On the Hydration of the Rare Earth Ions in Aqueous Solution

The totally symmetric stretching mode ν₁ ν1 Ln–(OH 2 ) 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 ν₁ ν1 Ln–(OH 2 ) peak positions and the Ln–(OH 2 ) 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 ν₁ ν1 Ln–(OH 2 ) and the Ln–(OH 2 ) bond distances of the RE hydrates has been given. Recent quantum mechanical calculations support the given interpretation.