MULTIDENTATE LIGAND KINETICS .13. LEAD(2) AND COPPER(2) SUPPRESSION OF THEIR RATE OF EXCHANGE WITH 1,2-DIAMINOCYCLOHEXANETETRAACETATOCADMATE(2)

The rate of Pb(II) substitution of Cd(II) in CdCyDTA2- (CyDTA is trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate) decreases as the concentration of Pb(II) increases. This is an example of an exchange reaction in which a reactant suppresses its own rate of exchange. The anomaly results from the steric requirements of CyDTA which prevent the direct exchange of metal ions. Instead the exchange rate is controlled by a hydrogen ion reaction with CdCyDTA2-. Lead(II) forms weak complexes with the acetate groups of CdCyDTA2- and these complexes block the hydrogen ion reaction. Hence excess Pb2+ slows the rate of appearance of PbCyDTA2-. Copper behaves in a similar manner. © 1969, American Chemical Society. All rights reserved.