Hf 2 B 2–2δ Ir 5+δ crystallizes with a new type of structure: space group Pbam , a = 5.6300(3) Å, b = 11.2599(5) Å, and c = 3.8328(2) Å. Nearly 5% of the boron pairs are randomly replaced by single iridium atoms (Ir 5+δ B 2–2δ ). From an analysis of the chemical bonding, the crystal structure can be understood as a three-dimensional framework stabilized by covalent two-atom B–B and Ir–Ir as well as three-atom Ir–Ir–B and Ir–Ir–Ir interactions. The hafnium atoms center 14-atom cavities and transfer a significant amount of charge to the polyanionic boron–iridium framework. This refractory boride displays moderate hardness and is a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient, and thermal conductivity. The metallic character of this system is also confirmed by electronic structure calculations revealing 5.8 states eV –1 fu –1 at the Fermi level. Zr 2 B 2–2δ Ir 5+δ is found to be isotypic with Hf 2 B 2–2δ Ir 5+δ , and both form a continuous solid solution.

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REMo 2 B 5 (RE = Ce, Pr, Nd) and CeW 2 B 5 have been synthesized by arc melting with further annealing at 1300°C. CeMo 2 B 5 crystallizes in a new structure type with space group Pnma, a = 11.0298(2) Å, b = 3.12080(5) Å, c = 9.9594(2) Å, Z = 4. Its structural arrangement reveals puckered [B 6 ]-hexagonal rings, trigonal-prismatic-and empty pyramidal-tetrahedralslabs. The structure can be related to the derivatives of AlB 2 and CrB prototypes. The boron atoms form a two-dimensional network of corrugated ribbons, each composed of four edgelinked [B 6 ] rings infinitely extending along the b-axis. CeMo 2 B 5[a]