A novel oxalate-based complex of the formula {Ba(2)(H(2)O)(5)[NbO(C(2)O(4))(3)]HC(2)O(4)}·H(2)O (1) was prepared from an aqueous solution containing the [NbO(C(2)O(4))(3)](3-) and Ba(2+) entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of 1 reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound 1 to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of 1 resulted in the formation of mixed-metal oxide phases, Ba(4)Nb(2)O(9) and Ba(5)Nb(4)O(15). Three stable polymorphs of Ba(4)Nb(2)O(9) were isolated: the known, hexagonal α- and orthorhombic γ-Ba(4)Nb(2)O(9), and another one, not previously reported, hexagonal δ-Ba(4)Nb(2)O(9) polymorph. The new, δ-Ba(4)Nb(2)O(9) polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Nb(2)O(9)(8-) face-sharing octahedral dimers are interconnected via corners to the regular BaO(6)(10-) octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba(5)Nb(4)O(15) oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba(4)Nb(2)O(9) form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba(4)Nb(2)O(9) and δ-Ba(4)Nb(2)O(9). Special focus was set on the electrical properties of the prepared mixed Ba(II)-Nb(V) oxides obtained by this molecular pathway in a single-step preparation.

A novel heterometallic oxalate-based compound, {Ba 2 (H 2 O) 5 [TaO(C 2 O 4 ) 3 ]HC 2 O 4 }•H 2 O (1), was obtained by using an (oxalato)tantalate(V) aqueous solution as a source of the complex anion and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. Compound 1 is a three-dimensional (3D) coordination polymer with the Ta atom connected to eight neighboring Ba atoms through the oxalate ligands and the oxo oxygen group. Thermal treatment of 1 up to 1200 °C leads to molecular precursor-to-material conversion that yields the mixed-metal γ-Ba 4 Ta 2 O 9 phase. This hightemperature γ-Ba 4 Ta 2 O 9 polymorph has the 6H-perovskite structure (space group P6 3 /m), in which the Ta 2 O 9 face-sharing octahedral dimers are interconnected via corners to the regular BaO 6 octahedra. To date, γ-Ba 4 Ta 2 O 9 has never been obtained at room temperature, because of the reduction of symmetry (P6 3 /m → P2 1 /c) that usually occurs during the cooling. Spectroscopic, optical, photocatalytic, and electrical properties of the obtained γ-Ba 4 Ta 2 O 9 phase were investigated. In addition to the experimental data, an absorption spectrum and band structure of the γ-Ba 4 Ta 2 O 9 polymorph were calculated using density functional theory.

Solvatomorphism has been found between two series of complexes of the composition [M(bpy)3]2[NbO(C2O4)3]Cl.nH2O [M = Fe2+ (1, 2), Co2+ (3, 4), Ni2+ (5, 6), Cu2+ (7) and Zn2+ (8, 9); bpy = 2,2'-bipyridine)], crystallizing in the monoclinic space group P2 1/c [3, 5, 8 (n = 11)] or in the orthorhombic space group P21 21 21 [2, 4, 6, 7 (n = 12)]. All the structures contain two symmetry independent [M(bpy)3]2+ cations, one [NbO(C2O4)3]3- anion, one Cl(-) anion, and crystal water molecules. The cations possess a trigonally distorted octahedral geometry, with an additional tetragonal distortion in 7. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [M(bpy)3]2+ cations--quadruple aryl embrace (QAE) contact. The contact is realized by the alignment of two molecular two-fold rotation axes, preserving the parallel orientation of the molecular three-fold rotation axes. The resulting two-dimensional honeycomb lattices of [M(bpy)3]2+ cations are placed between the hydrogen bonding layers made of [NbO(C2O4)3]3- and Cl(-) anions and the majority of the crystal water molecules. The temperature-dependent magnetic susceptibility measurements (1.8-300 K) show a significant orbital angular momentum contribution for 3 and 4 (high-spin Co2+), the influence of zero-field splitting for 5 and 6(Ni2+) and a substantially paramagnetic Curie behaviour for the Cu2+ compound (7).

The compound Rb 3 [NbO(C 2 O 4 ) 3 ]·2H 2 O (1) has been synthesized by two different methods and its exact chemical composition established. The niobium atom is heptacoordinated by oxygen atoms forming a distorted pentagonal bipyramid. Inspite of some similarities, the structure of 1 is not isotypic with the structure of (NH 4 ) 3 [NbO(C 2 O 4 ) 3 ]·H 2 O.

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