CMY 285 Practical 3: The isomerism of Chromium(III) Complexes A Kriel BSc Physics & Chemistry
The Synthesis of the tris(oxalate)Chromate(III) complex along with the two geometric cis- and transdiaquadioxalatoChromate(III) isomers and their subsequent properties and conversion in solution. Aim: In this experiment the synthesis and study of the geometric isomers of a Chromium(III)oxalate complex was the main objective with regard to comparing both physical and chemical properties of the two isolated isomers (such as appearance and purity test by means of a chemical reaction with dilute ammonia solution) as well as the aqueous conversion from the trans isomer in solution to the more soluble cisisomer and the rate of this conversion.
Abstract: The experimental procedure included synthesising several chromium(III) oxalate complexes, including the geometrical isomer pair cisK[Cr(C2O4)2(H2O)2].2H2O and trans –K[Cr(C2O4)2(H2O)2].3H2O. The two respective cis- and trans isomer crystals were collected and tested for purity using a dilute aqueous Ammonia solution to ensure that , in particular the trans-complex, was not contaminated by traces of the other isomer. Both isomers displayed the expected behaviour indicating relative purity. These prepared salts were made up in aqueous solutions that were used to determine absorption spectrums of both isomers. The absorption spectra of the cis and trans compounds are similar but differ greatly in intensity. The wavelength at which a maximum difference in maxima was observed (this occurred in the Ligand field area of the spectrum) ~ 420 nm, was chosen at which the absorption difference of two solutions of the trans complex at various stages of isomerisation to the cis complex was measured and manipulatedly plotted to obtain a rate constant (k= 0.0103 s-1) for this 1st order isomerisation equilibrium reaction.
Method:
The preparation of the trans isomer complex ( transK[Cr(C2O4)2(H2O)2].3H2O ) progressed well and after evaporating at a relatively steady pace at room temperature ( vessel covered with tin foil to avoid excessive evaporation which could contaminate the forming trans isomer crystals), the resulting solution of about a third the volume of the initial solution was filtered through a Buchner funnel and the crystals washed with ethanol and cold water. The crystals appeared pink-red and thus were deemed relatively pure and no further purification was needed. In the syntheses of the cis-isomer as well as the tris OxalatoChromate(III) complex no complications were noted and the method described in the Practical manual was followed regarding filtering and washing of crystals as well as the make-up of solutions for the UV/VIS analysis.
Results:
Preparation of tris(oxalate)chromate(III):
During preparation the solution had a very dark non-translucent colour. The crystals collected after re-crystallization were a dark moss-green with apparently fine texture. The yield recorded was 1.42 g.
Preparation of trans –K[Cr(C2O4)2(H2O)2].3H2O As discussed in ‘Method’ section above, the dark coloured solution was left to evaporate systematically at room temperature until only a third of the original volume remained, and the solid crystals were then filtered and washed yielding the red-pink coloured crystals of rough texture. The yield was recorded as 1.5 g.
K2Cr2O7 + 7H2C2O4.2H2O 13 H2O
2K[Cr(C2O4)2(H2O)2].3H2O + CO2(g) +
Theoretically, Potassium Dichromate is the limiting reagent. 2.0 g of K2Cr2O7 and 6.01 g of H2C2O4.2H2O was used in preparation. Thus, by taking stoichiometry into account, a yield of 4.8569 g of the trans-isomer salt is theoretically generated. The % Cr in the complex tri hydrate is 14.56 % by weight, and thus : 0.707 g Cr ought to be present in 100 % yield. From the obtained yield of 1.5 g and the 14.56 % Cr by weight, 0.22 g of Cr is present. Thus, % Yield based on Chromium = 0.22/0.707 x 100 = 31 % yield. *The poor yield may very well be attributed to the fact that the solution was filtered to collect crystals before an optimal amount of the crystals could form. However, the emphasis was placed on obtaining a trans isomer with as little cis-isomer contamination as possible.
Preparation of Cis-–K[Cr(C2O4)2(H2O)2].2H2O
The dark coloured solution mix was thick and appeared very viscous. When mixed with Ethanol, a clumpy solid formed. The mix was washed several times using ethanol. The powdery solid resulting from that was then filtered and washed with diethyl ether, and small minute amounts of orange granules appeared to be present (probably remnants of K2Cr2O7). To facilitate the removal of this, the filtered solid was washed with ethanol again. The final powdery solid/fine crystals appeared a as a pale black colour ,
The yield recorded was 2.94 g.
Testing the purity of the synthesised isomers by means of a test with dilute ammonia: Treatment of the cis salt with the ammonia solution resulted in a deep green solution spreading about the filter paper and all the solid salt dissolved into solution . Treatment of the cis salts with aqueous ammonia gives green cis-[aquahydroxobis(oxalato)chromate(III)] salts. Treatment then of the trans- crystals with a few drops of ammonia solution resulted in a minute hint of green solution at first contact where after no green solution appeared and only the solid brown crystal remained on the filter paper. Preparing solutions of the cis and trans isomers for UV/VIS analysis The solution containing the cis-complex appeared as a shade of redviolet whilst the solution of the trans-complex appeared light red.
See the following graphs detailing results from the spectrophotometric analysis.
• Note that Graph 2 displays the characteristic shape of a first order reaction when concentration is plotted vs. Time , rate of reaction is directly proportional to concentration.(and by Beer Lambert Law absorption may constitute the concentration of trans-isomer present.) Thus, one can commend the data up to this point as in agreement with theoretical interpretations.
Discussion The synthesised complexes (tris, cis and trans Chromium(III) oxalate complexes) appeared to exhibit the expected appearances from what is known in literature references. (ALAN G. MacDIARMID et al, 1977). The yield for the cis isomer in particular was fair whilst that of the tris complex was reasonable and the yield for the trans isomer was poor. However, as mentioned in the results section, the focus was primarily on isolating a pure trans isomer with the least amount of possible cis-isomer contamination. The purity tests confirmed that the synthesised isomer complexes were both quite pure respectively. In light of the reasonably pure isomers, the spectrophotometric analysis ought to have given concise results matching the expected trend – which is that the trans isomer in solution that is generally less soluble in water and hydrolyzes more rapidly than the corresponding salts of the cis isomer will gradually be converted to the cis isomer with time, and consequently the optical density of the solution declining since the data on Graph 1 suggests that the cis isomer is less optically active than the trans isomer of the complex. The graph generated by plotting the difference in absorption, D’,between the optically less dense solution (the solution left for 2 hours to inhibit a degree of isomerisation to cis) and the immediately prepared solution of trans isomer was then amended to a Ln D’ vs time plot which should have yielded a straight line plot with a negative slope. Whilst the various possible tangent slopes of this ln D’ vs time plot is negative (suggesting that isomerisation from trans to cis did take place), the plot is not a straight line as expected, but rather exhibits exponential properties. This suggests some error perhaps in the making up of the solutions or deviations in the time taken before taking UV/VIS measurements. Nonetheless a trendline was applied to it and the rate constant of the equilibrium reaction between the two isomers was calculated as the ( – slope).
References: ALAN G. MacDIARMID et al, 1977, INORGANIC SYNTHESES Volume XVII, McGraw-Hill, p. 147 – 151.
Prof. J.B.BARUAH et al,2010, Some Experiments for M. Sc in Inorganic Chemistry, p7 – 9.
Borer L. L. Et al, 1994, J chem.. Educ., 71, 332
Atkins P., De Paula J., 2006, Atkins’ Physical Chemistry 8e, Oxford University Press, p. 799
