Electronic beryllium compounds

The structure of dimethylberyllium is similar to that of trimethylaluminum except for the fact that the beryllium compound forms chains, whereas the aluminum compound forms dimers. Dimethylberyllium has the structure shown in Figure 12.3. The bridges involve an orbital on the methyl groups overlapping an orbital (probably best regarded as sp3) on the beryllium atoms to give two-electron three-center bonds. Note, however, that the bond angle Be-C-Be is unusually small. Because beryllium is a Lewis acid, the polymeric [Be(CH3)2] is separated when a Lewis base is added and adducts form. For example, with phosphine the reaction is... 

Beryllium chloride, an electron-deficient compound similar to aluminum chloride, is a Lewis acid. The anhydrous salt is used as a catalyst in organic reactions. Its applications, however, are limited. 

Only a limited number of structural studies have been reported on beryllium compounds. The simple alkyls appear to be polymeric with chain structures as shown in XVI (109). For comparison, the structure of di-(t-butyl)beryllium (XVII) is shown as determined from electron diffraction studies (6). In this case, the compound is a linear monomeric species with a Be—C bond length of 1.699 A. Similarly, dimethylberyl-lium has a Be—C bond distance of 1.70 A in the gas phase (5). Comparison of these beryllium structures with the polymer shows that the Be—C distance in the bridge is considerably greater than that in a normal Be—C single bond, a result similar to that observed for the aluminum derivatives. 

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). 

Use Electron tubes resistor cores windows in klystron tubes transistor mountings high-temperature reactor systems additive to glass, ceramics, and plastics preparation of beryllium compounds catalyst for organic reactions. 

In BeCl2, the chlorine atoms achieve the argon configuration, [Ar], and the beryllium atom has a share of only four electrons. Compounds such as BeCl2, in which the central atom shares fewer than 8 e, are sometimes referred to as electron deficient compounds. This deficiency refers only to satisfying the octet rule for the central atom. The term does not imply that there are fewer electrons than there are protons in the nuclei, as in the case of a cation, because the molecule is neutral. 

The octet rule predicts that atoms form enough covalent bonds to surround themselves with eight elechons each. When one atom in a covalently bonded pair donates two electrons to the bond, the Lewis structure can include the formal charge on each atom as a means of keeping track of the valence electrons. There are exceptions to the octet rule, particularly for covalent beryllium compounds, elements in Group 3A, and elements in the third period and beyond in the periodic table. 

Gaseous beryllium chloride (BeCl2) is a linear molecule (AX2). Gaseous beryllium compounds are electron deficient, with only two electron pairs around the central Be atom ... 

Boron and beryllium compounds sometimes do not fit the octet mle. For example, in BF3, the boron atom has only six valence electrons in its outermost shell, whereas in BeF2, the beryllium atom has only four electrons in its outermost shell. Other exceptions to the octet mle include any molecule with an odd number of valence electrons (such as NO or NO2). 

Table 4.1-103 Electronic transport in beryllium compounds, general description...

The metals of group 11 of the periodic table —beryllium, magnesium, calcium, strontium, barium, and radium—are called the alkaline-earth metals. Some of their properties are listed in Table 18-2. These metals are much harder and less reactive than the alkali metals because there are twice as many valence electrons. The compounds of all the alkaline-earth metals are similar in composition they all form oxides MO, hydroxides M(0H)2, carbonates MCO3, sulfates MSO4, and other compounds (M = Be, Mg, Ca, Sr, Ba, or Ra). 

---