All chemicals were of pure grade and were used without further purification. All solutions were prepared using bidistilled water. The ethylenediaminetetra-3-propinate was prepared using a previously described procedure [6]. Stock solution of (0.1 mol∙dm⁻³) of hexaaquachromium (III) was prepared by dissolving CrCl₃ in bidistilled water and leaving the solution for 48 hours at 45˚C, where upon green color of CrCl₃ changed to blue color of aquachromium(III) [7].

The absorbance measurements were performed using thermostatted 292 Cecil spectrophotometer and pH measurements were conducted with Griffin pH meter fitted with glass-calomel electrode standardized by potassium hydrogen phthalate.

Kinetic experiments were conducted by mixing thermostatted solutions of chromium(III) and the ethylenediaminetetra-3-propinate and adjusting hydrogen ion concentration to the required value with potassium hydroxide or perchloric acid. Ionic strength was adjusted by sodium perchlorate solution. The solution was then introduced into the reaction vessel, which was previously thermostatted to the desired temperature and the reaction was followed spectrophotometrically at l_max = 557 nm for the complex formed. The reaction rate was followed under pseudo first order conditions where at least ten fold excess of the ligand concentration over the reactant chromium(III) concentration was always ensured. Values of the observed first order rate constant, k_obs, were determined graphically for each run by plotting log (A_¥-A_t) versus time, t, where A denotes the measured absorbance and the subscripts refer to time of reaction. The absorbance (A_¥) was obtained directly after ensuring completion of the reaction. First order plots were linear for more than 85% of the reaction progress.

The pentaaquahydroxochromium(III) species is more

Table 1. Values of k_obs under various conditions.

reactive than the hexaaquachromium(III) due to the presence of OH⁻ which causes an increase of water labilities due to its π-bonding ability [15-22].

The results obtained can be explained by the following mechanism for the interaction between the predominant species of chromium(III) with the predominant species of EDTP in the pH range under investigation

Table 2. Kinetic data for the interaction of Cr(III) with EDTP at various temperature and pH; [Cr(III)] = 8.8 × 10⁻³ mol∙dm⁻³, [EDTP] = 11 × 10⁻² mol∙dm⁻³, I = 0.6 mol∙dm⁻³.

where IP₁-IP₄ are the hexaaquo and pentaaquohydroxy ion pair complexes of chromium(III) and EDTP.

The rate of exchange of the first ligand molecule, in the inner coordination sphere of the metal center is slow and therefore the rate determining Equations (7)-(10) [19-21]. As soon as one carboxyl group of the ligand enters into the inner sphere, the electron density on the chromium center increases owing to the inductive effect and as results the remaining ligands are labilized easily and its substitution is rapid. From the previous mechanism, the first order rate constant is derived as

by inversing Equation (11) we get equation

and a plot of 1/k_obs versus 1/[EDTP] gave s straight line with slopes

and intercepts, I

and

The values of the ion pair formation constants, K_IP and the rate constants of the rate determining steps, k, were calculating by plotting 1/k_obs versus 1/[EDTP] at different pH, (Figure 4). Values 7.11, 14.78, 10.15 and 16.81 mol⁻¹∙dm³ for the ion pair formation constants, KIP and 1.83, 4.81, 2.97 and 5.23 × 10⁻³ s⁻¹ for the rate determining steps, k, respectively were calculating by applying Equations (13)-(15) at different hydrogen ion concentrations and taking the values of K₁, K₂ and K_h from Equations (1) and (2). The unexpected values of ion pair formation constants, K_IP, () indicate the mechanism is not only via electrostatic attraction but also include hydrogen bonding between the acetate groups and the first coordination sphere H₂O.

The effect of temperature on the rate reaction was also studied at different hydrogen ion concentrations (Table 2). The activation parameters were calculating using Arrhenius plots values and the Eyring equation and were found to be 57.2 ± 3 kJ∙mol⁻¹ for the energy of activation and −128 ± 8 J∙K⁻¹∙mol⁻¹ for the entropy of activation.

It is well known that substitution reactions of hexaaquachromium(III) with a variety of ligands proceed by associative [12,20-22] and dissociative [23,24] mechanisms. Swaddle [25,26] and Lincoln [27] have reviewed that the activation parameters and mechanism of octahedral substitution and concluded that an associative mechanism is operative for octahedral cationic complexes of trivalent metal ions except for Co(III) with ionic radii greater than 60 pm, which demand associative character for substitution reaction of [Cr(H₂O)₆]³⁺ The associative mechanism is further supported by 1) lowering of enthalpy and large negative entropy for substitution of water by ligand compare to water exchange (for water exchange ∆H* = 109.6 kJ∙mol⁻¹ and ∆S* = +12 J∙K⁻¹∙mol⁻¹ [28]); 2) the straight line obtained from plotting of log k₁ with log k₂ [29], (Figure 5) (Where k1 and k₂ are the first order rate constants at different temperature) for the substitution of water in [Cr(H₂O)₆]³⁺ by valine [16], glycine [17], serine [18], Aspartic acid [19], L-glutamic acid [21], DL-lysine [21], DL-leucine [22]

and EDTP (this work).