The Use of NMR for the Study of Metal Chelates in Aqueous Solution

Abstract

Nuclear magnetic spectroscopy has been used to study a number of chelating agents and their metal chelates as a function of solution pH. This has permitted to determine when protonation of the ligand occurs and at what donor group the protons are attached. Such conclusions are made possible by observing the change in the chemical shift of the organic protons within the ligand adjacent to the donor group. The effect of varying the pH upon the NMR spectra of metal chelates permits determination of when metal-ligand bonding occurs in terms of pH and also permits establishment of which donor group of the ligand actually participates in the bonding. In many cases the chemical shifts for the free ligand are different from those for the coordinated ligand. When this is true NMR spectroscopy can be used to determine the correct formula for a specific metal chelate. Studies of chemical shifts as a function of pH also permit determination of stability constants which are related to a specific type of chelate bonding.