Mechanism of complex formation between benzene-1,2-diol and aquapalladium(II). Equilibria and kinetics in acidic aqueous perchlorate media

Abstract

Palladium(II) aqua-ions, in acidic aqueous perchlorate solution, react rapidly with benzene-1,2-diol (catechol) to form a 1 : 1 green complex. From equilibrium spectrophotometric data the reaction quotient K_eq.{Pd²⁺+ H₂cat ⇌[Pd(cat)]+ 2H⁺} is found to be (6 ± 2)× 10^–3 mol dm^–3 at 25 °C and l= 0.20 mol dm^–3. The rate of the forward reaction, first order in the concentration of each reagent and with a combined [H⁺]^–1 and [H⁺]^–2 dependence, is interpreted in terms of reactions involving [Pd(OH)]⁺ and Pd[OH]₂ with the diol to give a 1 : 1 chelate complex. The reverse reaction has been investigated independently by treating the complex with excess of HClO₄ and the two-stage mechanism is discussed with reference to a general scheme which takes into account the possible intermediate species.