Hydration of Lanthanoid(III) Ions in Aqueous Solution and Crystalline Hydrates Studied by EXAFS Spectroscopy and Crystallography: The Myth of the Gadolinium Break

The structures of the hydrated lanthanoid(III) ions including lanthanum(III) have been characterized in aqueous solution and in the solid trifluoromethanesulfonate salts by extended X-ray absorption fine structure (EXAFS) spectroscopy. At ambient temperature the water oxygen atoms appear as a tricapped trigonal prism around the lanthanoid(III) ions in the solid nonaaqualanthanoid(III) trifluoromethanesulfonates. Water deficiency in the capping positions for the smallest ions starts at Ho and increases with increasing atomic number in the [Ln(H2O)(9-x)] (CF3SO3)(3) compounds with x = 0.8 at Lu. The crystal structures of [Ho(H2O)(8.91)](CF3SO3)(3) and [Lu(H2O)(8.2)](CF3SO3)(3) were re-determined by X-ray crystallography at room temperature, and the latter also at 100 K after a phase-transition at about 190 K. The very similar Ln K- and L-3-edge EXAFS spectra of each solid compound and its aqueous solution indicate indistinguishable structures of the hydrated lanthanoid(III) ions in aqueous solution and in the hydrated trifluoromethanesulfonate salt. The mean Ln-O bond lengths obtained from the EXAFS spectra for the largest ions, La-Nd, agree with estimates from the tabulated ionic radii for nine-fold coordination but become shorter than expected starting at samarium. The deviation increases gradually with increasing atomic number, reaches the mean Ln-O bond length expected for eightfold coordination at Ho, and increases further for the smallest lanthanoid(III) ions, Er-Lu, which have an increasing water deficit. The low-temperature crystal structure of [Lu(H2O)(8.2)](CF3SO3)(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) angstrom) and two more distant capping sites corresponding to Lu-O at 2.56(1) angstrom, with occupancy factors of 0.58(1) and 0.59(1). There is no indication of a sudden change in hydration number, as proposed in the "gadolinium break" hypothesis.