Nuclear magnetic resonance studies of transition-metal complexes of ethylenediamine-NNN′N′-tetramethylphosphonate in aqueous solution
Nuclear magnetic resonance studies have been carried out on the structures of ethylenediamineNNN′N′-tetramethylphosphonate (edtmp) complexes of Cu2+, Fe3+, Co2+, Ni2+, and Mn2+ in aqueous solution. It is demonstrated that the hydration numbers in these complexes are higher than for the corresponding carboxylate complexes, approximating to 1 for complexes with Cu2+ or Fe3+ and between 1 and 2 for the other complexes. Evidently, replacement of carboxyl groups by binegative phosphonate groups produces additional strain within the complex arising from increased charge repulsions and this increases the tendency to have unco-ordinated ligand groups. As expected, this effect is less important for trivalent Fe3+ as both its ethylenediamine-NNN′N′-tetra-acetate (edta) and edtmp complexes contain one water molecule in their first co-ordination spheres. Although the water molecule is replaced by a hydroxyl group above pH 7 for the edta complex of Fe3+, this did not occur for the corresponding edtmp complex.