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ChemInform Abstract The stoichiometries and stability constants of the proton, Co(II), Ni(II), Cu(II), and Zn(II) complexes of the title phosphonic acids (I) and (II) are determined pH-metrically. From these data and the spectral parameters of the complexes it is established that these simple acids coordinate similarly to aminocarboxylic acids forming 1:1 and 1:2 chelate complexes. However, a species with only phosphonate group coordination also exist at low pH. The differences between the complex-forming properties of aminophosphonates and aminocarboxylates are explained by the difference in basicity, charge and size of the -PO32-and -COO-groups.