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.