Investigation of cis- and trans- Chromium(III) Chloride Aquation

It has been observed that chromium (III) ions in aqueous solutions freshly prepared from the chloride salt exhibit a slow yet distinct absorption shift corresponding to a color change from green to blue that takes place on a timescale of ~ 24 hours under ―normal‖ laboratory conditions. Previous literature studies have suggested that this color change is due to the stepwise replacement of the 2 coordinated chloride ions with water. While the cis- and trans- dichlorotetraaquochromium (III) isomers exist, only the trans- form is produced via the above procedure. However, both isomers can be produced in high chloride concentration solutions. Previous estimates of the trans → cis isomerization rate are not explained solely by one-step kinetics. It has been suggested that a multistep process involving the trichlorotriaquo- species may be the cause of this high rate, but to date this species has never been verified. Our previous investigation of chromium (III) aquation reactions utilized spectral factor analysis (FA) to monitor species and trans- kinetics (URC, 2007). The current study is an extension of this earlier work, and focused on the verification of the existence of the trichlorotriaquo- species, its corresponding kinetics, and the ability of spectra FA to track the concentrations of multiple species during isomerization and aquation. Kinetic data derived from the FA was also used to estimate the difference in aquation rates of the cis- and trans- isomers. It is hoped that these studies may form the basis for instructional laboratories on factor analysis, kinetics, and complexation reactions