Thermodynamics of the formation of metal complexes of 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid and of 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-N,N′-diacetic acid

Abstract

Thermodynamic functions for the formation of the alkaline-earth and some transition-metal complexes of the ligands L¹(1,4,1O-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid) and L²(1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-N,N′-diacetic acid) were determined by direct batch microcalorimetry. The ligand L¹ gives more stable transition-metal complexes than L² due to more favourable heats of reaction. The reverse situation is found for complexes of the alkaline-earth metals, although the differences in stability of the complexes of these metals with the two ligands are small due to less favourable entropy changes in the case of L². The two macrocyclic ligands behave similarly to their non-cyclic analogues L³ and L⁴(3-oxapentane-1,5-diamine-and 3,6-dioxaoctane-1,8-diamine-N,N,N′,N′-tetra-acetic acid) but form less stable complexes due to less favourable heats of reaction. The transition-metal complexes of L¹ do not follow the Irving-Williams order of stability, contrary to the situation with complexes of L². The reason for the inversion of the stability order is discussed in terms of the changes in crystal field stabilization energies when the octahedrally co-ordinated aqueous ions form complexes of a different structure, either five-co-ordinated or tetrahedral.