Eliana Tapuhi

THE KINETICS OF THE COMPLEX FORMATION BETWEEN Fe(III) AND CHLOROACETIC, DICHLOROACETIC AND TRICHLOROACETIC ACIDS. LIGAND DEPENDENCE OF THE RATE CONSTANTS

Abstract The kinetics of the reactions between Fe(III) and mono-, di-, and trichloroacetic acids have been investigated at 25°C and an ionic strength of 1 M, using the T-jump method. The rate constants for the reaction between Fe(OH)2+ and the anions of the acids exhibited values which are “normal” for this cation. Assuming those between Fe(OH)2+ and the undissociated acids to be also normal, the rate constants for the reaction of Fe3+ with the anions were evaluated, and found to depend on the basic strength of the ligand. For the rate constants of the reaction between Fe3+ and the undissociated acids, only very approximate values could be estimated. The results are compared with the rate of water exchange. The formation constants of the complexes were measured using a spectrophotometric method.

THE KINETICS OF THE REACTION BETWEEN V(III) IONS AND SALICYLIC ACID. MECHANISM AND ACTIVATION PARAMETERS

Abstract The kinetics of the reaction between V(III) and salicylic acid have been investigated in the temperature range between 7.5 and 45°C, at I=1 M. Both kinetic and equilibrium measurements show that, at hydrogen ion concentrations between 0.015 and 0.8 M, two complexes are formed, namely, VHSal2+ and VSal+. The reaction proceeds by two parallel paths, involving V3+ and HSal−, and V3+ and H2Sal, respectively. At 27°C, the corresponding rate constants are 1.61 × 103M−1 s−1 and 5.9 M−1 s−1. No contribution of a pathway involving VOH2+ could be detected. This fact, together with the ligand-dependence of the rate constants, tends to confirm the SN2 mechanism suggested by previous authors. The entropies of activation, however, are 2.2 e.u. and - within the experimental accuracy - zero, respectively. This apparent discrepancy is discussed in terms of the properties of salicylic acid. The reaction was monitored spectrophotometrically, using a stopped flow apparatus.

Mechanism of complex formation: kinetics and equilibria of the gallium(III)–5-nitrosalicylate ion system

The equilibria between gallium(III) and the 5-nitrosalycylate ion, HA–, have been investigated by spectrophotometry in acid solution (pH 1–2.5). Two complexes are identified, their formation constants being K1=[GaA+][H+]/[Ga3+][HA–]= 49 ± 2 and K1′=[Ga(HA)2+]/[Ga3+][HA–]= 500 ± 55 dm3 mol–1 at 25 °C and I= 0.1 mol dm–3. The kinetics of the formation and decomposition of the complexes have been investigated by the temperature-jump technique. The complex [GaA]+ is formed in two pairs of ‘proton-ambiguous’ paths. Attributing the observed rate exclusively to the two reaction involving HA– on the one hand, and Ga3+ and [Ga(OH)]2+ on the other, one obtains upper limits for the rate constants, namely (4.1 ± 0.2)× 102 and (6.4 ± 0.7)× 103 dm3 mol–1 s–1, respectively. A fifth reaction, involving [Ga(OH)]2+ and HA–, makes a small contribution to the observed rate. The deprotonation of [Ga(HA)]2+ is rapid in comparison with its formation from Ga3+ and HA–. Comparison of the results with the previous data provides evidence for an associative mechanism.

FORMATION CONSTANTS OF OUTER SPHERE COMPLEXES AT HIGH IONIC STRENGTH. A RE-EVALUATION OF KINETIC RESULTS

Abstract The formation constants of the inner sphere complexes of Fe(III) with Cl− and CHCl2COO− ions are determined kinetically, at 25°C and an ionic strength of 1 M. The formation constants of the ion pairs or outer sphere complexes are obtained from these results, together with previous data about the sum of the two constants. The values obtained are compatible with measurements concerning other complexes, but are lower than those calculated from the Fuoss equation. The results are discussed on the basis of the Eigen mechanism.

Mechanism of complex formation: a kinetic study of the gallium(III)–salicylate ion system

The kinetics of the reaction between gallium(III) and salicylic acid have been investigated by the temperature-jump technique at 25 °C. The measurements have been made in acid solution (pH 1.3–3) in order to avoid precipitation and to minimise polymerisation. The results are interpreted according to a reaction scheme consisting of five parallel paths : the species Ga3+, [Ga(OH)]2+, and [Ga(OH)2]+ react with the salicylate ion, and [Ga(OH)]2+ and [Ga(OH)2]+ react with salicylic acid, all giving rise to a 1 : 1 complex [Ga(OC6H4CO2)]+. The path involving [Ga(OH)2]+ and salicylate ion proceeds through an intermediate [Ga(OH)(OC6H4CO2)]. Among the reaction paths proposed, two pairs exhibit proton ambiguity; therefore, only upper limits far their rate constants can be estimated. The rate constants for the hydrolysed species are considerably larger than those for Ga3+. The equilibrium constant for the complex formation has been determined, both spectrophotometrically and kinetically, in the range pH 1.8–3.2.