STERIC EFFECTS IN CHELATION KINETICS . FORMATION AND DISSOCIATION OF NICKEL(2) COMPLEXES WITH BRANCHED POLY(AMINO ALCOHOLS) RELATED TO ETHYLENEDIAMINETETRAACETIC ACID

The formation and dissociation kinetics of the 1:1 nickel complexes of N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamiiic (TKED) and N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine (THPED) were investigated in aqueous solution at 25° over the pH range from 5.0 to 7.8 using spectrophotometric and pH-stat techniques. Formation and dissociation rate constants for both proton-independent and -dependent terms were evaluated for each compound. A comparison of these systems with previous results for nickel ion reacting with ammonia and the polyamines indicates that the formation rates are anomalously slow. Two alternate mechanisms are proposed to account for the kinetic behavior, both of which involve steric hindrance. Consideration of one of these mechanisms, involving the formation of a metastable intermediate where nickel is bonded to an alcoholic oxygen donor atom, permits the calculation of a minimum value of the rate constant for the rupture of a nickel-alcoholic oxygen bond in the aqueous environment. Based on the similarity in ligand structure, the results are correlated to the kinetic behavior of EDTA. © 1969, American Chemical Society. All rights reserved.