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Inorganic Chemistry Year 3
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LABORATORY MANUAL
INORGANIC CHEMISTRY
III
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Inorganic Chemistry Year 3
CONTENTS
EXPERIMENT
1:
ISOMERISM AND KINETICS IN COORDINATION
3
CHEMISTRY
2
THE PREPARATION AND CHARACTERIZATION
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OF MESITYLENETRICARBONYLMOLYBDENUM(0)
3
MAGNETIC SUSCEPTIBILITY
8
4
THE PREPARATION AND ESR STUDIES OF
BIS(ACETYLACETONATO)
OXOVANADIUM(1V) AND ITS PYRIDINE ADDUCT.
18
5
ELECTRONIC SPECTRA OF NICKEL(11)
COMPLEXES (a d8 SYSTEM)
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6
THE EFFECT OF SYMMETRY ON THE INFRARED
SPECTRUM OF THE SULPHATE GROUP.
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Inorganic Chemistry Year 3
EXPERIMENT 1:
ISOMERISM AND KINETICS IN COORDINATION
CHEMISTRY
INTRODUCTION
The purpose of this experiment is to prepare and study the
kinetics of the interconversion of some of the isomers of a typical
coordination compound.
Historically, isomerism has played a central role in establishing the
basic concepts of coordination chemistry upon which so much of modern
inorganic chemistry is based.
On the other hand, the reaction kinetics of coordination
compounds is an area of continuing intense research activity. Much of the
present interest in the mechanisms of the reactions of coordination
compounds stems from their relevance to enzyme reactions where the
active site frequently involves coordination to a metal ion.
The first part of this experiment involves the preparation of the cis
and trans isomers of dichlorobis(ethylenediamine)cobalt(III) chloride while
in the second part the rate of conversion of the racemic cis isomer to the
trans isomer is followed spectrophotometrically.
EXPERIMENTAL
(a) Preparation of Trans-Dichlorobis(ethylenediamine)cobalt(III)
chloride.
To a solution of 16 g of cobalt(II) chloride hexahydrate in 50 cm³ of
water, contained in a 250 cm³ Erlenmeyer (or conical) flask, add with
stirring 60 g of a 10% solution of ethylenediamine. A vigorous stream of air
is drawn through the solution for 1-2 hours and then 35 cm³ of
concentrated hydrochloric acid is added and the solution evaporated on a
steam bath until a crust forms over the surface ( ca. 75 cm³ ).
The solution is allowed to cool and stand overnight before the
bright green plates of the hydrochloride, trans-[Coen2CI2]CI.HCI are
filtered. These are then washed with alcohol and ether and dried at 110oC .
At this temperature the hydrogen chloride is lost and the crystals crumbled
to a dull green powder. Yield expected is ca. 8 g. (50 % based on
ethylenediamine used).
(b) Preparation of the cis isomer.
A solution of the trans form yields the less soluble cis form on
evaporation to dryness over a steam bath. The evaporation may need to be
repeated but not more than three times (further repetition of the
evaporation causes substantial decomposition) to effect cis formation.
Finally unchanged trans form may be washed out with a little cold water.
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Inorganic Chemistry Year 3
(c) The cis to trans isomerization.
In methanol solution the purple cis form is isomerized to the
(thermodynamically) more stable green trans form. In order to follow the
reaction by spectrophotometry, it is necessary to record the spectra of both
the cis and the trans isomer separately. To save time the following
procedure is adopted.
Prepare a solution (0.02M; 14 mg in 25 cm³) of the trans isomer in
methanol and record its UV-visible spectrum. Repeat this procedure for the
cis isomer, recording the UV spectrum immediately after the solution has
been prepared; then quickly place the solution, tightly stoppered, into a
constant temperature bath at 35˚. Meanwhile from an examination of the
spectra (discuss with your Demonstrator) select the most appropriate
wavelength to follow the reaction. Measure the optical density D at this
wavelength at 15 minutes intervals for about 90 min. by withdrawing
samples from the thermostated solution. D∞ may be measured by allowing
the mixture to stand overnight.
The isomerization reaction is first order. Then ln{cis} = -kt + c. it is
easily shown that ln (D-D∞) = -kt + C' so that a plot of log (D-D∞) against
time should give a straight line of slope –k/2.303. (Note that since the
reaction is first order neither the extinction coefficient for the two isomers
nor the initial concentration of the cis isomer need be known). From your
results, determine k and the half-life for the reaction.
REFERENCE:
1. Journal of Chem. Ed. , 1962, 39, 634.
2. J.C.S. 1953, 2696.
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Inorganic Chemistry Year 3
EXPERIMENT 2: THE PREPARATION AND CHARACTERIZATION OF
MESITYLENETRICARBONYLMOLYBDENUM(0)
INTRODUCTION
Isolated organometallic compounds of the transition elements
have been known for a long time (e.g. Zeize’s salt – 1827). However, it was
not until the 1950’s, following the accidental discovery of ferrocene, that the
organometallic chemistry of the transition metals became a major area of
inorganic research.
Perhaps the most versatile starting materials for the laboratory
synthesis of transition metal organometallic compound are the metal
carbonyls. In this experiment molybdenum hexacarbonyl is reacted directly
with an organic aromatic (mesitylene). In general, the reaction can be
represented as:
Ar + Mo(CO)6
→ ArMo(CO)3 + 3CO↑
EXPERIMENTAL
(a) Preparation
NOTE: THIS PREPARATION MUST BE PERFORMED IN A FUME
HOOD. METAL CARBONYLS ARE VERY TOXIC.
Assemble the apparatus as shown in
the diagram using a 50 cm3 round bottom flask
and a simple reflux condenser (water need not
be circulated through the condenser: air cooling
is sufficient).
Add 1.8 g Mo(CO)6 to the flask followed
by 10 cm3 of mesitylene and 10 cm3
decalin. Flush the apparatus with a moderate
stream of nitrogen for about 5 min. After the
nitrogen flow is turned off and the stopcock
closed, bring the mixture to a moderate boil with
the heating mantle. Reflux for 2-3 hr (yield
improves with stirring), then turn off the heat and
immediately flush with nitrogen. This facilitates
the removal of unreacted Mo(CO)6 from the
solution and also prevents the mineral oil in the
bubbler from being sucked back into the reaction
vessel.
Allow the mixture to cool to room temperature, then filter on a
medium frit (aspirator pump). For purification, the crude product is
dissolved in the minimum amount of hot CH2Cl2 (FUME CUPBOARD),
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Inorganic Chemistry Year 3
filtered hot (medium frit*, aspirator pump) and then allowed to cool. The
resulting lemon coloured crystals are collected and dried on a frit. If
necessary, additional product can be obtained by reducing the volume of
the mother liquor. Further purification of the recrystallized product may be
achieved by vacuum sublimation (100ºC, high vacuum).
(b) Characterization.
1. IR Spectrum.
Record the IR spectrum of the product as a nujol mull over the
whole range and assign the important bands. Also record the IR spectrum
of the compound in solution in CHCl3 in the region 1800-2000 cm-1.
Comment on the differences between the two spectra in this region.
2. NMR Spectrum.
Prepare 0.5 cm3 of a saturated solution of the product in the
specially purified CHCI3 (obtainable from the Laboratory Assistant) and
record the NMR spectrum. Comment on the spectrum particularly the
information it provides about the structure and bonding of the complex.
Explain why free mesitylene shows two peaks at 2.25 and 6.78 ppm (vs.
TMS in CDCI3 of relative intensity 9:3).
3. Mass Spectrum
A copy of the mass spectrum is given in Appendix 2A. Use it to
verify the molecular weight of the compound. Comment briefly on the
spectrum (molybdenum has seven naturally occurring isotopes ranging
from 9 to 24% abundance).
QUESTIONS
1. Use the spectral data to determine the structure of the
complex?
2. To what symmetry group does the complex belong?
3. What would an ESR spectrum of this compound reveal?
REFERENCES
1. G. E. Coates, M. L. H. Green and K. Wade:
Organometallic Compounds Vol. 11. (The Transition Elements)
1968.
2. B. Nicholls and M. C. Whiting, J.Chem. Soc., (1959) 551.
3. F. A. Cotton and Wilkinson, “Advanced Inorganic Chemistry”.
4. R. J. Angelici, J. Chem. Ed., 45 (1968) 119.
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Inorganic Chemistry Year 3
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Inorganic Chemistry Year 3
EXPERIMENT 3
MAGNETIC SUSCEPTIBILITY
Introduction
The aim of the experiment is (1) to acquaint the student with two
of the experimental techniques involved in magnetochemical
measurements, viz, the Gouy method for solids and the Evans’ method for
solutions, and (2) to examine the magnetic behaviour of a complex in the
solid and solution state, and to gain information about stereochemical
changes if any.
(The student may perform either PART A or PART B)
PART A
A study of manganese(III) acetylacetonate.
Preparation
To a stirred solution of 3.9 g of manganese chloride (MnCl2.4H2O)
and 10.2 g of sodium acetate (CH3COONa.3H2O) in water (150 cm3) add
slowly acetylacetone (18.8 cm3). Follow by adding slowly, with stiring, a
solution of potassium permanganate (0.79 g in 38 cm3 of water) and a few
minutes later in small amounts, a solution of sodium acetate trihydrate (9.8
g in 40 cm3 of water). Heat on a water bath for 20 minutes at about 60º,
cool in ice-cold water and filter off the dark solid. Wash the product with
ice-cold water, and dry in a vacuum desiccator over anhydrous calcium
sulphate. Dissolve the dry chelate in warm toluene (20 cm3), filter and
reprecipitate the complex by cooling and adding petroleum ether (40-60º)
(about 75 ml). Dry in a vaccum desiccator. Calculate the percentage yield.
Analysis
The analysis of manganese is particularly convenient to carry out
spectrophotometrically as permanganate ion.
Make up a solution of 100 mg precisely of potassium
permanganate in 1 litre of distilled water. Then by dilution, make up
solutions containing 1, 2, 3 and 4 mg of permanganate per 100 cm3.
Measure the absorbance of these solutions in a 1-cm cell at 528
nm on the UV-visible spectrophotometer , and plot a graph of absorbance
vs. concentration.
Weigh out accurately about 100 mg of the manganese(III)
acetylacetonate and dissolve it in 50 cm3 of nitric acid which contains 12
cm3 of concentrated acid for 5 minutes, cool, add about 0.5 g. of A.R.
sodium bismuthate and boil again for 5 minutes. At this stage the solution
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Inorganic Chemistry Year 3
should be purple and/or manganese oxides should have precipitated. If this
has not occurred, add a further 0.5 g. of sodium bismuthate and boil again
for 5 minutes. Very slowly add aqueous concentrated sodium sulphite with
stirring until the solution clears. Boil until all oxides of nitrogen are expelled,
cool to 15ºC and add sodium bismuthate in small quantities with stirring
until no further colour changes takes place (about 0.5 g. of sodium
bismuthate) and stir. Add 50 cm3 of the 3 in 100 dilute nitric acid and filter
through a sintered glass crucible. Wash the reaction flask and the sinter
with the dilute nitric acid until filtrate becomes colourless. Dilute the purple
filtrate and washing to exactly 1 litre. Measure the absorbance of this
solution at 528 nm, then dilute 50 cm3 of the solution to 100 cm3 with
distilled water and repeat the absorbance measurement on the diluted
solution. Using the previously prepared graph determine the percentage
manganese in the manganese(III) acetylacetonate.
Magnetic Study
(a) Measure the paramagnetic susceptibility of the complex
as a solid
(see Appendix 3A, p. 3-9)
(b) Run the NMR spectrum of a CHCl3 solution containing a
known concentration of the complex (ca. 0.01g/ml).
Insert a capillary tube containing pure CHCl3 as external
reference (see Appendix 3B).
From the frequency separation of the proton resonance
in the CHCl3, solution relative to the external reference
of pure CHCl3, calculate the magnetic susceptibility of
the complex in solution.
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Inorganic Chemistry Year 3
PART B
Structural Equilibrium in a Schiff-base complex of Ni(II).
In this experiment, we study the equilibrium between squareplanar and tetrahedral stereoisomers of a bis(alkylsalicylaldiminato)
nickel(II) complex in solution.
Preparation of bis(isopropylsalicylaldiminato)Nikel(II)
In a FUME HOOD add 7.0 cm3 (0.08 mol) of neat isopropylamine
to 4.2 cm3 (0.04 mol) of salicylaldehyde dissolve in 50 cm3 of methyl
alcohol contained in a 250 cm3 beaker. (Take care as the amine is volatile
and has a pungent odour). Allow the mixture to stand for 5-10 minutes at
room temperature covered with a watch-glass. Then add to the mixture a
solution of 5.0 g. (0.02 mol) of nickel(II) acetate tetrahydrate dissolved in 70
cm3 of distilled water VERY SLOWLY AND STIR CONTINUOUSLY.
When the addition is complete, add a solution of 5.2 g (0.04 mol) of sodium
acetate tri-hydrate in 50 cm3 of water. At this stage a light-green
suspension forms. This is an intermediate which is gradually replaced by
the darker green product complex. Heat the suspension at 50ºC with
continuous stirring for 10-20 minutes or until flocculation occurs. DO THIS
IN THE FUME HOOD, and DO NOT HEAT ABOVE 55ºC.
Cool the beaker and its contents in water to room temperature,
stirring the suspension during cooling. Filter this darker green precipitate
through a Buchner funnel, and wash it on the filter with several portions (3
x 20 cm3) of water. When washing the precipitate with water and also in the
subsequent washing with ethanol, mix the precipitate thoroughly using a
glass rod during each washing. Wash the precipitate with 15-20 cm3 of
ethanol, then suck dry on the filter for 10 minutes. DRY THE
PRECIPITATE THOROUGHLY BEFORE PROCEEDING ON TO THE
RECRYSTALLIZATION STEP.
Recrystallize the product from chloroform in the following manner:
IN THE FUME HOOD, add 50 cm3 of chloroform to the complex contained
in a 100 cm3 round bottom flask and add some boiling chips. Place a reflux
condenser on the flask and boil the mixture for about 5 minutes or until
almost all of the solid has dissolved. (The flask containing the chloroform
should be heated over a water-bath in the fume hood). Filter the hot
solution through a pre-heated Buchner funnel, and place the filtrate into an
evaporating dish and slowly evaporating the solvent until about 10-15 cm3
remain. (This can be done on a hot-plate in the fume hood). Filter off the
dark-green crystalline solid and wash it with 2 x 5 cm3 portions of ether,
then suck dry on the filter. Store the solid in a desiccator over silica-gel,
and record the yield of dry product. Calculate the percentage yield on the
basic of nickel acetate used.
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Inorganic Chemistry Year 3
Magnetic Study
(a) Measure the magnetic susceptibility of the solid by the Gouy
method (see Appendix 3A, p. 3-9).
(b) Obtain NMR spectra of an approximately 0.05M solution of the
complex in each of the solvent mixture* specified below. The
solution is prepared by dissolving an accurately weighed
sample in each of the solvent mixture in a 10 cm3 volumetric
flask:i)
ii)
10% toluene + 90% benzene (By volume. Prepared by
mixing 10 cm3 of toluene with 90 cm3 of benzene).
10% toluene + 10% pyridine + 80% benzene (By volume.
Prepared by mixing 10 cm3 of toluene, 10 cm3 of pyridine
and 80 cm3 benzene).
NOTE: NMR spectra of these solvent mixture are provided. See
Appendix 3C for the calculation of the mass susceptibility χo of these
solvent mixtures.
Diamagnetic correction of solvent: The frequency separation for the methyl
resonance in the solvent mixture relative to the pure toluene capillary is a
measure of the solvent susceptibility, hence this frequency shift is
equivalent to a diamagnetic correction and must therefore be subtracted
from the overall frequency separation measured for the metal complex.
From the corrected frequency separation calculate χg, χM, χMcorr
(corrected for the diamagnetism of the ligands. See Appendix 3D), and
hence the solution Bohr Magneton Number, µeff. (see Appendix 3B).
QUESTIONS
(All students are to attempt Q.1 and Q.2
Q.3 – Q.6 refer to PART B)
1. The ‘Spin-only’ formula for calculating magnetic moments is given in
Appendix 3A. Under what conditions is it applicable? Why does it
break down sometimes? (See Cotton and Wilkinson and Ref. 3 and
4).
2. What conditions should be satisfied by a complex whose magnetic
susceptibility we wish to determine in solution? Are these any
restrictions to the type of reference solvent we could use? (See Ref.
4, 5 and 6). On the basis of your answer, discuss the feasibility of
determining µeff for the following:(a) Fe(CN)63- in aqueous solution using the tert-butanol methyl
resonance as reference.
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Inorganic Chemistry Year 3
(b) CuCl42- in pyridine using the toluene methyl resonance as
reference.
(c) Ni(DMG)2 in benzene using the methyl resonance of 4methylpyridine as reference.(DMG = dimethylglyoxime).
3. Explain the change in magnetic behaviour of the Ni(salen)2 complex
on going from the solid to solution.
4. Is there any difference in magnetic behaviour on going from
toluene/benzene mixed solvent to pyridine/toluene/benzene mixed
solvent? Suggest a possible explanation
for your observations.
5. What is the generalized structural formula for a Schiff
base(salicylaldimine) ligand and how are they synthesized? What
is/are the usual structure(s) adopted by metal complexes containing
such ligands? (See Ref. 1 and 2)
6. Assuming that in toluene/benzene, an equilibrium exists between
square planar and tetrahedral forms,
Nisq
K
→
NiTd
Calculate the equilibrium constant K from the magnetic data.
(See Appendix 3E).
REFERENCES
1. R. H. Holm and M. O’ Connor, Prog. in Inorg. Chem., 14, 338-342
(1971)
2. R. H. Holm and K. Swaminathan, Inorg. Chem., 2, 181 (1963)
3. B. N. Figgis and J. Lewis, “Modern Coordination Chemistry” , Ed. By
J. Lewis and R. G Wilkins, Interscience, (1960), Ch.6.
4. B. N. Figgis and J. Lewis, “Technique of Inorganic Chemistry”, 4,
137 (1965), Ed. By H. B. Jonassen and Q. Weissberger,
Interscience.
5. D. F. Evans, J. Chem. Soc., 2003 (1959).
6. T. H. Crawford and J. Swanson, J. Swanson, J. Chem. Ed., 48, 382
(1971).
7. P. W. Selwood, “Magnetochemistry, “Interscience, John Wiley &
Sons, New York, 1956, Chapter 8.
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Inorganic Chemistry Year 3
APPENDIX 3A
THE MEASUREMENT OF MAGNETIC SUSCEPTIBILITY
THEORY AND DEFINITIONS
Magnetic susceptibility is defined as the ratio of the intensity of
magnetism induced in a substance to the magnetising force or intensity of
field to which it is subjected. It provides important electronic structural
information of the transition and rare earth metal complexes.
Traditionally, measurement of magnetic susceptibility has been made using
the Gouy and the Faraday methods. The original instruments were originally
made from conventional laboratory balances and large permanent magnets. The
magnets were brought towards the sample and the positive or negative change of
apparent weight was noted. The balances and sample holder were free of
ferromagnetic materials. The systems that evolved were very large relying on
heavy magnets, fixed in position, and a moving sample.
The late Professor Evans of Imperial College London introduced an
innovative step to the measurement. Instead of measuring the apparent weight
loss or gain of a suspended sample, the force acting upon a suspended magnet
was detected, thereby turning the method on its head. The advantage realised by
this step led to the development of the light and inexpensive magnetic
susceptibility balance, which is the MSB-MKI. The model used in our laboratory is
MSB-AUTO which has many improved features and which still relies on the
advantage first obtained by Professor Evans.
The three most common ways that magnetic susceptibility values are
expressed are with reference to the volume of sample, the weight of sample, and
the moles of sample. These terms are commonly referred to as volume
susceptibility, mass susceptibility, and molar susceptibility. The equations relating
these terms are provided below:
VOLUME SUSCEPTIBILITY
The volume susceptibility, designated χν, is expressed by the following
formula:
χν =
I
H
where I is the intensity of magnetism included in the substance and
H is the intensity of the applied external magnetic field
The volume susceptibility can be quite variable due to changes in the density of
the substance, particularly when the sample is a gas or solid. The mass
susceptibility, χg, introduces a density factor in the following manner:
MASS SUSCEPTIBILITY
χg =
χν
d
where d is the density of the substance
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Inorganic Chemistry Year 3
Note that χg has units of reciprocal density,
{
cm3
g
}
MOLAR SUSCEPTIBILITY
The most common way of reporting a magnetic susceptibility value in the literature
is by molar susceptibility, designated as χM.
The relationship of molar
susceptibility to mass susceptibility is shown below:
χM = χg x MW
where MW is the molecular weight of the substance.
The χM is the best value to use when comparing different materials qualitatively or
assessing the potential of a quantitative applications since the value of the term is
not subjected to variations due to the method of measurement or to sample
density.
Since the Auto, like other methods, measures Volume Susceptibility and the
literature is quoted in Molar Susceptibility, the conversion between the two values
is quite common in magneto chemical studies. The initial step in the conversion,
that is obtaining a χg value from χν , is done within the Auto provided the length
and weight of the sample are known and can be entered into the unit.
Calculation of magnetic moment
The molar susceptibility obtained has to be corrected for the inherent diamagnetic
contribution (χdia) from the ligands and metal ions using the table of Pascal’s
constants.
i.e.
χΜcorr =
χMe exp - χdia
The relationship between molar susceptibility and effective magnetic moment , µeff
is
µeff = 2.84 √ χΜcorr T
Bohr Magneton
It can also be shown that the effective magnetic moment, µeff is given by,
µeff = √n(n+2) B.M.
[Note : A system obeying Curie law would have no or negligible orbital contribution
towards the magnetic moment].
For the details on the operation of the MSB-AUTO magnetobalance, please refer
to the operating manual.
Cleaning and Storage of Sample Tubes
[Note : The sample tube used in this magnetobalance is a high precision tube
made of silica. It is expensive and should be treated with care!]
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Inorganic Chemistry Year 3
The good maintenance of the sample tubes is essential for good result.
Sample tubes can be cleaned with the conventional detergents and/or organic
solvents, depending on the nature of their sample. In difficult cases, strong acids,
viz. HCl or HNO3 may be required. Pipe cleaners have proven to be a useful tool.
Similarly an aerosol can of ethylene dichloride, supplied with a plastic capillary
spout (available for degreasing electronic components) has proven invaluable in
removing oil from the tubes when analysing for wear metals.
The tube should be stored in a dust free environment and its susceptibility
should be measured before introducing a sample. As a final precaution, the
outside of the tube should be wiped with a dust free wiper just prior to introducing
it to the balance, particularly if it has been allowed to lay on the bench.
Packing Sample into the Tube
The tube is packed as follows. The solid to be measured is ground to a fine
powder in a mortar and pestle. HgCo(NCS)4 forms very fine crystals and can
usually be used without grinding. Note that the greatest source of error in Gouy
measurements is in homogeneous packing. It is essential that an effective routine
for the packing of the powder into the tube is developed and adhered to. Good
packing requires time and patience.
A small amount of the compound is picked up from the mortar, placed in the
flares mouth of the tube, and tapped to the bottom of the tube. The closed end of
the tube is tapped for about a minute on two sheets of filter paper on a wooden
bench. (See a demonstrator about this). This process is continued until the tube
is filled to the mark - make sure that the sample does not pack down with further
tapping. Wipe away any excess compound from the mouth of the tube to remove
excess moisture.
Standard to calibrate the balance
Several standards can be used for checking the calibration of the balance.
Perhaps one of the most reliable trouble free standard is water (106 χg = -0.720),
since it is readily available in pure form. A singly distilled water sample should
suffice.
HgCo(SCN)4 (105 χg = +1.644 at 20oC) is considered one of the best solid
standsrds. At other temperature, use the relation
dχg
dτ
= -0.05 x 10-6 cgs/K
to obtain the exact χg value.
The ethylenediamine salts of nickel, Ni (en)3 S2O3 (105 χg = +1.104) is also a
useful secondary standard.
Many solid samples of paramagnetic substances, if properly dried and
ground and packed, usually give within 2% of their literature value.
If it is necessary to recalibrate, please refer to Section 5.2 of the original
operator’s manual.
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Inorganic Chemistry Year 3
APPENDIX 3B
EVANS' METHOD FOR MAGNETIC SUSCEPTIBILITIES
(Extracted from J. Chem. Ed., 46, 1969, 167)
This method is based on the principle that the position of a given proton
resonance iin the spectrum of a molecule is dependent on the bulk susceptibility
of the medium in which the molecule is found. The shift of a proton resonance
line of an inert substance due to the presence of paramagnetic ions is given by
the theoretical expressions5
δν
νo
=
2π
3
(χν - χ_')
(1)
where δν is the shift, νo is the applied field, χv is he volume susceptibility of the
solution containing paramagnetic ions, and χ’ is the volume susceptibility of the
reference solution. For example, if an aqueous solution of paramagnetic
substance with 3% of t-butylalcohol as an inert reference substance is placed in
the inner tube of a concentric cell, and an identical solution without the
paramagnetic substance is placed in the annular section of the cell, two
resonance lines will usually be obtained for the methyl protons of the t-butyl
alcohol due to the difference in the volume susceptibilities of the solutions. This is
in accord with eqn. (1). The gram susceptibility, χg of the dissolved substance is
given by the expression5.
χg =
3 δν
───────
2πνom
do - ds
+ χo + χo (────────)
m
(2)
where δν is the frequency separation between the two lines in cycles/sec, νo is
the frequency at which the proton resonances are being studied in cycles/sec, m
is the mass of substance contained in 1 ml of solution, χo is the mass (gram)
susceptibility of the solvent (-0.72 x 10-6 cc g-1 for dilute t-butyl alcohol solutions),
do is the density of the solvent, and ds that of the solution. For highly
paramagnetic substances, the last term can often be neglected5. The molar
susceptibility, the effective magnetic moment and the "spin only" number of
unpaired electrons may be calculated by the same procedure used for the Gouy
measurements (See Appendix 3A).
(SEE REFERENCES 5, 7 AND 8).
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Inorganic Chemistry Year 3
APPENDIX 3C
MASS SUSCEPTIBIILITY OF SOLVENT MIXTURES
For a mixture of non-interacting components,
Mass susceptibility, χo = Σm
f(i). χo' .
i
mi
where mf = ────── which is the mass fraction of the i-th component
Σmi
i
Use of this equation and the table below would enable us to calculate the mass
susceptibility of the solvent mixtures used in this experiment.
TABLE
MASS SUSCEPTIBILITY
χo x 10-6 (c.g.s. or Gaussian units)
DENSITY
g cm-3
Benzene
-0.702
0.876
Toluene
-0.7176
0.863
SOLVENT
Pyridine
-0.622
Chloroform
-0.497
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Inorganic Chemistry Year 3
APPENDIX 3D
DIAMAGNETIC CORRECTIONS
When making diamagnetic corrections, an addition to the measured value is
made for each atom present in the molecule.
The relevant diamagnetic susceptibilities are given in the table below:-
TABLE
DIAMAGNETIC CORRECTIONS (c.g.s. units)
(All values x 10-6/g atom
Element/Ion
H
-2.93
C
-6.00
C (aromatic)
6.24
O (alcohol)
-4.61
O (carbonyl)
+1.73
N (C=N)
-5.57
Mn3+
-10.0
Ni2+
-12.8
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Inorganic Chemistry Year 3
APPENDIX 3E
Consider at equilibrium we have n mol of each of the square planar and
tetrahedral forms present, each with a MOLAR magnetic susceptibility of χM, then for
the equilibrium:
K
_
NiSq
K =
NiTd
nTd
──────
nSq
We can write,
nsq
χΜobs = ───── . χMsq +
Σn
nTd
──── . χΜΤd
Σn
But, since
χMobs = 0,
then,
nTd
χMobs = ──────────── . χMTd
(nTd + nSq)
Now, µeff = 7.977 x 102 [ χM (SI).T]1/2
i.e.,
µeff2 = constant x χM at a given temperature, thus we can write
nTd
µeff2 = ───────────
(nSq + nTd)
µeff2(Td)
Assuming that the maximum Bohr Magneton Number, µeff, for a tetrahedral
[Ni(salen)2] complex in solution is about 3.3 (See Reference 1), it is then possible to
obtain an expression for K which can be solved using the observed value for
µeff,(obs) which you calculate from your data.
17
Inorganic Chemistry Year 3
EXPERIMENT 4:
THE PREPARATION AND ESR STUDIES OF BIS(ACETYLACETONATO)
OXOVANADIUM(1V) AND ITS PYRIDINE ADDUCT.
INTRODUCTION
Electron Spin Resonance Spectroscopy (ESR) sometimes known as
Electron Paramagnetic Resonance Spectroscopy (EPR) is a branch of
spectroscopy in which radiation of microwave frequency is absorbed by
molecules possessing electrons with unpaired spins. It is an important
technique for the investigation of electronic and kinetic studies of systems
which have a net electron angular momentum. These include free radicals,
triplet state systems most transition-metal ions and point defects in solids.
Free electrons (in the absence of a crystal field or magnetic field) are
aligned in a random manner and in the presence of an external field they are
ether aligned with the field (lower energy state) or in opposition with the field
(higher energy state).
This is called the first order Zeeman effect. The difference in energy between
the two states is proportional to the applied field (Fig. 1)
∆E = geβeHo
where ge is spectroscopic splitting factor or simply the g-factor and equals to
2.0023 for a free electron.
In quantum mechanical terms, the allowable spin states are
quantized and the component Ms of the election spin vector can have values
which are +s or –s i.e. Ms = +½(↑) or Ms = -½ (↓).
For a given applied field Ho (gauss) electron in a lower energy state
Ms = -½ can be excited to a higher energy state Ms = +½ and the energy
required for transition is given by
∆E = hν = geβeHo (ergs)
18
Inorganic Chemistry Year 3
where ν is the frequency of the applied radiation (cps or Hz). In the absence
of nuclear spin therefore we would expect an unpaired electron to give a line
spectrum corresponding to a Ms = -½ → Ms = +½ transition.
Ms=½
E = ½ geβeHo
Energy
Ms = ±½
∆E=hν=geβeHo
Ho
Ms = -½
E = -½ geβeHo
Applied field
(Fig. 1)
In ESR spectroscopy, a magnetic field of around 3000 gauss is normally used
and this brings the radiation frequency ν to lie in the microwave region of the
electromagnetic spectrum (~ 9000 MHz).
The nucleus also has associated with it a spin and the nuclear spin
quantum number I may have values 0, ½, 1, 3/2, 2 etc. In the presence of a
magnetic field the nuclear states are quantized and the component MI of the
nuclear spin vector may have the following values
1, (1-1), (1-2)…………………….-1
This is the nuclear Zeeman effect and the energy required for the transition is
given by
∆E = hν = gNβNHo. (It is interesting to note that the energies required to effect
a transition of an election is much higher than that of a nucleus. Thus the
resonance frequency in ESR is about 700 times larger than for the NMR
transitions).
Hyperfine Splitting.
When an unpaired electron comes in the vicinity of a nucleus with
spin I, an interaction takes place which causes the absorption signal to be split
into 2I+1 components. (e.g. three lines are expected for an unpaired electron
on N where
I = 1). This splitting results from an interaction between the nuclear spinelectron spin coupling and the energies of the levels are given by
19
Inorganic Chemistry Year 3
E = gβHMS + AMSMI
where A is referred to as the hyperfine coupling constant.
Using the above equation and selection rules of ESR i.e. ∆MI = 0 and
∆MS = +1, the energies for each level and the energies of the four transitions
for a system where S= ½ and I= 3/2 (Fig. II) can be calculated. The
difference in energies between each of the four transitions can also be
calculated.
+3/2
½ geβeHo
+½
-½
- 3/2
-3/2
Ms=±½
-½
-½ geβeHo
+½
+ 3/2
Fig. II
Tetravalent vanadium is a d1 system and therefore possesses an
electron spin of ½. The magnetic moment of this spin can interact with that
generated by the nuclear moment of this spin can interact with that generated
by the nuclear spin of V51 (I = 7/2) making it possible for it to be studied by
ESR spectrometry. In fact ESR studies of the vanadyl VO2+ system have led
to a clearer understanding of the structure and bonding of vanadium (IV)
complexes.
The number of hyperfine lines resulting from the interaction of the
lone electron with nuclear spin of V51 is given by (2I + 1). The spectra of VO2+
systems in solvents of low dielectric constants are relatively simple and show
eight well-defined lines, each separated by a hyperfine coupling constant Aiso
of around 110 gauss. The value of Aiso is a direct measure of the density of
the unpaired electron in the neighbourhood of the metal nucleus. Hence it is
an important parameter in the discussion of bonding in the system. For
example, most VO2+ complexes have the square pyramidal structure and are
therefore capable of coordination with Lewis bases at the remaining vacant
site. A decrease in the value of Aiso is attributable to an increase in the basicity
of the coordinating molecules (because of greater delocalization of the
unpaired electron).
EXPERIMENTAL
20
Inorganic Chemistry Year 3
(a) Preparation of Bis(acetylacetonato)oxovanadium (IV), VO(acac)2.
Reflex a solution of 2.5 g vanadium pentoxide in 6 cm3 water to
which has been added 4.5 cm3 concentrated sulphuric acid and 12.5 cm3
ethanol. After 1-2 hours, the colour of the slurry changes from green to
blue. Filter and add 6.5 cm3 acetylacetone to the solution. Neutralize the
solution by adding slowly and with stirring a solution of 10 g anhydrous
sodium carbonate in 60 cm3 water. Filter the product and dry in air.
Recrystallize from a chloroform-ether mixture. Dissolve the product in a
minimum of hot,dry chloroform, cool and add ether dropwise to complete
the precipitation.
(b) Preparation of Pyridine Adduct.
*The operation below must be carried out in the fume cupboard.
Dissolve a portion of the bis(acetylacetonato)oxovanadium(IV)
already prepared in benzene and reflux for 30 minutes with a large excess
of pyridine. Concentrate to a small volume by distilling off benzene and
pyridine and crystals should form on cooling in ice. The addition of small
amount of ether should improve the yield. Filter and dry in a desiccator.
(c) Infrared spectra:
Determine the infra-red absorption spectra of the complex and its
adduct as outlined below and compare the two. The band occurring at 995
cm-1 in the spectrum of the complex has been identified as a V=O
stretching mode. Determine the amount by which this band has shifted in
the spectrum of the complex and suggest the reason for this shift. Are
there any other differences between the two spectra?
PREPARATION OF THE SAMPLE FOR RECORDING THE INFRA-RED
SPECTRUM
Make sure that the compound to be investigated has been thoroughly
dried.
Grind a small amount (about 10-20 mg) very finely with an agate
mortar and pestle, add one drop of nujol (use a dropper for this purpose) and
grind again. If necessary, add a second drop of nujol and regrind. Wipe the
surface of the sodium chloride disc with a clean dry tissue, being careful not to
touch the flat surface of the disc with your fingers. Do NOT under any
circumstances allow the sodium chloride disc to become damp and see that it
is not scratched. Carefully smear the nujol mull onto the surface of the disc
and insert it into its holder. When the spectrum has been obtained, wipe off all
traces of the mull from the disc with a clean and dry tissue.
(d)
Preparation of Sample for ESR Spectrum*
21
Inorganic Chemistry Year 3
5 cm3 of a 0.05M solution of VO(acac)2 are prepared in redistilled
toluene. It is necessary to flush the system with dry nitrogen. Introduce the
sample to fill about 1 inch of the ESR quartz tubing as you would for running
NMR spectra. Stopper the tubing and record the ESR spectrum at room
temperature.
The spectrum of VO(acac)2py is similar to that of VO(acac)2 and has a
Aiso value of 105.3 g and g = 1.9699. Notice the separations between
subsequent lines (Aiso) are not constant and have values ranging from 98.3
gauss at lower fields to 118 gauss at higher fields. This is mainly due to the
fact that at weak fields the quantum numbers MS and MI are not pure. As a
result there is a non-liner divergence of energy levels as the field increases. In
the case of VO(acac)2, the hyperfine value is approximately equal to the
spacing between the fifth line and the fourth line.
i. e. h5 –h4 = 108 gauss (refer to the attached spectrum).
*NOTE: You need not carry out this procedure, but obtain a copy of the
spectrum from your lecturer/lab. assistant.
QUESTIONS
1. Using the selection rules for ESR, sketch the hyperfine energy levels
diagram obtained from the interactions of the lone electron with V51
nucleus.
2. Assuming the hyperfine splittings to be symmetrical calculate the ‘g
value’ for VO(acac)2 (Bohr magneton B = 0.92731 x 10-20 erg/gauss).
3. the following stretching frequencies and Aiso values are observed for
VO(hfac)2 and Vo(tfac)2.
ν(V=O) (cm-1)
Aiso(gauss)
VO(hfac)2
1024
112.6
VO(tfac)2
1010
110.1
Compare these values with those obtained for VO(acac)2 and discuss
the results.
(hfac = hexafluoroacetylacetonate; tfac = trifluoroacetylacetonate)
4. Comment on the Aiso and ν(V=O) values of VO(acac)2py relative to
that of VO(acac)2.
REFERENCES
1. R. S. Drago “Physical Methods in Inorganic Chemistry” (Reinhold)
2. J. P. Fackler and J. A. Smith, J. A. C. S. 92, 5787 (1970).
22
Inorganic Chemistry Year 3
3. C. M. Guzy et al., J.Chem. Soc. (A), 2791 (1969)
4. V. Selbin et al., J. Inorg. Nucl. Chem., 25, 1359 (1963).
5. R. A. Rowe and M. M. Jones, Inorg. Synth. 5, 115, (1957)
EXPERIMENT 5: ELECTRONIC SPECTRA OF NICKEL(11) COMPLEXES
23
Inorganic Chemistry Year 3
(a d8 SYSTEM)
Aim of experiment
To construct the energy level diagram for nickel (11) (within the limits
dictated by the various ∆ values of the ligands) from the spectra obtained for
the series of complexes, and to use this to derive spectral and other
information on nickel complexes in general.
Introduction
Because of the differing symmetry properties of d–orbitals, their initial
degeneracy is removed when they are brought under the influence of external
fields other than a uniform spherically symmetric one. Similarly the Russell –
Saunders states associated with the various d orbital configurations are also
split. In the case of d1 and d9 (by the ‘hole formalism’) this gives rise to the
energy level diagram shown in Figure 1.
d9 oct
2
T2
E
2/5∆
E
d1 oct
2
3/5∆
2
D
-3/5∆
-2/5∆
2
E
2
T2
∆
∆
O
Fig. 1
Other multi – electron systems are more complicated because the
interaction of several electrons gives rise to a series of Russell-Saunders
states, G, F, P, G etc., which are affected differently by the Oh field.
Thus for d2 and d8, we have the terms 3F,3P,1D,1G, and 1S, which are split by
the ligand field as shown in Figure 2.
When an electron undergoes a transition from the ground state to an
excited state, the molecule must absorb energy. In the case of the d - d
transitions the absorptions occur in the near UV, visible and the near IR. To a
first approximation these transitions are forbidden, but more refined theory
allows them to occur to give weak absorptions, as the Laporte rule is broken
down. The spin selection rule may also be overcome, and when it is we
observe very weak bands with an intensity of about 1/100 that observed for
spin – allowed transitions. For an octahedral nickel(11) complex, the following
spin - -allowed absorptions are expected:-
24
Inorganic Chemistry Year 3
3
A2g
3
→
3
T2g
A2g
3
→
3
T1g(F)
d8 oct
A2g
3P
1T
3A
1E
-1
-6
T1g(P)
2
3T (P)
1
2
3 /5
E
3
d2 oct
3T (P)
1
3T (F)
1
→
3T
2
/5
3F
/5
3A _
2
O
Fig. 2
Procedure
Obtain samples of the complexes listed in Table 1 from the lab
assistant (or if not available prepare a sample, see lecture/demonstrator in
charge) and prepare solutions as described in the Table 1.
Table 1: Solutions of complexes to be prepared.
Complexa
Solvent and Blank.
Concentration
1. [Ni(bipy)3]SO4.6H2O
Water
0.05M
2. [Ni(en)3]Cl2.2H2O
20% en
0.05M
3. [Ni(NH3)6]CI2
Aqueous NH3
0.05M
4. [Ni(H2O)6]SO4
Water
0.05M
5. [Ni(DMSO)6](CIO4)2
DMSO
0.05M
6. K4[Ni(NCS)6]4H2O
10M KSCN in water
0.05M
bipy = 2,2-bipyridyl ; en = ethylenediamine ; DMSO = dimethylsulphoxide.
25
Inorganic Chemistry Year 3
Record the spectrum of each solution in the region 200 – 1100 nm.
1. Plot the energy diagram E versus ∆ based upon the ground state 3A2g
as the energy zero for all values of ∆ (E and ∆ in units of cm-1) for the
complexes 1- 6,
Clearly labelling the various states (Note: The transition energy for the
lowest energy absorption (3A2g → 3T2g) for the d8 case gives the value
of ∆ directly for the complex under investigation.)
2. Assume that the free ion 3F – 3P separation is 15,836 cm-1 and that the
energy of the 3A2g ground state varies according to
E(3A2g) = - 6/5 ∆
(1)
Construct the Orgel diagram for nickel (II).
3. Answer the following questions:
i)
Assign the 2 bands in the observed spectrum of complex 5,
and estimate the frequency of the missing band.
ii)
Estimate the frequency of the absorption maximum for the
3
A2g → 3T1g(P) transition in complex 1. Why is it not possible
to get this directly from the spectrum? (Do not include
limitations of instrumentation as a valid reason).
iii)
Predict the position of the absorption maxima for Ni(o–
phen)3SO4.9H2O and KNiF3 and with the aid of Figure 3 ,
deduce the colour of these compounds.
Fig. 3
Near
Infrared
Red
13,300
Orange Yellow
15,400
17,000
Green
Blue
Violet
Ultraviolet
19,000 20,400 23,300 25,600cm-1
iv) Arrange the ligands in the spectrochemical series, that is in
order of increasing ∆.
v) Calculate the extinction coefficients for complexes 1 -5.
Comment on the differences, if any, between them and those
of d – d transitions in tetrahedral complexes, e. g. NiCl42- (٤ =
200) and for charge transfer transitions (٤ = 104 – 105) e.g.
MnO4-.
vi) Using equation 1, determine the mean energy (E) of the 3F
state, relative to the 3A2g level, for complexes 1 – 6. For
complex 6 use the diffuse reflectance data. Calculate the
26
Inorganic Chemistry Year 3
energy Eexp = (3T1g (P) – E) for each complex.
vii) Using Eexp calculated above, calculate P in the following
equation for each complex.
[3/5 ∆ P – 4/25 ∆2 ] + [ - 3/5 ∆ – P ] Eexp + Eexp2 = 0 (2)
This equation relates the energy of the 3T1g terms to the crystal
field splitting parameter ∆ and the free ion separation 3F – 3P,
given by P.
Since the complexes deviate from the point charge
electrostatic model and orbital interaction between the metal
and ligands occurs, the experimental energies of the various
states then differ from the theoretical energies. To take this
account, we can use equation 2 to calculate a value of the 3F –
3
P(P) separation in the complex rather than use the free ion 3F
-3
P separation. This we do by substituting an experimental
value (for 3T1g(F) or 3T1g(P) in equation 2.
viii) Obtain the Racah parameter B, for each ligand. The
Pcomplex
nephelauxetic ratio β = ---------Pfree ion
.
Use the value given above for Pfree ion.
ix) Arrange ligands in order of decreasing β value. This gives the
nephelauxetic series.
x) The energy versus ∆ correlations for 3T1g (P) and 3T1g(F)
deviate from linearity, why?
References:
Cotton & Wilkinson
Lewis & Wilkins
Cotton
Sutton
Manch & Fernelius
Forster & Goodgame
Drago
“Advanced Inorganic Chemistry”
“Modern Coordination Chemistry”
J. Chem. Educ.1964, 41, 466.
J. Chem. Educ. 1960, 37, 498.
J. Chem. Educ. 1961, 38, 192
Inorg. Chem. 1965, 4, 823.
“Physical methods in Inorganic
Chemistry”.
APPENDIX 5A
27
Inorganic Chemistry Year 3
Preparation of Nickel(II) Complexes:
1)
Ni(bipy)3SO4
Warm an aqueous solution containing 15 g of bipyridyl and 6 g of
NiCI2.6H2O. Cool and filter. Dissolve 5 g of the chloride and add 5 cm3 of
a 1M solution of Na2SO4. Ni(bipy)3SO4 crystallises out.
Reference: J. Chem. Soc., 1971 (A), 1637, 1640
2) Ni(en)3Cl2.2H2O:
2.8 g of 70% aqueous ethylenediamine is added to a solution of 2.38 g
(0.01 mol) NiCl2.6H2O in 100 cm3 of water . The purple solution is filtered
to remove the small amount of anhydrous iron oxide which precipitates
and is then evaporated to a volume of 60 to 70 ml on a steam bath. Two
drops of ethyelediamine are added, and the solution is cooled in an ice
bath. The orchid-coloured crystals that from are collected by suction,
washed twice with 95% ethanol, and air-dried. The yield is 2.86 g (80%).
Several more grams may be recovered from the mother liquor by adding
ethanol and cooling.
3) Ni(NH3)6CI2:
Dissolve 2 g NiCl2.6H2O in 3 cm3 water. Add 6 cm3 NH3 and heat for 10
min. but do not boil. (Use fume-hood.) Then cool the solution in an ice bath
and slowly add 6 cm3 ethanol with stirring. Filter with suction. Wash the
precipitate with a little cold conc. NH3, followed by alcohol and acetone.
Suck the solids dry. Record your yield.
4) Ni(DMSO))6Cl2:
Dissolve 2 g of NiCl2.6H2O in 20 - 50 cm3 of ethanol and mix with the same
volume of ether, follow by 3.9 g of dimethylsulfoxide (DMSO). Cool the
mixture in the refrigerator until crystals are formed. Filter and dry in
desiccator.
Reference: J. Inorg. Nucl. Chem., (1961), 16, 219.
5) K4Ni(CNS)6.4H2O:
Dissolve 0.01 mole of Ni(SO4).7H2O and KSCN in water (Work out
appropriate amounts). Evaporate to dryness. Extract with absolute
alcohol. Filter off the K2SO4 and concentrate the alcoholic solution to
obtain the complex.
Reference: Inorg. Chem. 4 (1965), 715, 823.
EXPERIMENT 6: THE EFFECT OF SYMMETRY ON THE INFRARED
28
Inorganic Chemistry Year 3
SPECTRUM OF THE SULPHATE GROUP.
INTRODUCTION
Any regular tetrahedral AB4 molecule or ion should exhibit only two
fundamental absorption in the infrared spectrum. The vibrational modes which
give rise to these absorption may be described roughly as stretching and
bending modes; both are triply degenerate. In the case of an ionic sulphate
two strong infrared absorption are in fact observed. If, in a given crystal, the
symmetry of the environment of the sulphate group is less than tetrahedral,
then additional weak absorption may be seen due to vibrations which are
forbidden when the fill tetrahedral symmetry is applicable.
Complexes are known in which the sulphate group is ionic,
monodentate or bidentate, and the splitting and changes of intensity of
absorption bands due to that group beautifully illustrate the effect of
progressively lowering the symmetry of the ion.
EXPERIMENTAL
Record the IR spectra of the following as ‘Nujol’ mulls (SEE
DEMONSTRATOR):
i)
ii)
iii)
iv)
a simple ionic sulphate
[Co(NH3)6] 2(SO4)3.5H2O
[Co(SO4)(NH3)5]Br
[Co(en)2(SO4)]Br
The second band of the sulphate ion (due to bending of the O-S-O angles) will
not appear in the region covered by our spectrophotometer.
Preparation of Complexes
(ii) Hexamminecobalt (III) sulphate pentahydrate.
First prepare hexamminecobalt(III) chloride as follows. Dissolve 12 g
of ammonium chloride and 18 g of cobaltous choride, CoCl2.6H2O, in 25 cm3
boiling water. Add 1 g of decolourising charcoal and cool in ice. Add 40 cm3
concentrated ammonia and the solution at 10°C or lower . Add slowly, in small
portions, 35 cm3 of ‘20 volume’ hydrogen peroxide, briskly shaking the
solution during the addition. Gradually raise the temperature to 50-60°C and
keep the flask at this temperature, with frequent shaking, until the last trace of
pink coloration is removed. Cool and filter. Transfer the crystals to a beaker
containing a boiling solution of 5 cm3 concentrated hydrochloric acid in 150
cm3 water. When all the solid, except the charcoal, has dissolved, filter the
liquid while still hot. Add 20 cm3 of concentrated hydrochloric acid to the
filtrate and cool the solution in ice. Collect the golden brown crystals and dry
them by washing with acetone. If necessary the complex may be
recrystallized from water containing a trace of hydrochloric acid.
29
Inorganic Chemistry Year 3
Dissolve 5 g of [Co(NH3)6]Cl3 in 50 cm3 hot water and mix with 50
cm of 20% sulphuric acid. Heat the solution to 60°C and add 25 cm3 of 95%
alcohol. Warm for a few minutes on a water bath to dissolve the solid and set
aside to crystallize. After 24 hr., crystallization should be complete; wash the
crystals at the pump with 95% alcohol until the filtrate is neutral. Dissolve the
salt in hot water and precipitate by addition of alcohol. Filter and wash with
alcohol until acid-free. Dry in air. If required, further recrystallisation may be
effected from hot water.
3
(iii) Sulphatopentamminecobalt (III)bromide.
First prepare chloropentamminecobalt(III) chloride as follows: (Note,
this compound is one of the most important of the cobaltammines and is the
starting substance for the preparation of many coordination complexes of
cobalt).
Co2+ + NH4+ +4NH3 + ½ H2O2 → [Co(NH3)5H2O]3+
[Co(NH3)5H2O]3+ + 3Cl-
→ [Co(NH3)5cCl]Cl2 + H2O
Dissolve 8 g of ammonium chloride in 48 cm3 of concentrated
aqueous ammonia in a 500 ml Erlenmeyer flask. While continuously agitating
the solution with a magnetic stirrer, add 20 g of finely powdered cobalt(II)
chloride hexahydrate in small portions. With continued stirring of the resulting
brown slurry, slowly add 13 cm3 of 30 per cent hydrogen peroxide from a
dropping funnel. When the effervescence has ceased, slowly add 48 cm3 of
concentrated HCl. Continue the stirring on a hot plate, holding the
temperature at about 85°C for 20 min; then cool the mixture to room
temperature and filter off the precipitated [Co(NH3)5Cl]Cl2. Wash with 30 cm3
of ice water in several portions, followed by 30 cm3 of cold 6M HCl. Dry the
product in an oven at 100°C for several hours. The yield is about 15 g.
Stir 8 g of [CoCl(NH3)5]Cl2 with 29 g of concentrated sulphuric acid in
a porcelain dish: add the acid in small portions so that the evolution of
hydrogen chloride gas is not too violent. When evolution has ceased, heat the
resulting oily mass on a boiling water bath for 4 hr., during which more
hydrogen chloride will be given off. Dilute with water and continue heating on
the water bath until there is no more evaporation. Pour the liquid into a beaker
and dilute with two volumes of water. If it is necessary to filter, do this as
quickly as possible. Leave it for 24 hours, whereupon the acid sulphate
precipitates in good yield, as shining, rectangular violet-red plates. Decant the
strongly coloured mother liquor and transfer the crystals to a sintered-glass
filter. Wash with 95% alcohol and dry in the air.
Dissolve 4 g of the acid sulphate in 120 cm3 cold water and treat the
solution with a mixture of 20 cm3 concentrated hydrobromic acid (b.p. 125°)
and 80 cm3 water. Gradually add alcohol, whilst stirring, to precipitate the
product as a fine violet-red crystalline powder. Wash with alcohol until acidfree and dry in air.
30
Inorganic Chemistry Year 3
iv) Sulphatobis(ethylenediamine)cobalt(III) bromide.
First prepare trans-dichlorobis(ethylenediamine)cobalt(III) chloride as
described in Experiment 1.
Add 10 g of trans-[Co(en)2CI2]Cl to 20 cm3 of concentrated sulphuric
acid in a beaker. When the initial effervescence has subsided, heat gently.
The complex slowly dissolves with evolution of hydrogen chloride and a violet
solution results. Raise the temperature to 120°C until the evolution of gas
ceases and then allow the oil to cool. Pour into 1 litre of alcohol which is
continually stirred to prevent formation of an oil. Filter off the solid, wash with
alcohol and then with ether. Dry in vacuo.
Dissolve the deliquescent solid in a minimum volume of water and
treat with a saturated aqueous solution of 5 g. lithium bromide then leave in a
refrigerator at 0°C for 3 days. Filter off the purple crystals, wash with alcohol
and then ether. Air dry the product. If the mother liquors are left for a further
three days a second crop of [Co(en)2(H2O)(SO4)] Br.H2O will be obtained.
Heat the powdered product at 110° for 24 hr. in an oven.
Recrystallise the [Co(en)2(SO4)] Br by dissolution in a minimum volume of
cold water and addition of sodium bromide. The product crystallizes as short
purple needles.
QUESTIONS
1. List the symmetry of the sulphate groups in each of the complexes.
2. Identify the bands due to the sulphate group in the IR spectra of the
complexes.
3. Draw out the rough form of the vibration giving rise to each absorption
REFERENCES.
K. Nakamoto et al. – J.A.C.S. 79, 4904 (1957).
C.G. Barraclough and M.L. Tobe – J.C.S. 1993 (1961).
31