Bonding borides

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

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. 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

Phosphides resemble in many ways the metal borides (p. 145), earbides (p. 297), and nitrides (p. 417), and there are the same diffieulties in elassifieation and deseription of bonding. Perhaps the least-eontentious proeedure is to elassify aeeording to stoiehiometry, i.e. (a) metal-rieh phosphides (M/P >1), (b) monophosphides (M/P =1), and (e) phosphorus-rieh phosphides (M/P < 1) ... 

Bonding is used extensively on steel. The reaction occurs with a high hydrogen dilution of the BCI3 to prevent substrate attack. An iron boride is formed. 1 1 Not all metals, however, are suitable to bonding. For instance, the bonding of titanium by CVD in a chloride-based system is more difficult since the titanium substrate is highly susceptible to HCl attack and the rate of diffusion is low. 

Unlike bonding, direct boride deposition does not require a reaction with the substrate to form the boride. Both boron and metal atoms are supplied as gaseous compounds. 

Borides of Group Via. As with the borides of Group Va, the incorporation of free metal in the Group Via borides is difficult to avoid. Both tungsten and molybdenum borides are obtained at high temperature by the hydrogen reduction of the mixed bromides.Bonding appears a more effective method to form these borides in thin layers (see Sec. 2.2 above). 

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

Figure 1 presents a scheme for the formation of B—B bonds in binary metal borides, i.e., the occurrence of one-, two- and three-dimensional B aggregates as a function of the periodic group of the metal constituent. 

Figure 1. Scheme of the formation of B — B bonds in binary metal borides B(0), borides with isolated B atoms 1b(2), borides with one-dimensional infinite B chains, each B atom having two nearest-B-neighbor atoms iB(3), borides with infinite two-dimensional B nets (three B neighbors for each B atom) formation of a three-dimensional B network (more than three nearest-neighbor B atoms for each B atom). 

With increasing B content, the covalent component of the bonding in boride lattices increases owing to the appearance of direct B—B bonds and a decrease in the metallic bond character, e.g., in the structural series of the CUAI2 family ... 

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

E. W. Hoyt, J. Chrone, Preparation of Self-Bonded Borides, Report USAEC-GEAP-3332, 1960 Chem. Abstr., 55, 1338c (1961). 

The c/two-boranes BnH - (5 < n < 12) and the carboranes Bf C2Hf +2 are showpieces for the mentioned Wade rule. Further examples include the B12 icosahedra in elemental boron (Fig. 11.16) and certain borides such as CaB6. In CaB6, B6 octahedra are linked with each other via normal 2c2e bonds (Fig. 13.13). Six electrons per octahedron are required for these bonds together with the 2n + 2 = 14 electrons for the octahedron skeleton this adds up to a total of 20 valence electrons. The boron atoms supply 3 x 6 = 18 of them, and calcium the remaining two. 

The B-B bond lengths in borides is close to that in pure boron crystals, and the latter are quite hard (=3000kg/mm2). Furthermore, the relative bond lengths in the borides are different from the carbides. For example, in TiB2 the... 

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