The complexation reaction of gallium(III) with 4-nitrocatechol: evidence for a dissociative-interchange mechanism on (H2O)5Ga(OH)2+

The equilibrium and kinetics for the reaction of excess gallium(III) with 4-nitrocatechol (H2L) to give the monochelate in aqueous solution: was investigated over the pH range 2–3. Measured spectrophotometrically, Kc=(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 pKa differs substantially from the pKh for Ga3+, 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 (H2O)5Ga(OH)2+ ion and H2L {rate constant, k (25°C, I=0.1 M)=(7.4±0.4)×103 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, ΔH‡=(65±3) kJ mol−1, ΔS‡=(−1±5) J K−1 mol−1. The kinetic data are consistent with the Id mechanism for ligand substitution.