On the Hydration of the Rare Earth Ions in Aqueous Solution

The totally symmetric stretching mode $$\nu_{1}$$ Ln–(OH2) of the first hydration shells of all the rare earth (RE) ions across the series from lanthanum to lutetium has been measured on dilute aqueous perchlorate solutions at room temperature. An S-shaped relationship has been found between the $$\nu_{1}$$ Ln–(OH2) peak positions and the Ln–(OH2) bond distances of the lanthanide(III) aqua ions. While the light rare earth ions form nona-hydrates, the heavy ones form octa-hydrates and the rare earth ions in the middle of the series show non integer hydration numbers between 9 and 8. A relationship between wavenumber positions $$\nu_{1}$$ Ln–(OH2) and the Ln–(OH2) bond distances of the RE hydrates has been given. Recent quantum mechanical calculations support the given interpretation.