The titration curves of repeated titrations for each system were treated in Hyperquad as a single set when refining stability constants. This means that the refinement procedure was run on both replicated curves at the same time to derive a single set of constants. The error reported for the stability constants is the standard deviation given in the Hyperquad output file. In figure and SI table 1, the log K values for PDMA/Metal complexes and mugineic acid family (natural ligand produced by plant) were compared.