Nuclear Magnetic Resonance Studies of Solutions of the Rare-Earth Ions and Their Complexes. III. Oxygen-17 and Proton Shifts in Aqueous Solutions and the Nature of Aquo and Mixed Complexes

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Abstract
A nuclear magnetic resonance survey has been undertaken in order to gain a better insight into the nature of the complex species present in aqueous solutions of the rare‐earth ions. Oxygen‐17 and proton chemical shifts of water are reported and the effects of concentration and added anions upon them discussed. Nuclear magnetic resonance evidence is presented, showing that the shifts observed in solutions of the perchlorate salts are due to a single hydrated species persisting over a wide range of concentrations and that no “inner‐sphere” complex is formed with perchlorate. It appears, also, that all the water molecules in the first solvation shell are equivalent with respect to their contribution to the shift, owing to their relatively short residence time. The sign, magnitude, and temperature dependence of the oxygen‐17 shifts as contrasted to those of protons support the suggestion that the former are mainly due to contact interaction, whereas the latter are dominated by dipolar interaction with the unpaired electrons of the rare‐earth ions. Examination of the chemical shifts of nuclei of the anions unequivocally demonstrates that “inner‐sphere” complexes are formed with nitrate and acetate. It is suggested that both nitrate and acetate act as bidentate ligands. A marked decrease of the number of water molecules contributing to the observed oxygen‐17 shift is noted in presence of acetate, nitrate, and sulfate. Chloride also gives rise to some dehydration of the rare‐earth ion, although in this case the mechanism of the process is as yet unclear. An approach to the estimation of equilibrium quotients for complex formation using water chemical shift data is outlined in an Appendix. Equilibrium quotients are estimated for the formation of nitrato–dysprosium complexes. A comparison of water and other ligand proton shifts of the acetate and tropolonate complexes along the lanthanide series suggests that in both complexes there is a similar ligand arrangement around a given cation and that a change of configuration occurs at erbium.