Metal – Aminopolycarboxylic Acid Complexes. I. Studies of Lead(II) and Cadmium(II) Complexes with Diethylenetriaminepentaacetic Acid in Aqueous Solution by Proton Magnetic Resonance Spectroscopy

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Abstract
Lead(II) and cadmium(II) each form a 1:1 complex with DTPA. Nuclear magnetic resonance (n.m.r.) spectroscopy was used to show that each complex has three metal–nitrogen bonds and labile metal–oxygen bonds to terminal acetate groups. The latter lead to proton equivalence for the methylene protons of the terminal acetate groups of the lead complex, and near equivalence for those of the cadmium complex. The central carboxylate group is coordinated to the lead but not to the cadmium. Protonation occurs below pH 5.5 for the lead complex and occurs predominantly at a terminal nitrogen atom with consequent breaking of the metal–nitrogen bond. In the cadmium complex protonation occurs below pH 5 at the terminal nitrogen, breaking the metal–nitrogen bond, and also to almost the same extent at the central, uncoordinated carboxylate group. These differences in behavior of the lead and cadmium complexes are attributed to differences in ionic size. Possible structures are proposed and allowances are made for the possibility of coordination number greater than six. In the protonated complexes, structures and interconversion of protonated forms are discussed in the light of the observed simplification of the n.m.r. spectra.

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