Complex formation of small organic ligands with uranium(VI) - comparison of stability constants with proton dissociation

During the past we have studied the complex formation behavior of uranium(VI) towards several aromatic carboxylic acids. These studies were continued, so that the formation constants of the monohydroxo- and dihydroxo- as well as some trihydroxobenzoic acids are available. A short review of these formation constants will be given.
Besides this we have compared the complex formation constants to the protonation constants of the carboxyl group. A linear relationship between these two properties was found. An decreasing protonation constant is connected to an increasing complex formation constant.
Nevertheless some exceptions of this rule were observed. These deviations can be explained by the different type of complex formation between uranium(VI) and the organic ligand. This concerns especially the number and position of the hydroxo groups in phenolic carboxylic acids. In the case of 2,3-dihydroxybenzoic the two OH-groups are involved in the complex formation already at relative low pH values. During the complex formation two protons are released from the organic ligand. The formed complex does not include the carboxylic group. The two phenolic hydroxo groups form a stable five atom ring with the uranylion. This result could be confirmed by EXAFS measurements.