BeF2

Beryllium. Beryllium [7440-41-7], Be, metal is produced by electrolysis of KCl—NaCl—BeCl2 melts. Temperatures up to 900°C are required. CeU voltages are 6 to 9 V (115). Electrolysis of mixtures of beryUium oxide [1304-56-9], BeO, ia lithium fluoride [7789-24-4], LiF, and beryUium fluoride [7787-49-7], BeF2, has produced beryUium metal at about 700°C and 2.6 V (116). DetaUs of fused salt metal winning processes are given ia Table 7. 

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

By contrast BeF2.xH20, TiF4.2H20 and ThF4.4H20 cannot be dehydrated without hydrolysis. 

There are a few species in which the central atom violates the octet rule in the sense that it is surrounded by two or three electron pairs rather than four. Examples include the fluorides of beryllium and boron, BeF2 and BF3. Although one could write multiple bonded structures for these molecules in accordance with the octet rule (liable 7.2), experimental evidence suggests the structures... 

Figure 7.5 (page 177) shows the geometries predicted by the VSEPR model for molecules of the types AX2 to AX. The geometries for two and three electron pairs are those associated with species in which the central atom has less than an octet of electrons. Molecules of this type include BeF2 (in the gas state) and BF3, which have the Lewis structures shown below ... 

Two electron pairs are as far apart as possible when they are directed at 180° to one another. This gives BeF2 a linear structure. The three electron pairs around the boron atom in BF3 are directed toward the comers of an equilateral triangle the bond angles are 120°. We describe this geometry as trigonal planar. 

Notice that the beryllium atom has no unpaired electrons, the boron atom has one, and the carbon atom two. Simple valence bond theory would predict that Be, like He, should not form covalent bonds. A boron atom should form one bond, carbon two. Experience tells us that these predictions are wrong. Beryllium forms two bonds in BeF2 boron forms three bonds in BF3. Carbon ordinarily forms four bonds, not two. 

The formation of the BeF2 molecule can be explained by assuming that, as two fluorine atoms approach Be, the atomic orbitals of the beryllium atom undergo a significant change. Specifically, the 2s orbital is mixed or hybridized with a 2p orbital to form two new sp hybrid orbitals. (Figure 7.12). 

In the BeF2 molecule, there are two electron-pair bonds. These electron pairs are located in the two sp hybrid orbitals. In each orbital, one electron is a valence electron contributed by beryllium the other electron comes from the fluorine atom. 

Linear molecule A triatomic molecule in which the bond angle is 180° examples include BeF2 and C02,176 Linear polyethylene, 612 Liquid scintillation counter, 518 Liquid-vapor equilibrium, 253-254q boiling point, 230-231 critical pressure, 231-232 critical temperature, 231-232 symbol, 227... 

Information exists about the use of measuring cells made entirely of diamond or graphite with or without embedded diamond windows. Diamond cells were used, for instance, by Toth and Gilpatrick [333] in the investigation of the Nb(IV) spectrum in a LiF - BeF2 molten system at 550°C. Windowless graphite cells for the IR spectroscopy of melts were developed by Veneraky, Khlebnikov and Deshko [334]. Diamond, and in some cases windowless sapphire or graphite micro-cells, were also applied for Raman spectroscopy measurements of molten fluorides. 

Quist, Bates and Boyd investigated Raman spectra of the molten systems NaF - LiF - BeF2 [351] and NaBF4 - NaF [352] and characterized the complexes BeF42 and BF4 as having a tetragonal configuration. Wave numbers are as follows (cm 1) ... 

Therefore we should expect in the gaseous state to find molecules such as BeH2 and BeF2. These molecules have been detected. On the other hand, beryllium has the trouble boron has, only in a double dose. It has two vacant valence orbitals. As a result, BeH2 and BeF2 molecules, as such, are obtained only at extremely high temperatures (say, above 1000°K). At lower temperatures these vacant valence orbitals cause a condensation to a solid in which these orbitals can participate in bonding. We shall discuss these solids in the next chapter. 

Now we can apply the process of combination shown in Figure 16-14 to BeF2. In the linear, symmetric BeFj molecule, the two bond dipoles point in opposite directions. Since the two bonds are equivalent, there is a complete cancellation, as shown in Figure 16-15. Hence the molecule has no net dipole the molecular dipole is zero. 

The general method for preparing the alkaline earth elements is to convert the mineral to a chloride or a fluoride by treatment with HC1 or HF. Then the molten salt is electrolyzed or, as in the case of BeF2, reduced with a chemical reducing agent such as Mg. 

Only two processes for the manufacture of Be are of industrial importance (i). the thermal reduction of BeF2 using Mg, and (ii) the electrolysis of BeCl2 in a molten chloride electrolyte. Direct reduction of the oxide is ineffective because of its thermodynamic stability only Ca reduces BeO to the metal unfortunately, Ca cannot be used since it forms a stable intermetallic compound with Be, BejjCa. 

The viscosity of molten BeF2 and LiF-BeF2 mixture has been measured with this method. In this experiment, cylindrical spindles of Inconel, varying in radius between 0.158 and 1.65 cm, were rotated in nickel vessels of 2.05 cm internal radius. 

BeF2, like quartz, has a polymeric structure with F atoms linking tetrahedrally coordinated Be atoms BF3 is monomeric. When cooling the liquid down to solidification, which of the two is more likely to form a glass ... 

There has never been a really clear understanding of what a bond line stands for. Originally it was meant to indicate simply that the two atoms between which it is drawn are held strongly together. However, it is now usually taken to indicate a shared pair of electrons, that is, a covalent bond. In contrast, the presence of ionic bonds in a molecule or crystal is usually implied by the indication of the charges on the atoms, and no bond line is drawn. This immediately raises the question of how polar a bond has to be before the bond line is omitted. Whereas the structure of the LiF molecule would normally be written as Li+F without a bond line, even the highly ionic BeF2 is often written as F—Be—F rather than as F Be2+ F . 

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