KINETIC STUDIES ON FORMATION OF METAL CHELATES .V. FORMATION OF TETRA-AQUO-MONO-MALONATO-CHROMIUM(III) ION FROM HEXA-AQUOCHROMIUM(III) ION AND MALONIC ACID

The rate of formation of tetra-aquo-mono-malonatochromium(III) ion from hexa-aquo-chromium(III) ion and malonic acid in aqueous HClO4NaClO4 has been followed spectrophotometrically under conditions in which no significant amount of bis-malonato-chromate(III) ion is formed. From the observed dependence of the rate on malonic acid and perchloric acid concentrations (acceleration by malonic acid and retardation by perchloric acid) a mechanism is suggested involving the rate-determining reaction of the ion-pair Cr(OH2)63+. HMal-(H2Mal = malonic acid) to form the product. A comparison of the values of the activation parameters, ΔH≠ and ΔS≠, for this system with the corresponding literature values for the water exchange and glycinato complex formation reactions of Cr(OH2)63+ ion suggests that in the present system the ion-pair reacts essentially by an SN1 process (SN1 IP mechanism). Evidence for ion-pair formation was obtained from observations on the ultraviolet spectrum. © 1969.