I1. 1
EXPERIMENT I1
PREPARATION OF SOME COBALTAMMINE COMPLEXES
The types of complexes to be prepared in this experiment have played a
considerable part in the development of coordination chemistry. Werner, for
example, developed his ideas of coordination number and geometry through
studying complexes of Co(III) and Pt(IV) with amines Cl, Br, I, pyridine, CN, etc.
Coordination compounds of Co(III) and Cr(III) have been of particular interest
because their complexes undergo ligand exchange very slowly compared with
complexes of many other transition metal ions. For example, Ni(NH 3)62+ reacts
virtually instantaneously with H2O to form Ni(OH2)62+. Under the same conditions,
the analogous reactions of Co(NH3)63+ and Cr(NH3)63+ occur very slowly. This
difference in behaviour of complexes of different metal ions has been qualitatively
accounted for by ligand field theory and molecular orbital theory.
The slow reactivity of Co(III) complexes has made them suitable for extensive
investigations. The structures of the octahedral Co(III) complexes which you will
prepare in this experiment are given below.
The synthesis of [Co(NH3)4CO3]NO3 will be carried out according to the unbalanced
equation:
Co(NO3)2 + NH3(aq) + (NH4)2CO3 + H2O2  [Co(NH3)4CO3]NO3 + H2O + NH4NO3
The Co(NO3)2 that is available commercially has the formula Co(NO3)26H2O and
very probably is a coordination compound having the ionic formulation
[Co(OH2)6](NO3)]2. Since Co(II) complexes, like those of Ni(II), react very rapidly by
ligand exchange, the first step in the reaction might be expected to be:
I1. 2
[Co(OH2)6]2+ + 3NH3 + CO32  [Co(NH3)4CO3] + 6H2O
This Co(II) complex could then be oxidised by the transfer of an electron to H 2O2 to
give the relatively unreactive Co(III) ion, [Co(NH3)4CO3]+.
The preparation of [Co(NH3)5Cl]2+ is accomplished from the carbonato complex
according to the following series of equations:
[Co(NH3)4CO3]+ + 2HCl  [Co(NH3)4(OH2)Cl]2+ + CO2(g) + Cl
[Co(NH3)4(OH2)Cl]2+ + NH3(aq)  [Co(NH3)5(OH2)]3+ + Cl
[Co(NH3)5(OH2)]3+ + 3HCl  [Co(NH3)5Cl]Cl2(s) + H2O + 3H+
The two complexes will be characterised by infrared and U/V-visible spectroscopy
and by electrical conductivity measurements.
EXPERIMENTAL PROCEDURE
No precautions are necessary to protect the reaction mixtures from the atmosphere.
(This is required for preparations that involve reactants or products that react with
moisture or oxygen in the air.) Operations that necessitate heating of the solutions
should be carried out in a fume cupboard.
Synthesis of [Co(NH3)4CO3]NO3
Dissolve 4 g (0.042 mole) of (NH4)2CO3 in 12 mL of H2O and add 12 mL of
concentrated aqueous NH3. While stirring, pour this solution into a solution
containing 3.0 g (0.0104 mole) of [Co(OH2)6](NO3)2 in 6 mL of H2O. Then slowly add
1.6 mL of a 30 per cent H2O2 solution. (Handle H2O2 with rubber gloves. If the
affected area is not washed immediately with water, hydrogen peroxide can cause
severe skin burns.) Pour the solution into an evaporating dish and concentrate over
a gas burner in a hood. Do not allow the solution to boil. During the evaporation
time add, in small portions, 1.0 g (0.01 mole) of (NH4)2CO3. Suction filter (with water
aspirator; for better control of the vacuum, used a pinch clamp on the rubber tubing
between the trap and filtration flask; see Figure I.1) the hot solution and cool the
filtrate in an ice water bath. Under suction, filter off the crystals of the red product.
Wash the [Co(NH3)4CO3]NO3 in the filtration apparatus first with a millilitre of water
(the compound is somewhat soluble) and then with a similar amount of ethanol. Dry
in desiccator overnight. Calculate the yield. Keep enough material to prepare
I1. 3
100 mL 5 x 103 M and 500 mL 0.001 M solution for characterisation purposes and
use the rest for the next part of the experiment.
Synthesis of [Co(NH3)5Cl]Cl2
Dissolve 0.5 g of [Co(NH3)4CO3]NO3 in 5 mL of H2O and add concentrated HCl (0.5
to 1.0 mL) until all the CO2 is expelled. Neutralise with concentrated aqueous NH3
and then add about 0.5 mL excess. Heat for 20minutes, again avoiding boiling;
[Co(NH3)5(OH2)]3+ is formed. Cool the solution slightly and add 7.5 mL of
concentrated HCl. Reheat for 10 to 15 minutes and observe the change in colour.
Purple-red crystals of the product separate on cooling to room temperature. Wash
the compound several times, by decantation, with small amounts of ice-cold distilled
water, then filter under a water aspirator vacuum with glass fritted funnel (medium
porosity). Wash with one millilitre of ethanol. Dry in desiccator overnight. Calculate
the yield.
CHARACTERISATION OF THE COMPLEXES
1.
U/V-Visible Spectra
Before you attempt to run the spectra of your complexes read the notes on
"Electronic Spectra of Some Transition Metal Complexes" (see Appendix).
Make up 100 mL (volumetric flask) of 5 x 10 3 M solutions of the two
complexes in water and run the spectra in the range 350 nm to 750 nm using
1 cm pathlength cells. The spectrum of [Co(NH3)5Cl]Cl2 may change slowly
with time so the spectrum should be run as quickly as possible after making
up the solutions. If you are unsure how to use the instrument ask a
demonstrator to help you.
Record the band positions and the absorbances at the band maxima and
work out the extinction coefficients (in M cm1). Compare your spectra with
I1. 4
those given in Table 3 of the notes on 'Electronic Spectra of Some Transition
Metal Complexes' (see Appendix) and comment on the results (see below).
2.
Infrared Spectra
The infrared spectra of your two samples must be run as nujol mulls between
sodium chloride plates.
If you do not known how to run a nujol mull ask the demonstrator to show you.
Two very important points:
(a)
The NaCl plates are expensive and require care in handling. Do not
drop them, wash with water or aqueous organic solvent mixtures, touch
the flat surfaces with your fingers or scratch with a spatula. Clean with a
little CHCl3, wipe with a tissue and store them in the desiccator after
use.
(b)
Run a spectrum of nujol by placing a small drop between the two NaCl
plates. Subtract these peaks from your spectrum to get the spectrum of
your sample.
The major peaks in each spectrum must be assigned. Help with your
assignments can be obtained from K Nakamoto, 'Infrared and Raman Spectra
of Inorganic and Coordination Compounds'. 3rd edition (see also the CRCV
Handbook).
3.
Electrical Conductivity of Solutions of Ionic Compounds
The determination of the number of ions constituting a given substance is
largely by a matter of comparing conductivity of known ionic substances with
those of the unknown compounds. The definitions usually begin with
resistance, since this is the quantity that is experimentally measured. The
resistivity, , is defined as the resistance in ohms of a solution in a cell that
has 1 m2 electrodes that are separated from each other by a distance of 1 m.
The reciprocal of  is the conductivity, K. The resistance, R, of the same
solution in a cell of nonstandard dimensions is obtained by multiplying by a
correction factor, C, (the cell constant), which depends upon the geometry of
the cell. Experimentally, C is evaluated from
R = C
(1)
I1. 5
i.e. by measuring R for a given solution whose  has been measured in a
standard cell and then calculating C from the preceding expression.
Since K = 1/, equation (1) is usually expression terms of measured
resistance, R, and the conductivity
R = C/K
(2)
The cell constant, C, is frequently obtained from equation (2) by measuring
the resistance, R, of a 0.02 M KCl solution whose conductivity at 25C is
0.2768 ohm1 m1. Having evaluated C for the cell used in the study, the
measurement of R will allow the calculation of the conductivity of any solution.
In determining the conductivities of solutions of electrolytes, it is desirable to
compare their conductivities for a given amount of electrolyte. Thus the molar
conductivity, Um, is defined as
m 
K
c
where c = concentration of the solution expressed in units of mol m3.
Comparisons of molar conductivities with those of known ionic substances
allow one to determine the number of ions present in a given salt. General
ranges of m for 2, 3, 4, and 5 ion conductors at 25C in water as solvent are
tabulated as follows:
Number of ions
m/ 1 m2 mol1 x 104
2
3
4
5
118 - 131
235 - 273
408 - 435
560
[see Atkins, 4th Ed., p. 750/751, for more details]
I1. 6
EXPERIMENTAL PROCEDURE
You will be provided with a conductance cell, conductivity bridge and a standard
0.02 M KCl solution.
Read carefully the instructions provided by the manufacturer for the operation of the
conductivity bridge. If you need help, ask the demonstrator. In making all resistance
measurements, thermostat the cell containing the solution at 25C for approximately
10 minutes before making a reading. Use distilled water in all solution preparations.
1.
Obtain the cell constant, C, from the resistance measured for the 0.02 M KCl
solution.
2.
Prepare 500 mL of 0.001 M aqueous solutions of [Co(NH3)4CO3]NO3 and
[Co(NH3)5Cl]Cl2 and measure their resistances. Make the measurements
immediately after the solutions are prepared, since significant decomposition
occurs on standing overnight.
Be certain to rinse the cell well with distilled water between measurements,
and when you have completed the experiment rinse it thoroughly and leave it
filled with water.
Calculate the molar conductivities of the two Co(III) complexes.
These
measurements are fairly sensitive tests of the ionic purity of your compounds.
REPORT
Include the following:
1.
Percentage yields of [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2.
2.
Values of m for the preceding complexes and your conclusions as to the
number of ions in each compound.
3.
IR spectra of [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2, indicating the
absorptions characteristic of NH3, CO3, and NO3. What are the spectral
similarities and differences between these two compounds?
4.
Visible spectra of the two complexes and comment on the results.
I1. 7
QUESTIONS
1.
Outline methods of analysing [Co(NH3)5Cl]Cl2 for the ionic chloride and for the
total chloride content.
2.
Conductivity of a solution of [Co(NH3)5Cl]Cl2 changes overnight. Give a
reason for this observation. Would you expect the conductivity to decrease or
increase?
3.
How do [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2 differ structurally?
How would you distinguish between these complexes experimentally?
Give the IUPAC names for the complexes.
4.
Cobalt(III) complexes are relatively inert and react very slowly whereas the
corresponding Cobalt(II) complexes are very labile (react very rapidly).
Explain this observation.
(Cotton and Wilkinson, "Advanced Inorganic Chemistry" should be consulted.)
REFERENCE
The above experiment has been adapted from Angelici, 'Synthesis and Technique in
Inorganic Chemistry' pp 13-25.