In natural waters, a significant fraction of heavy metal ions can be bound strongly in metal-organic complexes. Metal ion bioavailability is mediated by the introduction of chelating agents (eg. siderophores or synthetic chelating agents) which can capture metal ions via ligand exchange reactions. Timescales for ligand exchange reactions between two strong ligands can span many orders of magnitude. An “adjunctive”/”disjunctive” paradigm proposed by Herring and Morel describes two possible reaction pathways that represent the potential extremes of this range. On one hand, the “adjunctive” pathway proceeds via ternary complex formation. On the other hand, the “disjunctive” pathway proceeds via complete dissociation of the initial complex and rates are much lower. Rates of both pathways are predicted from rates of water exchange following the Eigen-Wilkins mechanism. We propose a third “interjunctive” pathway where a ternary complex forms following partial dissociation of the initial complex. This pathway occurs due to interactions between the incoming and outgoing ligands and yields intermediate rates that are not readily predicted. Using capillary electrophoresis, we explore how ligand structure influences the pathway of ligand exchange reactions between strong chelating agents. We provide several examples that illustrate the role of Lewis-base arrangement and ligand rigidity on reaction pathways.

Adjunctive, disjunctive, and "Interjunctive"? Influence of ligand structure on kinetic pathways of ligand exchange

249th National Meeting (2015)

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