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

Abstract

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.