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

Abstract

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(H(2)O)(9-x)](CF(3)SO(3))(3) compounds with x=0.8 at Lu. The crystal structures of [Ho(H(2)O)(8.91)](CF(3)SO(3))(3) and [Lu(H(2)O)(8.2)](CF(3)SO(3))(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 ninefold 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(H(2)O)(8.2)](CF(3)SO(3))(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) A) and two more distant capping sites corresponding to Lu-O at 2.56(1) A, 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.

Fragment Molecular Orbital-Based Molecular Dynamics Study on Hydrated Ln(III) Ions

“…[47], except for the followings. The CNs of the crucial first hydration shell in MD were set as nine for La(III) and eight for Lu(III), respectively [1,4,12]. Both types of the coordination setting were performed for the intermediate Eu(III), Gd(III) and Tb(III) ions.…”

“…Both types of the coordination setting were performed for the intermediate Eu(III), Gd(III) and Tb(III) ions. Namely, two MD runs were performed for these three ions to address the gadolinium break [8][9][10][11][12]. Total time integration was 3 ps for each MD run, and the last 2 ps trajectories were cast into the statistical evaluations of radial distribution function (RDF) of Ln-O, CN and angular distribution function (ADF) of O-Ln-O.…”

“…Spedding et al first addressed this issue and postulated the so-called gadolinium break [8][9][10][11], where a clear CN change could take place at around Gd whose trivalent occupation is the half-filled 4f 7 . In 2008, Persson et al [12] revisited this postulation for hydrated complexes by a systematic study of the extended X-ray near-edge fine structures (EXAFS) experiment. They found that the typical structures of nine-fold coordination and eight-fold coordination should be of tri-capped trigonal-prism (TTP) type and square anti-prism (SAP) type, respectively.…”

“…There have been some other papers of CN observations for Ln(III) [13][14][15]. Experimentally, it has been recognized that larger La-Nd ions have nine-fold coordination and smaller Ho-Lu ions do eight-fold one [1,4,12]. Pm-Dy, intermediate-sized ions, could be of both coordination types depending on conditions.…”

“…Note that FMO-MD is of a fully QM scheme where the energy and forces are evaluated at each step. A main purpose of the present study is to investigate the hydration structures related to the gadolinium break [8][9][10][11][12] as well as the lanthanide contraction [1,3], where the FMO-MD simulations were actually applied to t La(III), Eu(III), Gd(III), Tb(III) and Lu(III) ions in a droplet of 64 water molecules. The presence of 4f electrons was implicitly treated to reduce computational times, by using the model core potential (MCP) [37].…”