Musiri M. Balakrishnarajan

Structure and bonding in boron carbide: The invincibility of imperfections

Boron carbide, usually described as B4C, has the mysterious ability to accommodate a large variation in carbon composition (to as much as B10C) without undergoing a basic structural change. We systematically explore how the bonding varies with carbon concentration in this structure and the origin of the fundamental electron deficiency of the phase. As the carbon concentration is reduced, we find that the exo-polyhedral BEq–C bonds of the icosahedra in the structure become increasingly engaged in multiple bonding, and the repulsive steric interactions between the bulky B12 units surrounding the carbon atom are reduced. The short bond lengths observed within the three-atom C–B–C chains are then due to substantial π-bonding, while the carbon deficiency weakens its σ-framework significantly. We conclude that the idealized framework of boron carbide has to expel some electrons in order to maximize its bonding; disorder in the structure is an inevitable consequence of this partial oxidation. The localization of electronic states arising from the disorder leads to the semiconducting nature of boron carbide throughout its composition range.

Polyhedral Borane Analogues of the Benzynes and Beyond: Bonding in Variously Charged B12H10 Isomers

The effect of removing two protons, hydrogen atoms, or hydrides from the stable icosahedral B(12)H(12)(2-) is investigated theoretically. The resulting B(12)H(10)(q) (q = 4-, 2-, 0) isomers show interesting and understandable bond distance and stability variations, as well as special deformations associated with the apex-ring configuration typical of the underlying polyhedron. The dianions are analogous to o-, m-, and p-benzyne and have the special feature of distinct singlet and triplet states not far removed from each other in energy.

Thinking about metal-metal quadruple bonding in extended structures: A hypothetical A2M6E8 network

The electronic and structural possibilities of a recently conceived, as-yet unsynthesized, AM6E8 (or A2M6E8) structural type are explored. With A an alkaline earth metal, M a transition metal, and E a main group element, a range of geometries containing M–M pairs with very short separations is feasible. Density functional theory geometry optimizations and an extended Huckel analysis of the bonding in a representative Ca2W6O8 realization support the qualitative picture of quadruple metal-metal bonding in these hypothetical phases. The MM pairs interact moderately (metallic behavior is anticipated) through linking atoms. Geometry optimizations of a wide range of hypothetical compounds show that the M–M separation in these will be in the range of realistic quadruple metal-metal bonding, 2.20–2.30 A.