Komplexone XXXVI. Reakinsenthalpie und ‐entropie bei der Bildung der Metallkomplexe der höheren EDTA‐Homologen

Abstract

Abstract Complexing agents of the type ( − OOCCH 2 )N(CH 2 ) n N(CH 2 COO − ) 2 , with n values from 2 to 8, have been investigated. The stability constants and the heats evolved by formation of the 1:1‐complexes of Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ , Hg 2+ and La 3+ have been determined. From the behaviour in complex formation reaction these cations can be divided into two groups. In the first one, including the A‐cations Ca 2+ , Mg 2+ , Mn 2+ , Cd 2+ , Pb 2+ and La 3+ , the stability constants and −Δ H by formation of the 1:1‐complexes decrease rapidly when n is increased from 2 to 4. The second one contains all other metal ions investigated except Hg 2+ , and the drop in stability is observed between n = 4 and n = 5, whereas −Δ H slightly increases. Comparing the enthalpy values by formation of the complexes with cations of the first group of iminodiacetate and of EDTA (Table S), a considerable −Δ H increment is observed. The entropy gain by complex formation is approximately the same if the number of methylene groups in the chain connecting the two nitrogens is increased from 2 to 4. Replacement of one or two of these CH 2 − groups by 0 or S decreases generally the free energy and the enthalpy of complex formation to a great extent if new 5‐membered chelate rings are formed.