An Experiment in
trans-cis Isomerization
Synthesis and Kinetics of frans-Dichlorotetraamminecobalt(lll)Chloride
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Londa L. Borer and Howard W. Erdman
California State University, Sacramento, Sacramento, CA 95819
Cobalt(III) coordination complexes are ideal for in-depth
studies at the undergraduate level. The syntheses of the
isomers in this study are relatively short and simple, and
UV-visible spectroscopy, infrared spectroscopy, conductance, and kinetic studies can be done easily.
The
first
published synthesis of trans[Co(NH3)4Cl2]Cl»H20, performed by S. M. Jorgensen, took
approximately 96 h (1). Starting with reagents that can be
purchased easily, the procedure described in this paper
takes approximately 24 h to synthesize Co(NH3)4C03 |N03
from which the final product, fratts-[Co(NH3)4dJ2]ChH20,
is synthesized in 15 min.
[
Experimental
[Co(NH3)4C03]N03 was prepared by the method described in Synthesis and Technique in Inorganic Chemistry
(2). [Co(NH3)4Cl2]Cl was prepared according to the procedures described here.
Synthesis
A sand bath is pre-heated to a temperature high enough
to heat a solution to 80 °C. One gram (4 mmol) of carbonatotetraamminecobalt{III) nitrate is dissolved in 5 mL
of water in a 50-mL flask containing a thermometer and a
magnetic bar stirrer. The solution is heated to 50-60 °C in
the pre-heated sand bath for 3 min with stirring. Concentrated hydrochloric acid (3.3 mL) is added as quickly as
possible and with care to prevent the solution from froth-
ing.
The solution is heated to 80 °C with vigorous stirring for
5 min. (This temperature is critical!) A dark green precipitate of frans-dichlorotetraamminecobalt(III) chloride is
produced.
The solution is cooled quickly to room temperature in an
ice bath, and the crude product is collected by vacuum
filtration. The crude product is transferred to a 100-mL
beaker, swirled in 30 mL of ice-cold water to dissolve any
cis isomer, and filtered immediately to obtain the pure
trans isomer product.
The green trans isomer crystals are washed with approximately 2 mL of ice-cold methanol and air dried to yield
0.45-6.55 g (45-55%) of product. The cis complex,
[Co(NH3)4Cl(H20)Cl2, is obtained by reducing the temperature of the aqueous-methanol filtrate solution. The pur-
332
Journal of Chemical Education
ple crystals of the cis isomer are collected by vacuum
tion.
filtra-
Kinetics Study
A Perkin-Elmer Lambda Array 3840 UV/Vis spectrophotometer
was
used to follow the aquation of trans[Co(NH3)4C12]C1 in water, in 0.1 N HNO3, and in 1 N HC104
media at a temperature of 25 ± 0.5 °C. For the aquation of
the trans isomer in distilled water, 39.7 mg was dissolved
in 25 mL (6.32 x 10~3 M). The instrument was set at 630.4
nm, the most intense peak of the trans isomer, and the
aquation was monitored every minute for 90 min. Analysis
of the charge-transfer bands was made in the region from
190 to 390 nm using a 10 5 M distilled water solution. The
cis and trans isomer peaks were located at 213.7 and 253
nm, respectfully, along with an isobestic point at 230 nm.
Using the charge-transfer peak at 253 nm, analysis was
made using a solution of 18.8 mg of the trans isomer dissolved in 1000 mL of 1 N HC104 (7.48 x 10”5 M).
Conductance measurements were made using a ColePalmer digital conductivity meter, model 1481-90. A volume of 100 mL of water was added to a 25.1 mg sample of
the trans isomer (1.042 x 10~3 M). The solution was stirred
continuously during the aquation. Measurements were recorded every minute for the first 10 min and then taken
less frequently as the change in conductance decreased. A
plot was made of In (A„ -Af) versus time and a leastsquares analysis provided the rate constant, k.
The aquation of the cis isomer also was investigated by
conductivity to show that the reaction proceeds from an
aquachloro to diaqua complex.
Results and Discussion
The reaction of the trans isomer in aqueous solutions can
be followed easily in the UV-Vis region. Initially the spectrum has three peaks, as per a trans complex, but as the
reaction progresses, the splitting diminishes until there
are only two peaks indicating that the aquated cis complex
is formed (3). The steps of aquation proposed by Tsuchida
are as follows (4):
fTOas-[Co(NH3)4CI2]Cl
—>
rrans-[Co(NH3)4Cl H20]C12 -»
cis-[Co(NH3)4C] H20]CI2
Conductivity Measurements for the trans Isomer
-+
cis-[Ca(NH3>4(H20)2]Cl3
A plot of In (Ai
time for the disappearance
of the 630.4 nm or the 253 nm peak yields a straight line,
indicative of a first-order reaction. The rate constant can
be determined from the following equation:
In (Af A„) kt + constant
(1)
-
A„)
-
versus
=
where At is the absorbance at time t, A„ is the absorbance
when aquation of the trans isomer is complete, k is the rate
constant in s“\ and t is the time in seconds. A„ is determined by allowing the reaction to proceed for 90 min. The
constant, y intercept, is used to calculate A0, the absorbance at time zero, t ().
The aquation in water of the trans isomer over a period
of 30 min at 630.4 nm provided an aquation rate constant
of 1.85 x 10~3 s"1 which compared favorably to the literature value of 1.83 x 10 ~3 (5). Monitoring aquation of the
trans complex at wavelength 253 nm, with HCIO4 at pH 2,
provided a k value of 2.28 x 10-3 s-1 and an extinction coefficient of 2.39 x 104 (lit. 1.90 x 10“3 s”1 and 2.40 x 104, respectively) (5, 6).
Kinetics of a reaction also can be studied by following a
change in conductivity (7—9). Since there is an increase in
the number of ions during aquation, the conductivity of the
solution will change accordingly and, as when absorbance
is monitored, the rate constant k is determined from rate
of change. Conductance is the reciprocal of resistance
which is expressed as molar conductance, A, in units of
ohm_1em2mol_I. Digital conductivity meters are designed
to give a direct readout of molar conductance. For 0.001 M
solutions of cobalt ammines, the conductivity will be 415
ohm for four ions, 242 ohm 1 for three ions, and 98 ohmr1
for two ions (10). As the aquation proceeds from the trans
dichloro complex to the cis diaqua complex, the number of
ions changes from 2 to 4 at a specific rate. (See the table.)
The rate constant of aquation, k, is determined by measuring conductance over time and plotting In (A„ A() versus
time, where is the molar conductance when aquation is
complete and t is the molar conductance at time, t (7). The
value of 8.6 x 1CT4 s"1 determined for k from conductivity
measurements is somewhat smaller than that determined
from UV-visible spectra at 25 ± 0.5 C. This is expected
since, in the determination of k using UV-visible spectroscopy, only the disappearance of the trans isomer is consid1
Time/min
A/ohm
Number of ions
0
133
2
2
174
4
206
6
233
2-3
2-3
2- 3
8
258
3
10
3
12
269
300
16
331
20
357
28
395
417
428
3-4
3-4
3-4
36
90
3-
4
3—4
4
ered. In conductivity measurements, subsequent reactions
also are involved. Rund indicates that ion pairs must be
considered in conductivity measurements (9). If association occurs among the ions produced, the first-order rate
constant of the reaction appears to decrease with time, resulting in a nonlinear first-order kinetics plot. Because the
k value is derived by two different methods, comparison is,
at best, only relative.
There is a great deal of chemistry involved in this synthesis and kinetics experiment. Instructors may choose
only to do the synthesis of the trans complex and run UVVisible spectra of initial and final (after 90 min) aqueous
solutions. This shows the differences observed in a cis versus a trans octahedral complex. The more
in-depth reaction kinetics may be left for the more sophisticated student.
Literature Cited
1.
2.
-
3.
4.
5.
6.
7.
8.
9.
10.
Brauer, G., Ed. Handbook of Preparative Inorganic Chemistry; Academic Press: New
York, 1965; Vol. 2, p 1537.
Angelici, R. J. Synthesis and Techniques in Inorganic Chemistry; Saunders: Philadelphia, 1977, pp 17-19.
Wentworth, R. A. D.; Piper, T. S. Inorg. Chem. 1965,4, 709-714,
Tsuchida, R. Bull. Chem. Soc. Japan 1936, 11,721.
Pearson, R. G.; Boston, R. C.; Basolo, F. J. Phys. Chem. 1955, 59, 304-307.
Linck, R. G. Inorg. Chem. 1969,8, 1016-1018.
Marzik, J. V.; Sabatelli, A. D.; Fitzgerald, P. J,; Sameski, J. E. J. Chem. Educ. 1981,
58, 589-593.
Haake, R; Cronin, P. A. Inorg. Chem. 1963,2,879-880,
Rund, J. V.; Conrad, R. C. Inorg. Chem. 1972, 2,129-132.
Browning, D. R. Electro Metric Methods', McGraw-Hill: London, 1969, p 17.
Volume 71
Number 4
April 1994
333
