Metallic Borides, La2Re3B7 and La3Re2B5, Featuring Extensive Boron-Boron Bonding

La2Re3B7 and La3Re2B5 have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 degrees 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) angstrom, b = 6.755(1) angstrom, and c = 11.617(2) angstrom, and La3Re2B5 (space group Pmma) having lattice parameters a = 10.809(2) angstrom, b = 5.287(1) angstrom, and c = 5.747(1) angstrom. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron boron bonding. La3Re2B5 features fairly common B-2, dumbbells, whereas La2Re3B7 has unique onedimensional subunits composed of alternating triangular B-3 and trans-B-4 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 measured on single crystals, with behavior obeying the Bloch Gruneisen model and a room-temperature resistivity rho(300 K) of similar to 375 mu Omega cm. The electronic band structure calculations also suggest that La3Re2B5 is a regular metal.