Influence of Lewis base identity on rates of multidentate ligand exchange
Ligand exchange reactions influence the geochemical speciation of transition metal and chelating agents, which in turn affects bioavailability, solubility, and mobility of nutrient and contaminant metals. Chelating agents containing phosphonate groups are especially relevant because they are widely used as corrosion inhibitors, medications, and detergent builders. Here, we examine the influence of phosphonate Lewis base groups on ligand exchange rates. We employ capillary electrophoresis with spectrophotometric detection to follow the kinetics of nickel (II) capture by a strong chelating agent (CDTA) from weaker amino(phosphono)carboxylate chelating agents. A suite of structurally related chelating agents are examined. Replacing carboxylate groups with phosphonate groups increases chelating agent basicity and alters the pH-dependence of reaction rates. The influence of chelating agent structure on rates and pathways of ligand exchange are discussed.
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