Kinetics of Multidentate Ligand Substitution Reactions. VIII. Substitution Reactions of Diethylenetriaminepentaacetic Acid with Nickel(II) Iminodiacetate and Aspartate Chelates and of Cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetic Acid with the Nickel(II) Aspartate Chelate

Abstract

Abstract The substitution reactions of diethylenetriaminepentaacetic acid (DTPA) with nickel(II) chelates of iminodiacetic acid (IDA) and aspartic acid (Asp) and of cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetic acid (CyDTA) with the nickel(II) chelate of aspartic acid were studied by employing the polarographic method. All the reactions studied were found to proceed through the following two simultaneous reaction paths: (Remark: Graphics omitted.) where NiX0 and NiX22− denote nickel(II)-IDA or Asp chelates of 1-to-l and 1-to-2 compositions respectively, and where Zm− indicates a completely-deprotonated DTPA or CyDTA anion. From a comparison of the rate constants of substitution reactions involving the nickel(II)-aspartate chelate with the corresponding rate constants of substitution reactions involving the nickel(II)-iminodiacetate chelate, the six-membered ring of the nickel(II)-aspartate chelate, as compared with its five-membered ring, was concluded to be more labile in the substitution reaction.