Structures of the borate complexes with a tetritol, pentitols and hexitols in solution. A calorimetric study

Abstract

The molar heats for the reactions of borate with the polyhydroxy compounds pentaerythritol, L-arabinitol, xylitol, sorbitol and meso-inositol were determined by thermometric titrimetry. By means of the equilibrium constants, the corresponding Gibbs functions and entropies were also obtained. The results refer to aqueous medium at 25 °C and 0.1 M. The present data, together with previous data from this laboratory, lead to the conclusion that the differences in stability of these complexes are mainly determined by entropy, rather than by enthalpy factors. Unlike the other ligands (which are bidentate), pentaerythritol and meso-inositol seem to behave as tridentate ligands with borate. Nevertheless, their complexes exhibit very different stabilities, probably due to the different conformational freedoms of the two ligands in the uncomplexed form.