An Experiment in trans-cis lsomerization Synthesis and Kinetics of trans-Dichlorotetraarnrninecobalt(lll)Chloride Londa L. Borer and Howard W. Erdman California State University, Sacramento, Sacramento, CA 9581 9 Cobalt(II1)coordination complexes are ideal for in-depth ple crystals of the cis isomer are collected by vacuum fdtrastudies a t the undergraduate level. The syntheses of the tion. isomers in this study are relatively short and simple, and UV-visible spectroscopy, infrared spectroscopy, conducKinetics Study tance, and kinetic studies can be done easily. APerkin-Elmer Lambda Array 3840 W N i s spectrophoThe first published synthesis of tmnstometer was used to follow the aquation of trans[Co(NHllaCl9ICl.H9O. uerformed by S. M. Jorpensen. t w k [Co(NH&C121Cl in water, in 0.1 N HN03, and in 1N HC104 approxii&dy 96 h(if Starting 4 t h reagentsthat c.an be media a t a temperature of 25 k 0.5 'C. For the aquation of purchased easily, the procedure described in this paper the trans isomer in distilled water, 39.7 mg was dissolved takes approximately 24 h to synthesize [CdNH3)4C031N03 in 25 mL (6.32 x lo3 M). The instrument was set a t 630.4 *om which the fmal product, trans-[C~(NH~)~Cl~lCl~H~O, nm. the most intense peak of the trans isomer. and the is synthesized in 15 Gin. aq;ation was monitorei every minute for 90 min.'Analysis of the chame-transfer bands was made in the reeion from Experimental 190 to 390 -nm using a lo5 M distilled water sol&on. The [CO(NH~)~CO~INO~ was prepared by the method decis and trans isomer peaks were located a t 213.7 and 253 scribed in Synthesis and Technique in Inorganic Chemistry nm, respectfully, aloig with an isobestic point a t 230 nm. (2). [Co(NH&ClzICl was prepared according to the proceUsing the charge-transfer peak a t 253 nm, analysis was dures described here. made using a solution of 18.8 mg of the trans isomer dissolved in 1000 mL of l N HC104(7.48 x lo4 M). Synthesis Conductance measurements were made using a ColePalmer digital conductivitv meter. model 1481-90. A volA sand bath is pre-heated to a temperature high enough ume of 100mL of water was added to a 25.1 mg sample of to heat a solution to 80 'C. One gram (4 mmol) of carthe trans isomer (1.042 x lo3 MI. The solution was stirred bonatotetraamminecobalt(II1)nitrate is dissolved in 5 mL continuously during the aquation. Measurements were reof water in a 50-mL flask containing a thermometer and a corded every minute for the first 10 min and then taken magnetic bar stirrer. The solution is heated to 50-60 Y! in less frequently as the change in conductance decreased. A the pre-heated sand bath for 3 min with stirring. Concenplot was made of in (L-At) versus time and a leasttrated hydrochloric acid (3.3 mL) is added as quickly as squares analysis provided the rate constant, k. possible and with care to prevent the solution from frothThe aquation of the cis isomer also was investigated by ing. conductivity to show that the reaction proceeds from an The solution is heated to 80 T! with vigorous stirring for aquachloro to diaqua complex. 5 min. (This temperature is criticaL9 A dark green precipitate of trans-dichlorotetraamminecobalt(II1)chloride is Results and Discusslon pmduced. The solution is cooled quickly to mom temperature in an The reaction of the tmns isomer in aqueous solutions can ice bath, and the crude product is collected by vacuum be followed easily in the UV-Vis region. Initially the specfiltration. The crude product is transferred to a 100-mL trum has three peaks, as per a trans complex, but as the beaker, swirled in 30 mL of ice-cold water to dissolve any reaction uroaresses. the sulittine diminishes until there cis isomer, and filtered immediately to obtain the pure are only Gopeaks indicating that the aquated cis complex trans isomer product. is formed (31.The steps of aquation ~rouosedbv Tsuchida The green trans isomer crystals are washed with approxare as follows (4): imately 2 mL of ice-cold methanol and air dried to yield 0.45-0.55 g (45-55%) of product. The cis complex, trnn~-[Co(NH,)~C1,1C1+ [Co(NHJ4C1(H20)C12,is obtained by reducing the temperature of the aqueous-methanol filtrate solution. The purtran~-[Co(NH~)~Cl HaOIClz+
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Conductivity Measurementsfor the trans Isomer Timelmin
A plot of in (A, -Am) versus 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 he determined from the following equation:
In (A, -A,) = k t + constant (1) where A, is the absorbance a t time t, A is the absorbance when aquation of the trans isomer is complete, k is the rate constant in s-I, 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 &, the absorbance at time zero, t 0. The aquation in water of the trans isomer over a period of 30 min a t 630.4 nm provided an aquation rate constant of 1.85 x lo3 s-I which compared favorably to the literature value of 1.83 x 10 (5). Monitoring aquation of the trans complex at wavelength 253 nm, with HC104 at pH 2, provided a k value of 2.28 x lo3 s-' and an extinction coefs-' and 2.40 x lo4. reficient of 2.39 x lo4 (lit. 1.90 x spectively)(5, 6). Kinetics of a reaction also can be studied hv followine a change in conductivity (7-9).Since there is a n increaspin the number of ions during aauation, the conductivity of the solution will change acco;d&ly and, as when ahsdrbance 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 ~ h - ' c m ~ m o l -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 o h - ' for four ions, 242 ohm-' for three ions, and 98 o h - ' for two ions (10).As the aauation ~roceedsfrom the trans dichloro complex to the cis d~aquacomplex, the number of i c (See the table., i w s chances from 2 to 4 at a s ~ e c ~ frate. The rate constant of aquation, &, is determined by measuring conductance over time and plotting In (L- At) versus time, where is the molar conductance when aquation is complete and t is the molar conductance a t time, t (7). The value of 8.6 x lo4 s-' determined for k from conductivity measurements is somewhat smaller than that determined from UV-visible spectra a t 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 consid-
*
0 2 4 6 8 10 12 16 20 28 36 90
ohm-' 133 1 74 206 233 258 269 300 331 357 395 417 428
Number of ions
2 2-3 23 2-3 3 3 34 34 34 34 34 4
ered. In conductivity measurements, subsequent reactions also are involved. Rund indicates that ion pairs must he considered in conductivity measurements (9).If association occurs among the ions produced, the fxst-order rate constant of the reaction amears .. to decrease with time. resulting in a nonlinear first-order kinetics plot. ~ectlusethe k value is derived by two different methods. com~arisonis. a t 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 WVisible spectra of initial and final (after 90 min) aqueous solutions. This shows the differences ohsewed in a cis versus a trans octahedral complex. The more in-depth reaction kinetics may he left for the more sophisticated student. Literature Cited 1. Brauer,G..Ed.H&ofPrepmfiwl~rgonlcChemiat'y;Aeade~ePrrss:New York,1965; Val. 2, p 1537. 2. Angeliei, R. J. Synthssis end nchnlpua in Inorgenie Chemiafm;Sanders: Philadelphk 1977, pp 17-19. 3. Wenfworth. R. A D.; Piper, T S lnorg. Chem. 1985.4.709-714. 4. Tsuchids. R.Bull. C h m . Soe.Jaoon 1838.. 12.721. . 6. Pearson, k.G.; Boston, R. C.: ~ s & l oF, J Phys Chem. 1955.69, SOPS07. 6. Linek R.G.Inorg.Chem 1968,8,10161018. 7. Manik. J.V.; Sabatel1i.A.D.;Rtzgerald, P.J.; Smeald. J. E. J. Chem Educ 1881, kR k%5Cc3
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