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 °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) Å, b = 6.755(1) Å, and c = 11.617(2) Å, and La₃Re₂B₅ (space group Pmma) having lattice parameters a = 10.809(2) Å, b = 5.287(1) Å, and c = 5.747(1) Å. 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 measured on single crystals, with behavior obeying the Bloch-Grüneisen model and a room-temperature resistivity _{300 K} of ∼375 μ cm. The electronic band structure calculations also suggest that La₃Re₂B₅ is a regular metal.