Coordination Polyhedra LaOn in Crystal Structures

Abstract

Within the framework of our research on the stereochemistry of lanthanides in the structure of oxygencontaining compounds [1][2][3][4], this study is devoted to crystal chemical analysis of the LaO n coordination polyhedra. As previously, structural data were taken into account provided that the following conditions were fulfilled simultaneously: no randomly distributed lanthanum or oxygen atoms were present in a structure; X-ray diffraction analysis had been performed to R ≤ 0.1 , and the lanthanum atoms occured in LaO n coordination polyhedra. In databases [5,6], these conditions are met for 553 compounds containing 762 crystallographically nonequivalent sorts of La(III) atoms. The full list of compounds is available from the authors: vologzhanina@fromru.com.Since in 96 compounds, all or some of the hydrogen atoms have not been located during the structure determination, the coordinates of the missing atoms were calculated at the first investigation stage using the HSite program [7] included in the structure-topological TOPOS package [8]. Using the method of intersecting spheres [9], according to which any two atoms are considered to be chemically bonded if the common face of their Voronoi-Dirichlet polyhedra (VDP) is described by the I 4 , I 3 , or I 2 intersection, the coordination numbers (C.N.s) of all atoms were determined. The lanthanum C.N.s were found to vary from 4 to 12, the most frequently encountered values being 8 and 9, which is consistent with the published data [10]. Using the TOPOS software [8], characteristics of the lanthanum VDP were calculated for all 553 compounds (Table 1) and statistical analysis of the results was carried out.In the structures of the compounds in question, the VDP of 762 La atoms have in total 9571 faces. Of this set, the method of intersecting spheres [9] distinguished 6625 faces that correspond to La-O interatomic contacts and are regarded as lanthanum-oxygen valence bonds in the classical description. In addition, 19 more faces with I 4 , I 3 , or I 2 type intersections correspond to La-Q valence contacts (Q = Li or Cu, Table 2). For the La-Li bonds found, the interatomic distances are 0.36-0.99 Å shorter than the sum of the orbital, atomic, or Slater radii of atoms, while the solid angles ( W ) of the faces range from 2.0 to 6.9%. Here and below, the Ω angles are expressed in percent of the full solid angle, equal to 4 π steradian. For La-Cu bonds, Ω are 1.8 and 2.7%, and the interatomic distances are approximately equal to the sum of the orbital or atomic radii but are shorter than the sum of the Slater radii (Table 2). Taking into account the La-Q contacts, isle (tri-or hexanuclear), chain, and framework fragments consisting of La and Q atoms bound by valence bonds can be distinguished in the metal sublattices of the corresponding structures. Thus, the method of intersecting spheres allows the chemical bonds both between atoms that sharply differ in electronegativity (e.g., La-O) and those with similar electronegativities to be revealed from a unified standpoint. ...