On the Hydration of Heavy Rare Earth Ions: Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Lu³⁺-A Raman Study

Raman spectra of aqueous Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺-perchlorate solutions were measured over a large wavenumber range from 50-4180 cm^-1. In the low wavenumber range (terahertz region), strongly polarized Raman bands were detected at 387 cm^-1, 389 cm^-1, 391 cm^-1, 394 cm^-1, and 396 cm^-1, respectively, which are fairly broad (full widths at half height at ~52 cm^-1). These isotropic Raman bands were assigned to the breathing modes, ν₁ Ln-O of the heavy rare earth (HRE) octaaqua ions, [Ln(H₂O)₈]³⁺. The strong polarization of these bands (depolarization degree ~0) reveals their totally symmetric character. The vibrational isotope effect was measured in Yb(ClO₄)₃ solutions in H₂O and D₂O and the shift of the ν₁ mode in changing from H₂O to D₂O further supports the character of the band. The Ln-O bond distances of these HRE ions (Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺) 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(OH₂)₈]³⁺. Furthermore, the force constants, k, were calculated for these symmetric stretching modes. Ytterbium perchlorate solutions were measured over a broad concentration range, from 0.240 mol·L^-1 to 2.423 mol·L^-1, 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^-1), the fully hydrated ions [Yb(H₂O)₈]³⁺ exist, while at higher concentrations (C_T > 2 mol·L^-1), ion pairs are formed. The concentration behavior of Yb(ClO₄)₃ (aq) shows similar behavior to the one observed for La(ClO₄)₃(aq), Ce(ClO₄)₃(aq) and Lu(ClO₄)₃(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^-1. The 1:1 chloro-complex in YbCl₃(aq) is very weak, diminishing rapidly with dilution and vanishing at a concentration < 0.4 mol·L^-1.