Nuclear magnetic resonance studies of transition-metal complexes of ethylenediaminetetra-acetic acid (EDTA) in aqueous solution

Abstract

The structures of EDTA complexes of transition-metal ions in aqueous solution have been investigated by n.m.r. It is shown that whereas complexes with Fe³⁺ and Mn²⁺ possess a single water molecule in their coordination spheres, the coordination number of water in complexes of Co²⁺, Ni²⁺ and Cu²⁺ are 0.19, 0.33 and 0.38, respectively. These are interpreted in terms of a dynamic equilibrium between two complexes, one containing hexa-coordinate EDTA and the other penta-coordinate EDTA. It appears that the predominant solution structures adopted by these complexes are similar to those determined for the crystals. Although the coordinated water becomes replaced by a hydroxyl group for the EDTA complex of Fe³⁺ above pH 7, this did not occur for complexes of divalent ions below pH 10.