Metal Chelates of Aminoalkylphosphonic Acids

Stabilities Alkylphosphonate groups are monodentate donors of intermediate hardness that may replace carboxylate groups in chelating agents such as nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). The corresponding ligands nitrilotrimethylenephosphonic acid (NTP), 1, ethylenediaminetetraraethylenephosphonic acid (EDTP), 2, and diethylenetriaminepentamethylenephosphonic acid (DTPP), 3, form metal chelates that are generally more stable than those of the analogous aminopolycarboxylates. The stability constant data in Table I ( 1 ) for representative alkaline earth and transition metal chelates of these ligands show mixed effects. The alkaline earth NTP chelates are more stable than those of NTA, while the reverse is the case for EDTP and EDTA. For both NTP and EDTP, the transition metal chelates are considerably more stable than Table I Comparison of Stabilities of Metal Chelates of Aminopolyacetate and Aminopolymethylenephosphonate Ligands* High Resolution Image Download MS PowerPoint Slide those of the aminopolycarboxylate analogs, except for Ni(II), which shows a curious and thus far unexplained reversal. The metal chelates of hydroxyethylethylenediaminetriacetic acid (HEDTA) seem to be more stable than those of hydroxyethylethylenediaminetrimethylenephosphonic