the Edit menu
(primary axis to z, secondary to x) before performing your calculation.
Carefully look at all the oribtals in the MO diagram. They are situated in three large groups.
What is the nature of the lower group? Should you ignore these in any interaction diagram?
Which set of orbitals consitute net bonding between the chloride ions and tellurium? Which
correspond only to the 18 lone pairs expected on the halogens?
Using your analysis of the PM3 MO diagram, construct your own interaction diagram. Use only
those symmetry adapted orbitals for the halogens that interact with Te. Ignore all that are lone
pair in character. Show bonding and antibonding interaction, and occupy with the right number of
net electrons (i.e. ignore core or lone pair electrons). Identify the HOMO. What is the nature of
the HOMO?
Report
Hand in your products as well as all original spectra. Identify the bands in the IR spectra according to
their origin, and attempt a vibrational analysis of the Te-X bands. Write a full report according to the
instructions given at the front of the laboratory manual.
At the end of your report, address the following additional questions:
1.
2.
3.
4.
What is meant by "stereochemical inactive lone pair". Discuss how this concept applies to
[TeX6]2- ions, and to the isoelectronic compounds BrF6- , IF6- and XeF6. Is there any
evidence that the lone pair in these EX6L molecules can be streochemically active?
Briefly describe the principles of Mössbauer spectroscopy. (See ref. 20 for a simplified
discussion of this technique.) Discuss the results reported in the literature for the Mössbauer
spectra of [TeX6]2- salts. Contrast the three halides.
Briefly discuss the origin of the UV-visible absorption bands using a molecular orbital approach.
(Hint: use the octahedral MO diagram you developed in the computational exercise, and
determine the number of filled orbitals. Transitions between filled and empty orbitals show up
as absorptions in the UV-visible region of the electromagnetic spectrum.)
Consult with your colleagues preparing the other halide complexes of tellurium(IV). Use their
results as well as your own to describe the colours of the Cl, Br and I complexes. Put this on a
quantitative basis by preparing a table of (i.e.) for the various bands of the three types of
complexes. What does this data suggest about the strength of the Te-X bonds?
References
1.
F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th edition (Wiley, N.Y.,
1988), p 508. [QD151.2 C68]
2.
N.N. Geenwood and A. Earnshaw, Chemistry of the Elements (Pergamon Press, Oxford,
1984), p 908. [QD466.G74]
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Experiment 9
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Part III - Groups 17 and 18 elements
G. Wulfsberg, Principles of Descriptive Inorganic Chemistry (Brooks/Cole, Monterey, Cf.,
1987), p 40. [Reserve]
D.F. Shriver, P.W. Atkins, C.H. Langford, Inorganic Chemistry (Freeman, N.Y., 1990), p.
387-388.
G. Wilkinson, R.D. Gillard and J.A. McCleverty, Comprehensive coordination chemistry
(Oxford: Pergamon Press, 1987). Vol 1, p. 213-330 & Vol. 3, p.302-303. [QD474.C65]
H. Marshall, Inorganic syntheses (1946) 2, 188-190. [QD151.A1.I5]
I.D. Brown Can. J. Chem. (1964) 42, 2758-67. [QD1.C285]
N.N. Greenwood and B.P. Straughan, J. Chem Soc. (A) (1966), 962-4. [QD1.C6]
T.C. Gibb et. al., J. Chem Soc. (A) (1970), 212-17. [QD1.C6]
J.D. Donaldson et. al., J. Chem. Soc. Dalton Trans. (1975) 1980-83. [QD1.C62]
D.S. Urch, J. Chem. Soc. (1964), 5775-81. [QD1.C6]
L.S. Bartell, J. Chem. Phys. (1967) 46, 4530. [QC1.J94]
R.M. Gavin, Jr., and L.S. Bartell, J. Chem. Phys. (1968) 48, 2460. [QC1.J94]
H. Kim, H.H. Claasen and E.Pearson, Inorg. Chem. (1968) 7, 616. [QD1.I615]
K. Seppelt, Acc. Chem. Res. (1979) 12, 211. [QD1.A275]
R.D. Burbank and G.R. Jones, J. Am. Chem. Soc. (1974) 96, 43. [QD1.A5]
I.S. Butler and J.F. Harrod, Inorganic Chemistry (Benjamin/Cummings, Cf., 1989).
[QD151.2.B88]
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