On the anomalous concentration dependence of the inner-sphere hydration number change of aqua lanthanide ions

Contrary to the expectation from the simple equilibrium reaction: [Ln(H2O)(8)](3+) + H2O = [Ln(H2O)(9)](3+), where Ln(3+) is a lanthanide ion, nona-aqua Ln(3+) species anomalously increase in population with a decrease in water content in aqueous europium and gadolinium chloride solutions. From the enormous data reported by Spedding er al. (Chem. Eng. Data 1975, 20, 72), the anomalous behaviour is shown to be a general phenomenon in aqueous lanthanide chloride and perchlorate solutions at all concentration ranges. It is concluded from the Raman experiment that the involvement of the outer-sphere complex formations in the equilibrium reaction for hydration number change is the main cause for the anomalous behaviour.