Structural chemistry borides

Boron (like silicon) invariably occurs in nature as 0X0 compounds and is never found as the element or even directly bonded to any other element than oxygen. The structural chemistry of B-O compounds is characterized by an extraordinary complexity and diversity which rivals those of the borides (p. 145) and boranes (p. 151). In addition, vast numbers of predominantly organic compounds containing B-O are known. 

RogI, P. (1985). Structural Chemistry and Phase Equilibria of Ternary Rare Earth-Platinummetal Borides. J.Less-Common Met. 110, 283-294. 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

Rogl, P., K. Hiebl and M. J. Sienko, 1982, Structural chemistry and magnetic behavior of RM4B4 borides, paper presented at the 7th Intern. Conf. on Solid Compounds of Transition Elements, Grenoble (June 21-25), Proceedings, II A4. 

Nickel atoms in BajNi B form distorted, puckered 3.6.3.6-kagome nets stacked in six layers perpendicular to the c axis. The densely packed framework of trigonal-Ni prisms again result in boron-pair formation, although Ba atoms are too large to be sandwiched between two Ni layers, and only four Ba can be accommodated within six Ni layers. Superconductivity is found for ( a, Sr, Ba)2pt9Bg borides with a structure related to Ba2Ni9Bfi and e o,B2 however, with respect to crystal chemistry and boron coordination, only the subcell is derived so far. 

Parth6, E. and Chabot, B. (1984) Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues. In Handbook on the Physics and Chemistry of Rare Earths, ed. Gschneidner Jr., K.A. and Eyring, L. (North-Holland, Amsterdam), Vol. 6, p. 113. 

E. Parthe and B. Chabot, Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues 113... 

Borides Sohd-state Chemistry Carbides Transition Metal Solid-state Chemistry Electronic Structure of Sohds Quasicrystals Structure Property Maps for Inorganic Solids Superconductivity Zintl Compounds. 

This article covers only a part of the chemistry of boron. Boron-carbon compounds are covered in other articles in this volume see Boron Organoboranes Boron Metallacarbaboranes, and Boron Polyhedral Carboranes). The main subject of the latter two articles, and the separate one on Boron Hydrides is the extensive chemistry of the multicenter bonded boron-hydride systems. This area has been a major focus of boron research for the past 60 years. There is some direct overlap between the two articles Borides Solid-state Chemistry and Borates Solid-state Chemistry, and this more general one covering the inorganic chemistry of boron. Boron-Nitrogen Compounds are also covered separately. These articles should be consulted for more detailed discussions of the structure, bonding, and properties of borides, solid-state borates, and boron-nitrogen compounds. 

Fhst, the number of valence functions (or frontier orbitals) and valence elechons (frontier orbital occupancy) determines the tendency toward cluster bonding. It is instructive to recall that the structural motif in elemental boron is the icosahedron with six-connected boron atoms see Borides Solid-state Chemistry), it is the tetrahedral carbon atom in the diamond form of elemental carbon with four-coimected carbon atoms and it is three-connected phosphorus atoms in the sheets of elemental black phosphorus (see Phosphides Solid-state Chemistry). Boron has more valence orbitals than valence elechons, naturally leading to orbitally rich cluster formation for example, BH has three orbitals and two elechons and forms... 

The second example comes from solid-state chemistry, and the coimection with cluster chemistry is less obvious. Despite this, it is an excellent example of how E/M variation can lead to systematic variation in structure and, consequently, to properties. Although the example is taken from solid-state metal borides, the silicides see Silicon Inorganic Chemistry) and phosphides (see Phosphides Solid-state Chemistry) could have been used. 

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