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

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Abstract
Complexing agents of the type (−OOCCH2)N(CH2)nN(CH2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 Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+ and La3+ 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 Ca2+, Mg2+, Mn2+, Cd2+, Pb2+ and La3+, the stability constants and −ΔH 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 Hg2+, and the drop in stability is observed between n = 4 and n = 5, whereas −ΔH 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 −ΔH 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 CH2− 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.