Molecular structures of tris(dipivaloylmethanato) complexes of the lanthanide metals, Ln(dpm)(3), studied by gas electron diffraction and density functional theory calculations: A comparison of the Ln-O bond distances and enthalpies in Ln(dpm)(3) com

摘要

The molecular structures of tris(dipivaloylmethanato)neodymium(III), Nd(dpm)(3), and tris(dipivaloylmethanato)ytterbium(III), Yb(dpm)(3), have been determined by gas electron diffraction (GED) and structure optimizations through density functional theory (DFT) calculations. Both molecules were found to have D-3 molecular symmetry. The most important structure parameters (r(a) structure) are as follows (GED/DFT): Nd-O = 2.322(5)/2.383 angstrom, Yb-O = 2.208(5)/2.243 angstrom, O-Nb-O = 72.1(3)/71.3 degrees, and O-Yb-O = 75.3(2)/75.8 degrees. The twist angles of the LnO(6) coordination polyhedron, defined as zero for prismatic and 30 for antiprismatic coordination, were theta = 19.1(3)/14.2 degrees for Nd and 20.4(2)/19.2 degrees for Yb. Structure optimizations of La(dpm)(3), Gd(dpm)(3) Er(dpm)(3), and Lu(dpm)(3) by DFT also yielded equilibrium structures of D-3 symmetry with bond distances of La-O = 2.438 angstrom, Gd-O = 2.322 angstrom, Er-O = 2.267 angstrom, and Lu-O = 2.232 angstrom. The Ln-O bond distances in 12 Ln(dpm)(3) complexes studied by GED decrease in a nearly linear manner with the increasing atomic number (Z) of the metal atom, as do the Ln-O bond distances in the cubic modifications of 14 sesquioxides, Ln(2)O(3). The bond distances in the dpm complexes are, however, about 2% shorter. The mean Ln-O bond rupture enthalpies of the cubic sesquioxides calculated from thermodynamic data in the literature vary in an irregular manner with the atomic number; the La-O, Gd-O, Tb-O, and Lu-O bonds are nearly equally strong, and the remaining bonds are significantly weaker. The Ln-O bond rupture enthalpies previously reported for 11 Ln(dpm)(3) complexes are on the average 13 kJ mol(-1) or about 5% smaller than in the sesquioxides, but they vary in a similar manner along the series: it is suggested that the pattern reflects variations in the absolute enthalpies of the gaseous Ln atoms.