TRANSITION-METAL COMPLEXES OF AMINOPHOSPHONIC ACID ANALOGS OF METHIONINE AND ALANINE IN AQUEOUS-SOLUTION

The stoichiometries and stability constants of the nickel(II), copper( II) and zinc(II) complexes of 1-amino-3-methylthiopropanephosphonic ac id (MetP) and 1-amino-ethanephosphonic acid (alpha-AlaP) have been det ermined pH-metrically at 25 degrees C at an ionic strength of 0.2 mol dm(-3) (KCl). From the stability data and the absorption spectra of th e complexes it has been established that simple aminophosphonic acids coordinate to the nickel(II) and copper(II) ions forming chelate compl exes in which the metal binding mode is bidentate with the [NH2, PO32- } donor set. P-31 and H-1 NMR measurements showed that MetP and alpha- AlaP exhibit similar properties in the presence of zinc(II)) ions, but the ligand reacts to form a cyclic phosphonoamidate in neutral and sl ightly alkaline solution in the Zn(II)-alpha-AlaP system and at slight ly acidic conditions in the Zn(II)-MetP system. This difference reveal s that the latter ligand at pH > 7 prefers Zn(II) coordination involvi ng all possible (amino, phosphonate and thioether sulfur) donor groups .