Potassium Dioxalato Diaquo Chromium (III) Complexes: Trans–Cis Isomerization

Abstract

THE X-ray diffraction data of van Niekerk and Schoening¹, obtained from a sample of red potassium chromi oxalate prepared in these laboratories and corresponding analytically to K[Cr(H₂O)₂(C₂O₄)₂].3H₂O, led these workers to affirm that this salt has the trans configuration, as deduced by Werner² from other evidence. We have investigated the colour and other changes which occur when aqueous solutions of this salt are aged under different conditions, and our results show that the changes are due to trans–cis isomerization, uncomplicated by olation phenomena. Since the same ultimate molar extinction coefficient is attained over a wide range of temperature and concentration, we conclude that the transformation to the cis configuration proceeds to practical completion. Kinetically, the reaction 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 × 10⁹ and E = −17.9 k.cal./mol. (see graph). Confirmation of the practical completion of the trans–cis conversion was obtained by isolation of the blue cis salt, K[Cr(H₂O)₂(C₂O₄)₂].2H₂O, moderately dilute solutions of which are optically stable when olation is excluded.