The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k(1(obs))' = k(0) + k(1(obs))[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: DeltaH(1(obs)) ( not equal) = 51.2 +/- 11.5 kJ mol(-1), DeltaS(1(obs)) ( not equal) = -97.3 +/- 28.9 J mol(-1) K(-1) (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are DeltaH(2(obs)) ( not equal) = 44.5 +/- 5.0 kJ mol(-1), DeltaS(2(obs)) ( not equal) = -175.8 +/- 70.3 J mol(-1) K(-1). The observed stages are proposed to proceed via interchange dissociative (I(d), first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.

中文翻译：

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铬 (III) 与 3,4-二羟基苯甲酸的反应：弱酸性水溶液中的动力学和机理。

研究了铬 (III) 和 3,4-二羟基苯甲酸 (3,4-DHBA) 之间的相互作用，导致在铬 (III) 配位球中取代水分子后形成氧键配合物。实验结果表明，该反应至少分三个阶段进行，涉及各种中间体。发现第一阶段与配体浓度线性相关 k(1(obs))' = k(0) + k(1(obs))[3, 4-DHBA]，相应的活化参数计算如下: DeltaH(1(obs)) (不等于) = 51.2 +/- 11.5 kJ mol(-1), DeltaS(1(obs)) (不等于) = -97.3 +/- 28.9 J mol(-1) K (-1)（复合激活参数）。在动力学上无法区分的第二和第三阶段不依赖于配体和铬 (III) 的浓度，分别考虑异构化和螯合过程。对应的激活参数为 DeltaH(2(obs)) (不等于) = 44.5 +/- 5.0 kJ mol(-1), DeltaS(2(obs)) (不等于) = -175.8 +/- 70.3 J mol( -1) K(-1)。建议观察到的阶段通过交换分离（I（d），第一阶段）和关联（第二和第三阶段）机制进行。反应伴随着质子释放，如 pH 值下降所示。