Nickel and chelating agent speciation by capillary electrophoresis: Exploration of pathways, rates, and structure-reactivity relationships pertaining to exchange reactions

Nickel concentrations in contaminated soils and wastewater effluent are often high enough adversely impact plants and organisms. Since nickel has one slowest rates of ligand exchange of the +2 transition metals, nickel speciation is often governed by the rate of a slow ligand exchange reaction, NiL + Y → NiY + L. Here we consider reactions where L represents a weak chelating agent such as IDA, NTA and EDDA and Y represents a stronger chelating agent such as EDTA or CDTA. Reaction takes place through bond-making and bond-breaking involving Lewis Base groups. Bond strengths increase and rates decrease in the order: water, carboxylate (O), amine (N).;The effects of rigidity and steric hindrance on pathways of multidentate ligand exchange are explored. EDDA has four Lewis Base groups arranged in linear fashion, which can be depicted as O-N-N-O. NTA possesses four Lewis Base groups arranged in tripodal fashion, N(O)₃. Loss of Ni from Ni-EDDA is proportional to the concentration of CDTA, depicted as (O) ₂N-N(O)₂, indicating ternary complex formation. Loss of Ni from Ni-NTA, in contrast, is independent of CDTA concentration. The rigidity of NTA causes a shift in the reaction pathway to complete dissociation of NiNTA prior to CDTA attack. Chelating agent analog structures were synthesized to test hypotheses regarding mechanism.;Additional experiments with N-substituted IDA chelating agents, R-N(O) ₂, examine steric effects on ternary complex formation. Reactions proceed via parallel reaction pathways: a ternary complex formation pathway and a complete dissociation pathway. Increasing the bulkiness of the R group, increases the percent of product formation that occurs through complete dissociation.;We demonstrate the ability of capillary electrophoresis to efficiently separate amine-, carboxylate-, and -phosphonate-bearing chelating agents and their nickel complexes. We discuss the impact of the background electrolyte that fills the capillary during separation on detection and separation. With regards to electric field-induced dissociation, a simple method is presented for predicting the stability of nickel(II)-complexes. We also discuss how capillary electrophoresis enhances the utility of the Job's Method to systems where multiple metal-ligand species are present. Job's Plot electropherograms provide evidence of a novel 3:2 nickel diethylenetriaminepentaacetate complex. A provisional stability constant is given.