Substitution Reactions of Some Rare Earth Metal(III)-Polyamine-N-polyacetate Complexes with Lead(II) Ions

Abstract

Kinetics of the substitution reactions of lanthanum(III), neodymiurm(III), samarium(III), gadolinium(III) and holmium(III) complexes of ethylenediaminetetraacetate(edta), N-(2-hydroxyethyl)ethylenediamine-N, N′, N′-triacetate(edtaoh) and 1, 2-cyclohexanediamine-N, N, N′, N′-tetraacetate(cydta) with lead(II) ions has been studied in the pH range of 2.38-5.97, at an ionic strength of 0.1, in the presence of excess metal ions over a temperature range of 15-30°C. The initial rate equations and activation parameters have been obtained from the measurement of change in absorbance in a region of 240-245 nm by stopped-flow method. Observed rate constants are in the order : La>Nd>Sm>Gd>Ho for the edta and cydta systems, and La>Nd≈Sm≈Gd≈Ho for the edtaoh system. The order of the effect of the ligands on the rate is edtaoh>edta>cydta for holmium and edta>edtaoh>cydta for the other metals. The rate constants for the formation of MHX^(n-4)- where M denotes the rare earth metal(III) and X^n- a completely deprotonated ligand anion, are in the order of La<Nd<Sm>Gd<Ho and are lower than those expected from the Eigen-Tamm mechanism.