M. Fontbardia et al., SODIUM-ION COMPLEXES WITH ETHYLENEDIAMINETETRAACETIC ACID, Acta crystallographica. Section C, Crystal structure communications, 49, 1993, pp. 1452-1456
The crystal structures of tetrasodium (1,2-ethanediyldinitrilo)tetraac
etate pentahydrate and disodium dihydrogen (1,2-ethanediyldinitrilo)te
traacetate dihydrate have been determined. Na ions prefer to act as br
idges between different edta ligands and only in the tetrasodium salt
is the edta ligand hexadentate to one Na ion. Na ions display seven-co
ordination, when coordinated to one edta ligand (distorted pentagonal
bipyramid), and six-coordination, when linked to several edta ligands
(from distorted octahedral to trapezoidal bipyramidal geometry). An Na
ion in the tetrasodium salt shows five-coordination, because the sixt
h site is blocked by the edta ligand. H atoms in the disodium salt are
linked to amine groups. Only these two salts can be obtained on varyi
ng the pH of an aqueous solution containing Na+ and ethylenediaminetet
raacetic acid. The first is obtained around pH 2-5 and the second arou
nd pH 12.