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The synthesis and results of IR spectroscopy and X-ray diffraction analysis of new complexes of biurete NH2CONHCONH2 (BU) with the composition LnCl3 · 2BU · 4H2O, where Ln = La (I), Pr (II), Ho (III), Er (IV), and Lu (V), are presented. Crystals of complexes I–V include complex cations [Ln(H2O)4(BU)2]3+ and uncoordinated chloride ions. The coordination mode of biurete molecules is bidentate through the oxygen atoms, and upon coordination the BU molecules are transformed from the initial trans to cis configuration. Water molecules are also coordinated through the oxygen atom (the shape of the polyhedron of the Ln atoms is a two-capped trigonal prism). The oxygen atoms of both BU molecules and the oxygen atoms of the first and second water molecules form a trigonal prism, whereas the oxygen atoms of the third and fourth water molecules form two caps of the coordination polyhedron. The coordinated BU molecules are joined with the chloride ions and water molecules of the adjacent complex cations by hydrogen bonds. The degree of conversion of trans-BU to cis-BU in the lanthanide series of complexes of this type is discussed.

New complexes of yttrium and lanthanum bromides and erbium chloride with carbamide (Ur), [Y(Ur)4(H2O)4]Br3 (I) and [La(Ur)4(H2O)4]Br3 (II), [La(Ur)6(H2O)2]Br3 (III), and [Er(Ur)6Cl]Cl2 (IV), are synthesized and studied by X-ray diffraction analysis. In structures I–III, the coordination of the ligands (water and carbamide) by the metal occurs through the oxygen atoms. The coordination polyhedron of the rare-earth metal atoms is a distorted square antiprism (coordination number 8), and the bromide ions are not included into the internal coordination sphere of the complexes. In compound IV, the internal coordination sphere of the complex includes six carbamide molecules and one chloride ion, and the coordination polyhedron of Er is a distorted pentagonal bipyramid (coordination number 7). Specific features of the structures of the crystalline carbamide derivatives of chlorides, bromides, and iodides of the considered rare-earth metals are compared. A distortion of the planar structure of some carbamide ligands, depending on their number in the complex cation and on the nature of the complex, is observed.

Data on the synthesis and the IR spectroscopic and X-ray diffraction analyses of new complexes of yttrium chloride with carbamide (Ur), [Y(Ur)4(H2O)4]Cl3 (I) and [Y(Ur)6(H2O)2]Cl3 (II), and with acetamide (AA), [Y(AA)5(H2O)2]Cl3 (III), are presented. The coordination of the ligands occurs through the oxygen atoms. For complexes I and II, the coordination polyhedra of the Y atoms are distorted tetragonal antiprisms. For structure III, the coordination polyhedron of the Y atom is a distorted pentagonal bipyramid. The coordination of four Ur molecules in complex I does not change their planar structure, and two Ur molecules in structure II have the dihedral angle N-C(O)-N different from 180°. The chloride ions are in the external sphere. Many hydrogen bonds are observed in the structures of complexes I–III.

Data on the synthesis, IR spectra, and X-ray diffraction analysis of the thiocarbamide complexes of europium, holmium, and erbium iodides, [Ln(H2O)9]I3 · 2CS(NH2)2 (Ln = Eu (I), Ho(II), Er (III)), are presented. The crystal structures of the complexes contain nonaaqualanthanide cations (the polyhedron shape is a monocapped tetragonal antiprism), outer-sphere thiocarbamide molecules, and uncoordinated iodide ions. The thiocarbamide molecules form hydrogen bonds with the aqua cations and join them into continuous layers or network ensembles. The thiocarbamide molecules are disordered in the crystal structures of complexes II and III.

Scandium, yttrium, lanthanum, cerium, and terbium bromides and iodides of the formulas [(H2O)5Sc(μ-OH)2Sc(H2O)5]Br4 · 2H2O (I), [(H2O)5Sc(μ-OH)2Sc(H2O)5]I4 (II), [Y(H2O)8]Br3 · 2H2O (III), [(H2O)7La(μ-Br)2La(H2O)7]Br4 (IV), [Ce(H2O)9]I3 (V), and [Tb(H2O)9]I3 (VI) were obtained and examined using X-ray diffraction and IR and Raman spectroscopy. It was found that the compositions and structures of complexes II and III differ from the literature data. Crystallographic parameters were obtained for isostructural complexes V and VI. It was demonstrated that the conditions of the synthesis and the possibility of protolysis are decisive for the formation of polynuclear oxo and hydroxo complexes of rare-earth metal halides.

The synthesis and data of the study of new complexes of erbium and lutetium bromides with carbamide, [Ln(Ur)4(H2O)4]Br3 (I and III) and [Ln(Ur)6(H2O)2]Br3 (II and IV) (Ln = Er and Lu), by IR spectroscopy and X-ray diffraction analysis are presented. For all four compounds, coordination with metal occurs through the oxygen atoms of the water and carbamide molecules. The coordination polyhedra of the Ln atoms are distorted square antiprisms. Coordination of four carbamide molecules does not change their planar structures. The bromide ions are not coordinated, existing in the external sphere. Many hydrogen bonds are observed in the structures of complexes of both types.

New acetamide and carbamide complexes LnI3 · 4Ur · 4H2O (Ln = La, Eu, Dy, Ho, Y; Ur is carbamide) and LnI3 · 4AA · 4H2O (Ln = Nd, Eu, Dy, Ho, Y; AA is acetamide) are synthesized. The complexes are characterized by the data of chemical analysis, IR spectroscopy, and X-ray diffraction analysis. The ligands (water, carbamide, and acetamide molecules) are coordinated by the rare-earth element atoms through the oxygen atom, and the coordination polyhedron is a distorted square antiprism. The iodide ions are not coordinated and are located in the external sphere. The structural characteristics of the complexes are compared in the series [Ln(L)4(H2O)4]I3 (Ln = La, Nd, Eu, Gd, Dy, Ho, Er; L = AA, Ur).

New acetamide complexes of lanthanum, gadolinium, and erbium iodides of the composition LnI3 · 4AA · 4H2 O (Ln = La, Gd, Er; AA = CH3CONH2) are synthesized and studied. The synthesized complexes are characterized by the data of chemical analysis and IR spectroscopy and are studied by X-ray diffraction analysis. The coordination of the ligands (water and acetamide molecules) by the lanthanum, gadolinium, or erbium atom occurs through the oxygen atoms. The coordination polyhedron of the Ln atom is a distorted square antiprism. The iodide ions are not coordinated and exist in the external sphere.

The novel carbamide complexes of gadolinium and erbium iodides of the composition LnI34Ur·4H2O (Ln = Gd, Er; Ur = carbamide) were synthesized and studied by X-ray diffraction and IR spectroscopy. The ligands are coordinated to the central Gd or Er atom through the O atoms of water or carbamide molecules. The coordination polylhedron of Ln atoms is a distorted square antiprism. The iodide ions are not coordinated and lie in the outer sphere.

New hexanuclear complexes of lanthanum and neodymium iodides, [La6(H2O)23(OH)10]I8 · 8H2O (I) and [Nd6(H2O)23(OH)10]I8 · 8H2O (II), are synthesized and studied by X-ray diffraction analysis. The isostructural crystals of complexes I and II are orthorhombic: a = 13.197(4) A, b = 15.152(3) A, c = 15.302(4) A and a = 13.060(4) A, b = 14.967(5) A, c = 15.098(4) A, respectively; Z = 2, space group Pnnm. The lanthanum (neodymium) atoms coordinate the aqua and hydroxo ligands and enter the composition of the Ln6 -containing complex cations. The coordination polyhedron (ignoring the central oxygen atom) of each atom of the complexing agent is somewhat distorted square antiprism with the aqua and hydroxo ligands being in the vertices. Four bridging ligands link this atom of the complexing agent with the four adjacent atoms.

Physicochemical properties and X-ray diffraction data for [Ln(C11H12N2O)6]I3 were considered. The properties and stability of the complexes were compared along the lanthanide series.