| 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. |
