BIURET COMPLEXES OF PRASEODYMIUM AND HOLMIUM BROMIDES

Abstract

We present the synthesis of new complexes of praseodymium and holmium bromides and biuret (NH2CONHCONH₂ - BU), PrBr₃2BU4H₂O (I) и HoBr₃2BU4H₂O (II) and the results of IR spectroscopy and X-ray diffraction analysis. In the crystals of I and II we find complex cations [Ln(H₂O)4(BU)₂] ³⁺ (Ln = Pr, Ho) and uncoordinated bromide ions existing in the external sphere. Biuret molecules serve as bidentate ligands (through the oxygen atoms) and are transformed from the original cis- to trans-configuration. Four molecules of water are coordinated in the internal sphere through the oxygen atoms too. The coordination polyhedron is a two-capped trigonal prism. The oxygen atoms of two BU molecules and the first and second oxygens of the water ligands form a trigonal prism. The third and fourth oxygens of the water ligand form two caps of the coordination polyhedron. The coordinated BU molecules are joined with the bromide ions and water molecules of the neighbour complex cations by hydrogen bonds. We discuss the conversion of trans-BU to cis-BU in the complexes of this type for the lanthanide chlorides and bromides.

Keywords

• complexes, structure, biuret, praseodymium, holmium, bromide, coordination polyhedra, thermal decomposition.