On the Hydration of Heavy Rare Earth Ions: Ho, Er, Tm, Yb and Lu-A Raman Study.

Raman spectra of aqueous Ho, Er, Tm, Yb, and Lu-perchlorate solutions were measured over a large wavenumber range from 50-4180 cm. In the low wavenumber range (terahertz region), strongly polarized Raman bands were detected at 387 cm, 389 cm, 391 cm, 394 cm, and 396 cm, respectively, which are fairly broad (full widths at half height at ~52 cm). These isotropic Raman bands were assigned to the breathing modes, ν Ln-O of the heavy rare earth (HRE) octaaqua ions, [Ln(HO)]. The strong polarization of these bands (depolarization degree ~0) reveals their totally symmetric character. The vibrational isotope effect was measured in Yb(ClO) solutions in HO and DO and the shift of the ν mode in changing from HO to DO further supports the character of the band. The Ln-O bond distances of these HRE ions (Ho, Er, Tm, Yb, and Lu) follow the order of Ho-O > Er-O > Tm-O > Yb-O > Lu-O which correlates inversely with the band positions of the breathing modes of their corresponding octaaqua ions [Ln(OH)]. Furthermore, the force constants, , were calculated for these symmetric stretching modes. Ytterbium perchlorate solutions were measured over a broad concentration range, from 0.240 mol·L to 2.423 mol·L, and it was shown that with increasing solute concentration outer-sphere ion pairs and contact ion pairs were formed. At the dilute solution state (~0.3 mol·L), the fully hydrated ions [Yb(HO)] exist, while at higher concentrations (C > 2 mol·L), ion pairs are formed. The concentration behavior of Yb(ClO) (aq) shows similar behavior to the one observed for La(ClO)(aq), Ce(ClO)(aq) and Lu(ClO)(aq) solutions. In ytterbium chloride solutions in water and heavy water, representative for the behavior of the other HRE ions, 1:1 chloro-complex formation was detected over the concentration range from 0.422-3.224 mol·L. The 1:1 chloro-complex in YbCl(aq) is very weak, diminishing rapidly with dilution and vanishing at a concentration < 0.4 mol·L.