# Why does only the cis-isomer of potassium dioxalatodiaquochromate(III) form?

A friend of mine performed a standard synthesis reaction of potassium dioxalatodiaquochromate(III) by adding potassium dichromate to ethandioic acid dihydrate and a little water. The synthesis instructions suggested that only the cis-form is created. When looking online about this, I found numerous dodgy documents corroborating this idea, such as this or this.

Why is this? I can't find an explanation as to why only one stereoisomer forms nor any mechanisms anywhere.

• There is only one isomer. How could it be otherwise? – Ivan Neretin Jul 6 '19 at 14:45
• @IvanNeretin There are numerous well-known reactions across organic chemistry that lead to the production of various stereoisomers of products and both stereoisomers here are stable (in fact, there is an equilbrium between the forms set up that leans slightly towards the cis-form in aqueous solution). So why is only the cis formed in the reaction? – Isky Mathews Jul 6 '19 at 14:51
• @IvanNeretin If it's clear, could you write an answer with a mechanism? – Isky Mathews Jul 6 '19 at 14:54
• @IvanNeretin there seem to be a number of research papers and educational labs discussing the cis and trans isomers of this compound. I'm confused why you say it doesn't exist. This image shows the cis form. The trans would have the H2Os both axial. – Tyberius Jul 6 '19 at 17:52
• It was a misunderstanding on my part; sorry about that. Yes, there are two isomers and they can be produced separately. – Ivan Neretin Jul 6 '19 at 18:23

I'm not an inorganic chemist, but to my understanding, both cis- and trans-dioxalatodiaquochromate(III) ions exist (Note that some also called it as Diaquabis(Oxalato)Chromate (III)). According to what I found online is that solid state reaction gives cis-form while aqueous reaction predominantly forms trans-isomer (it is mentioned in one of your attachments). However, it's still up to debate.

Hamm in 1952 had shown that transcis Isomerization using UV-vis spectroscopy ($$trans \rightarrow cis$$; Ref.1). According to Hamm, all previous workers have prepared trans-isomer because its lack of solubility. Yet, the existence of cis- and trans-isomers has been first recognized by Werner and has presented the preparation methods of two isomers individually (Ref.2), thus it is not true that the your concept of only the cis-stereoisomer forms.

Werner had mentioned that aqueous solution of trans-isomer is stable only in the cold temperatures. That may be the reason why only cis-isomer forms in solid state preparations because the preparation needs heating at high temperature.

Using the method given by Werner, trans-dioxalatodiaquochromate(III) isomer has been prepared and its crystal structure has been studied (Ref.3 & 4).

Later studies have showed this trans-isomer converted to cis-isomer by long standing in aqueous solutions (Ref.5). Based on their results, the authors have concluded that these color changes (dark red trans-isomer to blue cis-isomer) are due to transcis isomerization, uncomplicated by olation phenomena and that the transformation to the cis configuration proceeds to practical completion. They also stated that:

Kinetically, the reaction (transcis isomerization) obeys a first-order law, and the dependence of the observed velocity constant upon temperature is found to be adequately described by the Arrhenius equation, where $$A = 3 \times 10^9$$ and $$E = \pu{−17.9 kcal/mol}$$.

Finally, in recent studies have been done on computational analysis of trans- and cis-Isomers of Potassium dioxalatodiaquochromate(III). The studies revealed that cis-Isomer is slightly stable than trans-Isomer (Ref.6):

The trans- and cis-isomers of potassium diaquabis(oxalato)chromate (III) were studied computationally and experimentally. The structures of trans- and cis-configurations of $$\ce{[Cr(H2O)2(C2O4)2]-}$$ were optimized by DFT methodology with various functionals namely: B3LYP, CAM-B3LYP, TPSS, PBE, M06-L and ωB97X-D along with the more sophisticated MP2 method. The calculations show that the most stable forms for both isomers are in quartet states. The results from all DFT methods reveal that the cis-isomer is literally more stable than the trans-isomer with the lower average relative energy of $$\pu{2.1 kcal/mol}$$. These are consistent with the results from MP2 calculation and experimental observation. The absorption wavelengths for the excited states of trans- and cis-structures were calculated by the time-dependent density functional theory (TDDFT) method. For the experiments, the trans- and cis-isomers of potassium diaquabis(oxalato)chromate (III) were synthesized and characterized by UV-Vis spectrophotometry. Both isomers have two maximum absorption wavelengths at $$415$$ and $$\pu{560 nm}$$.

References:

1. Randall E. Hamm, “Complex Ions of Chromium. II. transcis Isomerization of Potassium Dioxalatodiaquochromate(III),” J. Am. Chem. Soc. 1953, 75(3), 609-611 (https://doi.org/10.1021/ja01099a026).
2. A. Werner, W. J. Bowis, A. Hoblik, H. Schwarz, H. Surber, “Über Metallverbindungen mit komplex gebundener Oxalsäure. Zweite Mitteilung: Über Dioxaloverbindungen),” Justus Liebigs Annalen der Chemie 1914, 406(3), 261-331 (https://doi.org/10.1021/ja01099a026).
3. J. N. van Niekerk, F. R. L. Schoening, “Structure of Trans-Potassium Dioxalato-Diaquo-Chromiate,” Nature 1950, 166, 108 (https://doi.org/10.1038/166108a0).
4. J. N. van Niekerk, F. R. L. Schoening, “The crystal structure of trans potassium dioxalatodiaquochromiate, $$\ce{K[Cr(C2O4)2(H2O)2].3H2O}$$,” Acta Cryst. 1951, 4(1), 35-41 (https://doi.org/10.1107/S0365110X51000064).
5. G. E. Cunningham, R. W. Burley, M. T. Friend, “Potassium Dioxalato Diaquo Chromium (III) Complexes: TransCis Isomerization,” Nature 1952, 169, 1103 (https://doi.org/10.1038/1691103a0).
6. Songtham Ruangchaithaweesuk, Juthathip Chorkate, Thana Maihom, Potjaman Poolmee, Piti Treesukol, Tipawan Rungsawang, Phornphimon Maitarad, Bundet Boekfa, "Combined Computational and Experimental Studies of Trans- and Cis-Isomers of Potassium Diaquabis(Oxalato)Chromate (III)," Key Engineering Materials 2017, 757, 103-107 (https://doi.org/10.4028/www.scientific.net/KEM.757.103).
• I was just going to say that I never meant to claim that only the cis-form occurs in reactions, simply in the reaction mentioned above. Nevertheless, an amazingly informative answer with all kinds of information previously stuck behind paywalls for me! – Isky Mathews Jul 8 '19 at 11:17