ChemInform Abstract: Metallic Borides, La2Re3B7and La3Re2B5, Featuring Extensive Boron-Boron Bonding

La2Re3B7 and La3Re2B5 have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La2Re3B7 (space group Pcca) having lattice parameters a = 7.657(2) A, b = 6.755(1) A, and c = 11.617(2) A, and La3Re2B5 (space group Pmma) having lattice parameters a = 10.809(2) A, b = 5.287(1) A, and c = 5.747(1) A. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron–boron bonding. La3Re2B5 features fairly common B2 dumbbells, whereas La2Re3B7 has unique one-dimensional subunits composed of alternating triangular B3 and trans-B4 zigzag chain fragments. Also observed in La3Re2B5 is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La2Re3B7 is a semimetal, which is observed in the electrical resistivity data as ...