The complexation reaction of gallium(III) with 4-nitrocatechol: evidence for a dissociative-interchange mechanism on (H 2O) 5Ga(OH) 2

• Published in: Inorganica Chimica Acta Vol. 311; no. 1; pp. 33 - 39
• Main Author: Macdonald, C.J
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2000
• Subjects: 4-Nitrocatechol; Complexation; Equilibrium; Gallium(III); Kinetics and mechanism; Gallium(III); Kinetics and mechanism; Complexation; Equilibrium; 4-Nitrocatechol
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(00)00294-2

The equilibrium and kinetics for the reaction of excess gallium(III) with 4-nitrocatechol (H 2L) to give the monochelate in aqueous solution: Ga 3++ H 2 L ⇌ K c GaL ++2 H + was investigated over the pH range 2–3. Measured spectrophotometrically, K c=(0.034±0.003) M, is in excellent agreement with the independently determined kinetic value of (0.034±0.001) M (25°C, I=0.1 M). Use of the ligand 4-nitrocatechol, whose p K a differs substantially from the p K h for Ga 3+, eliminated the proton ambiguity problem that has complicated many similar investigations, and allowed identification of the reaction modes. Over the pH range studied, the complex formation reaction involves the monohydroxo (H 2O) 5Ga(OH) 2+ ion and H 2L {rate constant, k (25°C, I=0.1 M)=(7.4±0.4)×10 3 M −1 s −1}. For the reverse (decomposition) reaction, whose rate varies directly with [H +], k (25°C, I=0.1 M)=(65±3) M −1 s −1, Δ H ‡=(65±3) kJ mol −1, Δ S ‡=(−1±5) J K −1 mol −1. The kinetic data are consistent with the I d mechanism for ligand substitution.