Early stages of the hydrolysis of chromium(III) in aqueous solution-XII. Kinetics of cleavage of the trimer and tetramer in acidic solution

The kinetics of cleavage of the hydrolytic trimer and tetramer of chromium(III) have been studied in acidic solution and at elevated temperatures using spectrophotometric methods. For both reactions, the variation in absorbance with time was consistent with the presence of two consecutive rate-determining processes. The two reactions were concluded to proceed according to: Product analyses and consideration of the stability constants for these oligomers indicated that their overall cleavage reactions were irreversible under the conditions studied. However, it was unclear whether the initial step in tetramer cleavage was irreversible or not. The dimer was identified as an intermediate in the cleavage of the trimer. Each of the bridge-cleavage processes (k₁ and k₂) occurred via acid-dependent pathways, involving protonation processes in addition to bridge-cleavage processes, while only some of them displayed acid independent pathways. The acid dependence of the rate constants and associated parameters at 298 K are: (i) for trimer cleavage k₁ = k_b [H⁺], where the dimer to monomer was found to be the rate-determining step at higher temperatures (≈350 K), while cleavage of the trimer to dimer and monomer becomes rate determining at lower temperature (≈ 300 K). This change in the rate-determining step is attributable to the much higher value for k₁ compared with k₂. Possible mechanisms for trimer and tetramer cleavage are discussed and their rates compared to previously studied cleavage reactions of Cr^III oligomers.