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The composition of the complex formed by the interaction of hexaaquochromium(III) with acetate ions was determined by Job's method of continuous variations and by conductivity measurements. It was found to be (1:2). Kinetic studies were carried out at 30, 35 and 40°C while the [H+] of the medium was varied from 0.1 × 10−4 M to 31.6 × 10−4 M. The variation of the pseudo-first order rate constant (kobs) with [H+] and the ligand concentration indicated a mechanism based on ion-pair formation and the following rate equation was derived on this basis: kobs=[kanKIKa[H+]+k′anK′IKa][acetate]T[H+]2[H+]Ka+[H+]K′I+KaK′I+[KIKa+[H+]+K′IK′aKa][acetate]T The reaction was found to be of the associative interchange (Ia) type for the reaction of Cr(H2O)63+ and dissociative interchange (Id) type for that of Cr(H2O)5OH2+ with acetate ions. The activation parameters, calculated using the Eyring equation are: ΔH∗ = 103 kJ mol−1; ΔS∗ = 31 JK−1mol−1 for the formation of Cr(H2O)5(OOCCH3)2+ and ΔH = 104 kJ mol−1; ΔS∗ = 41(±2)JK−1mol−1 for the formation of Cr(H2O)4OH(OOCCH3)+.

The interaction of hexaaquochromium(III) with salicylic acid was studied in the pH range 2.5-4.2. The composition of the complex formed was one mole of salicylic acid to one mole of hexaaquochromium(III). The variation of pseudo first order rate constant with pH, ionic strength and ligand concentrations indicated a mechanism based on ion-pair formation. The rate equation derived on the basis of this mechanism is: k obs = k an K 1 K a [salicylic] T [H + +K a +K 1 K a [salicylic] T An associative interchange mechanism is suggested for the reaction. The activation parameters calculated by Eyring equation are ΔH ∗ = 74.48 (±0.33) kJ mol−1; ΔS ∗ = −60.49 (±1.11) JK−1 mol−1 for anation and ΔH = −9.00 (±0.43) kJ mol−1; ΔS = −13.60(±1.42) KJ−1 mol−1 for ion-pairing.