Borides and Hydrides

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

Borides and Hydrides. Ti and either B or H3BO3 in a plasma arc gave TiBj. The structural data on a boron-rich titanium boride have been reinterpreted in terms of the occupation of interstitial holes in B12 icosahedra by Ti atoms. The enthalpy of formation of TiB2 has been measured and the kinetics of its oxidation by O2 studied. 

The above has allowed the author, Tolstopyatova, and Naumov (390) to eonelude that the bond energies of the atoms of the organic substrates with the surface of oxide catalysts found by the kinetic method, are chiefly organometallic. Probably, this conclusion can be extended to sulfides, selenides, nitrides, borides, and hydrides of metals. 

Gerasimov et have provided a reference book on the thermodynamic properties of tungsten, molybdenum, titanium, zirconium, niobium, and tantalum, and their more important compounds, viz. oxides, sulphides, halides, carbides, nitrides, silicates, borides, and hydrides. 

Derivatives such as borides, carbides, nitrides, and hydrides are best prepared by direct reaction between the elements. These metaHoid-type compounds, which often show variable composition, are colored and sometimes semiconducting. 

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. 

Another highly active non-pyrophoric nickel catalyst is prepared by reduction of nickel acetate in tetrahydrofuran by sodium hydride at 45° in the presence of tert-amyl alcohol (which acts as an activator). Such catalysts, referred to as Nic catalysts, compare with P nickel boride and are suitable for hydrogenations at room temperature and atmospheric pressure, and for partial reduction of acetylenes to civ-alkenes [49]. 

Other reactive forms of nickel including nickel boride and nickel alkoxide complexes can also be used for desulfurization. Tri-w-butyltin hydride is an alternative reagent for desulfurization.204... 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

All the hydrides of boron, except B9H15 recently isolated by Schaeffer, were discovered by Stock (1914-20). The starting material was the gaseous product of the reaction between magnesium boride and dilute hydrochloric acid. His remarkable success was due to a vacuum technique which he developed for handling compounds sensitive to oxygen and moisture, such as the hydrides of boron and silicon. Monomeric BH3 was not found because there are insufficient electrons to stabilise the bonding in such a compound, as it is stabilised in the monomeric boron halides. 

G.16 W. B. Pearson. A Handbook of Lattice Spacings and Structures of Metals and Alloys (New York Pergamon Press, 1958). A most useful source of information. Gives the crystal structures of intermediate phases, and the variation of lattice parameter with composition in solid solutions, of binary and ternary alloys. Also gives the crystal structures of metal borides, carbides, hydrides, nitrides, and binary oxides. 

A curious compromise is reached in many ionic crystals. The crystal NaCl,f or example, is based on two interpenetrating close-packed (fee) lattices. The positions of one lattice are occupied by positive ions, while those of the other are occupied by negative ions. Consider the unit cube of the fee structure in Fig. 27.7(a). There is a void, or hole, outlined by the octahedron, at the center of the cube. An identical octahedral hole is centered on each edge of the unit cube (Fig. 27.7b). Each hole is at the center of an octahedron, which has atoms at each of the six apices. The centers of the octahedral holes occupy the positions of an fee lattice, which interpenetrates the lattice on which the atoms are located. Small foreign atoms, such as H, B, C, N, can occupy these holes. Many carbides, hydrides, borides, and nitrides of the metals are interstitial compounds formed in this way. 

Borides, carbides and hydrides. The usual notation has been used, for example, TaC (not CTa). The separate treatment of borides, carbides and hydrides is justified by the special characteristics of many of their structures and chemical properties. 

Borides, carbides, hydrides — Boride, Carbide, Hydride 3.1 Table of the structures of borides - Tabelle der Strukturen der Boride (System containii B, B-C and B-H but not O, N or a halogen- B-, B-C- nnd B-H-haltige Systeme ohne O, N oder Halogen )) ... 

Oxoninm. Magnesium boride and 4-M HCl at 60 °C give small yields of boron hydrides, otherwise prepared in non-aqueous reactions. 

Very interesting are the results of recent investigations on the mechanisms of Co(II) mediated reductions of nitriles, alkenes and alkyl halides by LiAlH4 and NaBH4. Those studies have unambiguously identified borides and aluminides of cobalt as catalysts in all three reductions, a finding clearly at odds with commonly held notions about the mechanisms of such processes and which could also be relevant to other transition-metal—hydride systems [12]. 

Reactions of HCl and nitrides, borides, silicides, germanides, carbides, and sulfides take place at significant rates only at elevated (>650° C) temperatures. The products are the metal chlorides and the corresponding hydrides. The reactions most studied are those involving nitrides of aluminum, magnesium, calcium, and titanium, where ammonia (qv) is formed along with the corresponding metal chloride. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

CH2CN)4Yb[( J.-H)2BH]2, and (CgH N)4Yb[( J.-H)2BH4]2 have been stmcturally characterized by x-ray crystallography and shown to contain ytterbium to boron hydride Yb—H—B linkages. Thermal decomposition of lanthanaboranes can be used to generate lanthanide metal borides. 

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... 

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. 

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