BeF2, structures

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... 

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. 

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 . 

Some atoms are able to form compounds even though the resulting structure doesn t provide eight valence electrons. For example beryllium and boron do not complete their octet in their covalent compounds because these atoms have less than four valence electrons. For example, in BeF2 (F - Be - F) beryllium shares its two valance electrons but it doesn t complete its octet, it is only surrounded by four electrons. In BF3, the boron atom shares its three valence electrons but does not complete its octet as it has just three electron pairs (six electrons) surrounding it. 

When the ratio of the radii increases, then a new type of crystal structure occurs, in which each divalent ion is surrounded by six univalent ions, and each univalent by three divalent ones. This arrangement, shown in Figure 14, is called the rutile type, named after the mineral Ti02. While CaF2 still has the structure with 8 4 coordination, the rutile structure is observed in MgF2 because of the smaller radius of the Mg2+ ion. After the rutile type there is a further reduction of the coordination to 4 2 this type of structure occurs in BeF2 and Si02 in the different modifications of silica, each silicon ion is surrounded by 4 O2" ions, and each O2 ion is between two Si4+ ions. The ionic ratio r+/r can also be decreased if,... 

If we consider the fluorides, for example, which form pure coordination lattices (p. 33), then those from the alkaline earth metals with the exception of magnesium and beryllium crystallize in the fluorite structure, in which the cation is surrounded by eight fluorine ions for CaF2 and CdF2, which have the same structure, r+/r is 0.71 and 0.69 respectively just at the limit V 3— 1 — 0.73. The fluorides of other divalent ions, such as Mn, Fe, Co and Ni and also Mg, crystallize in a structure with coordination number six (rutile type). It is only for BeF2 that the ratio r+/r = 0.23 lies below the limit of this coordination number and it has a structure similar to that of cristobalite (Si02) with four neighbours (see also p. 66). 

As an example, in liquid BeF2-LiF, one can interpret the characteristics of the NMR absorption as being due to the existence of BeF. The ion is stable to 820 K. However, no evidence of the ion s rotation is seen, and a probable interpretation of this is that BeF groups are bonded into bigger structures, which prevent rotation of individual units in the structure. 

Predict a structure of BeF2 in the gas phase. What structure would you predict for BeF2(s) ... 

According to Walsh (3 ), molecules with not more than 16 valence electrons are linear in their ground states. Since LiF2 (g) has 16 valence electrons, we assume its molecular structure is linear. The Li-F bond distance is estimated by comparison with that of LlF(g) reported by Wharton et al, ( ). The vibrational frequencies are estimated from those of the Isoelectronic gaseous molecules, CO and BeF2. 

Binary compounds are formed with all nonmetallic elements, many by direct combination. Beryllium is exceptional as its coordination is almost always tetrahedral, giving structures that may be regarded as polymeric rather than highly ionic. Thus BeO has the wurtzite structure (see Topic D3). BeF2 is... 

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