Boride Structures Isolated Boron Atoms

Metal Boride Structures (Isolated Boron Atoms). 

Borides with Isolated Boron Atoms 6.7.2.1.3. Metal Boride Structures (Isolated Boron Atoms). 

Borides with isolated boron atoms. These include most of those with low B to M ratios such as M4B, M3B, M2B, M5B2 and M7B3. In the M4B and M2B structures, boron atoms lie in triangular-prismatic or square-antiprismatic holes between multiple layers of metal atoms. In the others, the metal atoms are arranged in approximately close-packed arrays, with the boron atoms in triangular-prismatic interstices. 

Table 2. Structure Types, Boron Coordination and Representatives op Metal Borides with Isolated B Atoms (Filled Metal Host Lattice Compounds)...

Table 2. Crystal Structures and Boron Coordination of Platinum Metal Borides WITH Isolated B Atoms (Owing to Defect Boron Sublattice)...

There are no sharp metal boron ratios for which the transition from isolated boron atoms to boron pairs to boron chains and so on occurs. For example, isolated boron atoms solely occur in borides, crystallizing in the relatively metal-poor REFC2B2 (ThCr2Si2)-type structure (M/B = 1.5), while... 

The proper metal boride structures with isolated boron atoms include approximately 20 structure types with many ternary representatives. The boron coordination is trigonal prismatic (in the Fe3C, TisP, ResB ThvFeB structure types), octahedral (in RhsB4), or Archimedian prismatic (in the CuAl2-type structure with many M2B representatives). 

A large number of binary metal borides have been prepared and characterized. Their stoichiometries vary from M5B to MBioo, but the most common are M2B, MB, MB2, MB4, MBe, and MB12. The structures of metal borides vary from isolated boron atoms in borides from M4B to M2B, to chains of boron atoms in MB and M3B4, to two-dimensional networks of boron atoms in MB2 and M2B5, to complex three-dimensional arrays of boron atoms in MB4, MBe, and MB12. In general, metal borides exhibit characteristics of typical metals, for... 

MP2, MAs2 and MSb2 all have a compressed form of the marcasite structure, while the carbides MC have trigonal prismatic coordination in the WC structure. Several borides are known MB2 has nets of boron atoms. RunBg has branched chains while RU7B3 has isolated borons. 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

A large number of metal borides have been prepared and characterized. Several hundred binary metal borides M Bj, are known. With increasing boron content, the number of B-B bonds increases. In this manner, isolated B atoms, B-B pairs, fragments of boron chains, single chains, double chains, branched chains, and hexagonal networks are formed, as illustrated in Fig. 13.3.1. Table 13.3.1 summarizes the stoichiometric formulas and structures of metal borides. 

We have described the structures of some boron-rich borides in which there are extensive 3D systems of B—B bonds. At the other extreme there are crystalline borides with low boron content in which there are isolated B atoms, that is, B atoms surrounded entirely by metal atoms as nearest neighbours. With increasing boron content the B atoms link together to form first B2 units, then chains, layers, or 3D frameworks extending throughout the whole crystal (Table 24.3). 

Fourteen years passed between the report of the boride from Schmid and cow-orkers, and that of the next fully encapsulated boron atom in a discrete molecular environment Shore s group described [HRu6(CO)nB], the octahedral structure (Fig. 1) being confirmed by the results of an X-ray diffraction studyj The metal-bound H atom is readily removed, and Shore also reported the isolation of [(Ph3P)2N][Ru6(CO)nB].f Independently, we reported the synthesis and spectroscopic characterization of the salt [Me3NH][Ru6(CO)i7B]. These studies opened the door to the development of the chemistry of interstitial borido clusters. 

Similar to silicates, the crystal structures of borides can easily be classified according to the arrangement of the boron atoms. Boron may occur as an isolated atom or form B-B bonds with an increasing degree of interconnection in the chains, double chains, layers and frameworks and combinations thereof (Fig. 1). Due to the strong covalent bonding between the boron atoms and the electron deficiency of the three-center bond a number of complex and unique structures result which... 

This kind of classification has been used by Nowotny and Rogl (1977) and Rogl and Nowotny (1978) in a survey of ternary metal borides. The structures have been classified according to the occurrence of isolated B atoms, B-B dumbbells, B chains, or B nets and so on. A similar classification of binary transition metal borides had been proposed many years before by Kiessling (1950). The type of homonuclear boron linkage can be correlated with the metal to boron ratio of the compound (see table 20). 

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