Borides halogens

Borides are relatively inert, especially to non-oxidizing reagents. They react violently with fluorine, often with incandescence. Reaction with other halogens is not as violent and may require some heat. Resistance to oxidation, acids, and alkalis is summarized in Table 17.5. In oxidation conditions, a layer of boric oxide is formed on the surface which passivates it to some degree. Boric oxide melts at 450°C and vaporizes at 1860°C. It offers good protection up to 1500°C in a static environments but it has low viscosity at these temperatures and tends to flow under stress and the protection it offers is limited.f k l... 

There are few useful reactions in which new B—H bonds are formed. Although the formation of boranes from the protolysis of borides or the reduction of boron compounds with Hj, either in electrical discharges or in the presence of active metals, have historical importance, these methods have no importance or utility today. Indeed, the preparation of boranes is so dominated by the single common starting material, the tetrahydroborate ion, that the only important reactions in which B—H bonds are formed are those in which hydride ion either reduces species with B—O or B-halogen bonds to form boranes or adds to trifunctional boron compounds to form hydroborates. 

The stereochemistry of boron is simple in many of its compounds with the halogens (but note BgClg), oxygen, nitrogen, and phosphorus, three coplanar or four tetrahedral bonds being formed. The more complex stereochemistry of the element in electron-deficient systems (elementary B, some borides, and the boranes) is dealt with separately. The planar arrangement of three bonds from a B atom has been demonstrated in many molecules BX3 and BR3 (Table 24.1), in cyclic molecules of the types noted in the previous section, in many oxy-ions (see later section), and in crystals such as the graphite-like form of BN (p. 847) and AlBj (p. 842). 

In Chapters 20 and 21 we shall look at individual elements of the c -block in detail. However, a few general points are given here as an overview. In general, the metals are moderately reactive and combine to give binary compounds when heated with dioxygen, sulfur or the halogens (e.g. reactions 19.1-19.3), product stoichiometry depending, in part, on the available oxidation states (see below). Combination with H2, B, C or N2 may lead to interstitial hydrides Section 9.7), borides Section 12.10), carbides Section 13.7) or nitrides Section 14.6). 

Although rather unreactive at ordinary temperatures, titanium combines directly with most non-metals, for example, hydrogen, the halogens, oxygen, nitrogen, carbon, boron, silicon, and sulfur, at elevated temperatures. The resulting nitride, TiN, carbide, TiC and borides, TiB and TiB2, are interstitial compounds which are very stable, hard and refractory. 

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. 

Boride (B-, B-C- und B-H-haltige Systeme ohne O, N Oder Halogen) 3.1... 

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 )) ... 

Organosilylboron compounds are prepared by the reaction of haloorganosilanes with either boron compounds containing halogen and the Na/K alloy, or an alkali metal boride (Scheme 3.28) ... 

Boron carbide is chemically inert, although it reacts with oxygen at elevated temperatures and with white hot or molten metals of the iron group, and certain transition metals. B4C reacts with halogens to form boron halides—precursors for the manufacture of most nonoxide boron chemicals. B4C also is used in some reaction schemes to produce transition metal borides. Boronizing packings containing B4C are used to form hard boride surface layers on metal parts. 

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