Effect of pressure on the coordination structure of acetate-rare earth complex in water

Pressure-induced coordination structural changes of the acetate-rare earth complex in water (LnCl₃·20H₂O·CH₃COOLi) have been investigated using Raman spectroscopy. From the analysis of Raman CC stretching band of acetate ion, the integrated Raman intensity of polymeric chain structure decreases with increasing pressure throughout the rare-earth series. The monodentate ligand appears at around 0.6 GPa in the meddle-heavy rare earth region. We have determined the difference in the partial molar volume (PMV) (ΔV) among three coordination structures. The rank ordering of the PMV of three coordination structures is as follows: monodentate ligand <bidentate ligand <polymeric chain structure. The absolute values ΔV^{Polymeric→ Bidentate} and ΔV^{Bidentate→Monodentate} become larger with decreasing ionic radius of Ln³⁺ ion, and theses are probably due to the size effect of Ln³⁺ ion.