Beryllium compounds hydrides

SAFETY PROFILE Confirmed carcinogen. A dangerous fire hazard. When heated to 220°C it liberates explosive hydrogen gas. Reacts violently with methanol, water, and dilute acids. When heated to decomposition it emits toxic fumes of BeO. See BERYLLIUM COMPOUNDS and HYDRIDES. 

The hydrides are excellent fuels on an energy basis. Lithium and beryllium hydrides are very good fuels but both are solids and both produce solid oxides. All beryllium compounds ate very toxic. The hydrides of boron and carbon form many complex compounds containing multiple atoms of both boron... 

Several compounds of beryllium have important applications. The most commercially important beryllium compound is beryllium oxide (BeO), which is used in high-temperature applications, such as crucibles, microwave ovens, ceramics, and insulators. Beryllium oxide also finds use in gyroscopes and military vehicle armor. Beryllium chloride (BeCl2) is used as a catalyst in the synthesis of organic chemicals. Beryllium hydride (BeH2) is a source of hydrogen gas when mixed with water. Beryllium carbide (Be2C) is a source of neutrons in nuclear reactors. 

The nature of a binary hydride is related to the characteristics of the element bonded to hydrogen (Fig. 14.8). Strongly electropositive metallic elements form ionic compounds with hydrogen in which the latter is present as a hydride ion, H. These ionic compounds are called saline hydrides (or saltlike hydrides). They are formed by all members of the s block, with the exception of beryllium, and are made by heating the metal in hydrogen ... 

Compounds with H H—H Hydrogen H—Be—H Beryllium hydride H—B—H Boron hydride H 1 H—C—H 1, Methane H—N—H Ammonia H— H Water H—F Hydrogen fluoride... 

Theoretically, both aluminum hydride, AIH3, and beryllium hydride, BeH2, are attractive fuels because of their high heat release and gas volume contribution. Both axe difficult to manufacture and both deteriorate chemically during storage, due to loss of hydrogen. Because of these difficulties, coupled with relatively modest Isp gains, these compounds remain experimental. 

According to this simple picture, beryllium hydride should have two different types of H-Be bonds —one as in 1 and the other as in 2. This is intuitively unreasonable for such a simple compound. Furthermore, the H-Be-H bond angle is unspecified by this picture because the 2s Be orbital is spherically symmetrical and could form bonds equally well in any direction. 

Apart from the vast number of compounds containing B-H-B bridge bonds, a good many other three-centre E-H-E links are found, especially where E or E is B, Be or Li. Beryllium hydride BeH2, is a onedimensional polymeric solid (isostructural with BeCl2 and SiS2 see Section 3.3), whose structure can be rationalised in terms of Be-H-Be (3c, 2e) bridge bonds ... 

Examples of compounds that have fewer than eight valence electrons are beryllium hydride, BeH2 and boron trifluoride, BF3. 

Most of the Group IA and IIA metals react with hydrogen to form metal hydrides. For all of the metals in these two groups except Be and Mg, the hydrides are considered to be ionic or salt-like hydrides containing H ions (see Chapter 6). The hydrides of beryllium and magnesium have considerable covalent character. The molten ionic compounds conduct electricity, as do molten mixtures of the hydrides in alkali halides, and during electrolysis of the hydrides, hydrogen is liberated at the anode as a result of the oxidation of H ... 

Consider how we might explain the bonding in a compound of divalent beryllium, such as beryllium hydride, BeH2. The beryllium atom, with only four electrons, has a configuration of ls22s. 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

The discussion above was limited to alanates, borohydrides, amides, and combinations of these materials. Other hydrides or alternative approaches have also been proposed for storage applications. Zaluska et al. ° studied lightweight lithium-beryllium hydride and showed a reversible hydrogen capacity of over 8 wt%. They also showed that the hydride may be usable down to 150°C. Although these results are rather promising, it is unlikely that any beryllium-containing compound would be considered for vehicular use because of the toxicity of this element, even though the hydride may be quite stable. 

Magnesium, strontium, and barium form similar compounds. Beryllium hydride cannot be formed by direct combination of the elements but can be prepared by the following reaction ... 

Hydrogen bridges between the beryllium atoms produce a polymeric structure for BeH2, as shown in Fig. 18.6. The localized electron model describes this bonding by assuming that only one electron pair is available to bind each Be—H—Be cluster. This is called a three-center bond, since one electron pair is shared among three atoms. Three-center bonds have also been postulated to explain the bonding in other electron-deficient compounds (compounds where there are fewer electron pairs than bonds), such as the boron hydrides (see Section 18.5). 

Compounds in which hydrogen is bonded to the alkali and alkaline-earth metals except beryllium are prepared by direct synthesis from the metals or amalgams. Beryllium hydride is prepared by pyrolysis or reduction of organic derivatives. 

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