The lanthanide hydrated ions [Ln(H2O9)]^3+(Ln=Ce, Pr, Nd, Pm, Ho, Er, Tm, Yb) have been studied by means of density functional theory (DFT). The fully optimized geometrical structure and the binding energy between lanthanide ion and H2O of these complexes have been obtained. The calculated results are in fair good agreement with experiments, showing that DFT method can also give reliable results for the calculations of the complex systems forming from lanthanide ions and neutral molecules. Based on analysis of the calculated results it is shown that the binding between Ln^3+ and H2O is mainly due to the interaction of Ln 5 d orbitals and the lone pairs in oxygen atoms. The contribution to the binding energy by other lanthanide orbitals is small. This conclusion is in accordance with the bonding model of lanthanide compounds proposed by one of the authors before. This bonding model can also be used to explain the fact that the binding energy between Ln^3+ and H2O increases from La to Lu, while the bonding energy of the neutral lanthanide compounds decreases along the lanthanide series.

Key words: LANTHANIDE SERIES COMPOUNDS, HYDRATED ION, CONFIGURATION, CHARGE DISTRIBUTION, BINDING ENERGY