BeF2, bonding

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... 

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. 

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... 

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. 

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 . 

Figure 7.9 gives contour plots of L for the molecules LiF, BeF2, BF3, and CF4. When there is a large difference in the electronegativities of two bonded atoms, most of the electron den-... 

Consider, for example, BeF2, which is a symmetrical, linear molecule. Each Be—F bond is polar (because fluorine has a greater electronegativity than beryllium). Due to the linear shape of this molecule, however, the polarities of the two bonds are directly opposite each other. The two bonding polarities exactly counteract each other, so that BeF2 is a non-polar molecule. The shape of a molecule, combined with the polarity of its individual bonds, therefore, determine polarity. 

The HSAB rule works and the reaction is exothermic as written. If we look at the individual heats of atomization of the species (from bond energies. Appendix E) we find BeF2 = +1264 HgFa = +536 Bel, = +578 Hgl, = +291 kJ moT. The driving force in Eq. 9.82 is almost entirely the strong bonding in the hard-hard interaction. 

For the C02 Cjo cluster, the translational frequencies e = 54 cm-1 and a [ = 122 cm-1 are smaller than for C2H2 C7o both these vibrations are coupled to the deformation vibrations of the guest and the cage. According to the Mulliken population analysis, there is some electron density redistribution and an increase of polarity of the both M-X bonds in the encapsulated guests, as compared to the free molecules BeF2 and CO2 (induced polarization). 

As compared to CO2 and BeF2, the translational frequencies for the bulkier guest molecules increase (50 and 175 cm-1 for Si02,173 and 229 cm-1 for CS2) and are coupled to deformations of the XMX bond angle. The librational frequency e [ for CS2 is considerably lower than its translational frequencies, whereas, for SiC>2, e [ is intermediate between e and a ... 

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. 

Gas phase Bep2 is monomeric and linear. Prepare a molecular orbital description of the bonding in BeF2. 

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. 

Al3+. Wc would therefore expect that the bonds in such crystals would possess considerable covalent character. It is not possible to determine the nature of the bond directly from the melting point of the crystals as will be apparent from the following discussion. The melting points of several fluorides are given in Table CLIII the values for LiF, BeF2, NaF,... 

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