Beryllium covalency

The hydrides of beryllium and magnesium are both largely covalent, magnesium hydride having a rutile (p. 36) structure, while beryllium hydride forms an electron-deficient chain structure. The bonding in these metal hydrides is not simple and requires an explanation which goes beyond the scope of this book. 

As a consequence of the high ionisation energy of beryllium its halides are essentially covalent, with comparatively low m.p.. the melts being non-conducting and (except beryllium fluoride) dissolving in many organic solvents. 

Beryllium Halides. The properties of the fluoride differ sharply from those of the chloride, bromide, and iodide. BeryUium fluoride is essentiaUy an ionic compound, whereas the other three haUdes are largely covalent. The fluoroberyUate anion is very stable. 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

The hydrides of the later main-group elements present few problems of classification and are best discussed during the detailed treatment of the individual elements. Many of these hydrides are covalent, molecular species, though association via H bonding sometimes occurs, as already noted (p. 53). Catenation flourishes in Group 14 and the complexities of the boron hydrides merit special attention (p. 151). The hydrides of aluminium, gallium, zinc (and beryllium) tend to be more extensively associated via M-H-M bonds, but their characterization and detailed structural elucidation has proved extremely difficult. 

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. 

Beryllium, at the head of Group 2, resembles its diagonal neighbor aluminum in its chemical properties. It is the least metallic element of the group, and many of its compounds have properties commonly attributed to covalent bonding. Beryllium is amphoteric and reacts with both acids and alkalis. Like aluminum, beryllium reacts with water in the presence of sodium hydroxide the products are the beryl-late ion, Be(OH)42, and hydrogen ... 

Beryllium compounds are very toxic and must be handled with great caution. Their properties are dominated by the highly polarizing character of the Be2+ ion and its small size. The strong polarizing power results in moderately covalent compounds, and its small size limits to four the number of groups that can attach to the ion. These two features together are responsible for the prominence of the... 

Beryllium compounds have a pronounced covalent character, and the structural... 

Self-Test 14.7A Explain why beryllium compounds have covalent characteristics. 

In a search for CVD precursors for beryllium, the air sensitive, volatile solid CpBe(SiMe3) 17 was prepared from the reaction of Li[SiMe3] with CpBeCl in pentane.62 Characterized with single crystal X-ray diffraction, multinuclear NMR, and mass spectra, the compound displays a Be-Si bond length of 2.185(2) A that is somewhat longer than the sum of covalent radii (2.01 A). The lengthening is similar to that observed in the related Be(Si(/-Bu)3)2 (Be-Si = 2.193(1) A),63 so there is not a pronounced effect from the Cp ligand on the Be-Si interaction. 

Although the formula for beryllium chloride is BeCl2, the compound exists in chains in the solid state. The bonding is covalent, and the environment around each Be is essentially tetrahedral with each Cl bridging between two Be atoms separated by 263 pm. 

Compounds of beryllium and aluminum are substantially covalent as a result of the high charge -to-size ratio, which causes polarization of anions and very high heats of hydration of the ions ( —2487kJ mol-1 for Be2+ and — 4690kJ mol-1 for Al3+). 

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

As it does not have any unshared electrons, beryllium would not be expected to form a covalent bond. But experimentally it is found that beryllium is able to form two covalent bonds. To form these bonds one electron moves from the 2s orbital to the 2p orbital leaving the atom in an excited state with two unpaired electrons (Figure 4b). 

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. 

Beryllium and magnesium belong to the 2nd group of the Periodic Table together with calcium, strontium, barium and radium. Characteristic differences, however, may be noticed between the chemistry of Be and Mg and that of the alkaline earth s proper. Be has a unique chemical behaviour with a predominantly covalent character. The heavier elements (Ca, Sr, Ba, Ra) have a predominant ionic behaviour in their compounds. Mg has a chemistry in a way intermediate but closer to that of Be. Analogies between the Mg and Zn chemistries may also be underlined. 

Beryllium is normally divalent in its compounds and, because of its high ionic potential, has a tendency to form covalent bonds. In free BeX2 molecules, the Be atom is promoted to a state in which the valence electrons occupy two equivalent sp hybrid orbitals and so a linear X—Be—X system is found. However, such a system is coordinatively unsaturated and there is a strong tendency for the Be to attain its maximum coordination of four. This may be done through polymerization, as in solid BeCk, via bridging chloride ligands, or by the Be acting as an acceptor for suitable donor molecules. The concept of coordinative saturation can be applied to the other M"+ cations, and attempts to achieve it have led to attempts to deliberately synthesize new compounds. 

The Third-Group Elements.—The B—F bond has about 63 percent ionic character, B—O 44 percent, B—Cl 22 percent, and so forth. Bor,on forms normal covalent bonds with hydrogen. The aluminum bonds are similar to those of beryllium in ionic character. 

---