The reactivity of co-ordinated oxalate. Part VII. Specific salt effects in the racemisation of the trisoxalatochromium(III) anion {[Cr(ox)3]3–}

Abstract

The effect of changes of cation (M) in the salt M₃Cr(ox)₃ on the observed rate of racemisation of the [Cr(ox)₃]^3– anion have been investigated in aqueous solution at 25 °C, (where M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, Me₄N⁺, Et₄N⁺, Prⁿ₄N⁺, and Buⁿ₄N⁺). In the presence of the alkali-metal ions and the ammonium ion the observed rates of [Cr(ox)₃]^3– racemisation were similar but decreased with increasing size of the R₄N⁺ ion. Increasing the concentration of Et₄N⁺(Et₄NI) caused further decreases in the observed racemisation rate. These data are interpreted in terms of specific ion interactions, with [Cr(ox)₃]^3– participating in ‘water structure enforced ion-pairing’ with the R₄N⁺ ions. The Et₄N⁺⋯[Cr(ox)₃]^3– ion-pair formation constant and the specific racemisation rate constant of [Cr(ox)₃]^3– in the ion-pair have been determined by curve fitting to be 26 and 18·7 × 10^–3 min^–1 respectively.

Changes in concentration of the added salts NaClO₄, NH₄NO₃, and Nal on the observed rate of [Cr(ox)₃]^3– racemisation have also been investigated in aqueous solution at 25 °C and the presence of a residual racemisation rate observed. This racemisation is thought to occur by interaction with the solvent. This view is supported by rate studies in mixed solvents and results of these are compared with earlier results.