Thermodynamische Untersuchungen zur Chelatbildung zwischen Aminoterephthalsäure-N,N-diessigsäure und 3 d- bzw. Erdalkalimetallionen. Eine Betrachtung zum Einfluß einer koordinativ inaktiven Carboxylgruppe auf die komplexe Bindung von Metallionen durch Aminopolycarbonsäuren

N-(2-Carboxyphenyl)iminodiacetic acid (H₃A) and N-(2,5-dicarboxyphenyl)iminodiacetic acid (H₄B) are tetradentate ligands and form complexes of the composition MA- and MB^2? with M^II ions. These compounds differ by the additional charge of the second carboxylic group only, which is fixed to the benzene nucleus and which is unable for coordination for steric reasons. Using an anisothermal calorimeter ΔH values for the formation of the complexes MA- and MB^2? in aqueous solution have been measured at an ionic strength 0.1 m KNO₃. From these data, and from the stability constants of the complexes, entropy changes ΔS have been calculated. In all cases investigated (M^m+ = H⁺, Mg²⁺, Ca²⁺, Sr²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) the ΔH values are more negative for the complexes MA^m-3 than for complexes MB^m-4, whereas the ΔS values are greater for complexes MB^m-4. Using a simple model for the molecules of the complexes MB^m-4 and empirically determined dielectric constants of the medium between the central ions and the noncoordinated ionized carboxylic group, the electrostatic attraction between these charges was calculated. Basing on these results the influence of the noncoordinated carboxylic group on the central atom by the mesomeric and inductive effect is discussed.