Beryllium polymeric

Table 7.4 Data for the stability constants of beryllium polymeric hydrolysis species (reaction (2.5), M = p > 1). 

Beryllium Hydride. BeryUium hydride [13597-97-2] is an amorphous, colorless, highly toxic polymeric soHd (H = 18.3%) that is stable to water but hydroly2ed by acid (8). It is insoluble in organic solvents but reacts with tertiary amines at 160°C to form stable adducts, eg, (R3N-BeH2 )2 (9). It is prepared by continuous thermal decomposition of a di-/-butylberylhum-ethyl ether complex in a boiling hydrocarbon (10). 

Chemical Reactivity - Reactivity with Water Reacts vigorously as an exothermic reaction. Forms beryllium oxide and hydrochloric acid solution Reactivity with Common Materials Corrodes most metals in the presence of moisture. Flammable and explosive hydrogen gas may collect in confined spaces Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water and rinse with dilute solution of sodium bicarbonate or soda ash Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Figure 8.9 Examples of four-coordinated molecules of beryllium (a) BeCl2 2Et20 and (b) the polymeric molecule BeCl2.

A polymeric structure is exhibited by "beryllium dimethyl," which is actually [Be(CH3)2] (see the structure of (BeCl2) shown earlier), and LiCH3 exists as a tetramer, (LiCH3)4. The structure of the tet-ramer involves a tetrahedron of Li atoms with a methyl group residing above each face of the tetrahedron. An orbital on the CH3 group forms multicentered bonds to four Li atoms. There are numerous compounds for which the electron-deficient nature of the molecules leads to aggregation. 

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... 

Nucleophilic carbene 4 (R = Me, R = H) has been shown to disrupt the polymeric structure of beryllium dichloride to form an ionic carbene complex, 10 (15). The crystal structure of 10 revealed that the cation possesses a distorted tetrahedral coordination geometry at the beryllium center. The average Be-Cl bond distance (2.083(7) A) is... 

To investigate complexes of the Lewis acid BeO may seem strange from the experimental point of view. BeO is a polymeric solid with a high melting point and it is very difficult to obtain monomeric BeO. Moreover, beryllium is very poisonous and its compounds are difficult... 

The fact that NHCs form stable compounds with beryllium, one of the hardest Lewis acids known and without p-electrons to back donate, shows the nu-cleophilicity of these ligands. Reaction of l,3-dimethylimidazolin-2-ylidene with polymeric BeCl2 results in the formation of the neutral 2 1 adduct 23 or the cationic 3 1 adduct 24. The first NHC-alkaline earth metal complex to be isolated was the 1 1 adduct 25 with MgEt2- Whereas l,3-dimesitylimidazolin-2-ylidene results in the formation of a dimeric compound, the application of sterically more demanding l,3-(l-adamantyl)imidazolin-2-ylidene gives a monomeric adduct. ... 

The Ziegler polymerizations of olefins (92, 5) and the aluminum (108), gallium, beryllium, and indium (ill) alkyl growth reactions also seem to be examples of olefin insertion reactions of metal-carbon compounds. Despite great effort concerning the mechanism of the polymerization reactions, relatively little has been learned about the actual catalytic species involved. 

If pseudoanionic polymerization occurs anywhere else in the periodic table, beryllium and lithium are the most likely candidates. 

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. 

The Mg—C distance in the bridge are somewhat longer than the Mg—C single-bond distances observed in the Grignard-type reagents, but comparisons cannot be made with a simple dialkyl since none of the structures have been reported. The Mg—Mg distance is about 2.70 A, which is relatively short and could permit metal-metal interaction for stabilization of the polymeric materials, as suggested for the beryllium derivatives, but neither sufficient experimental data nor theoretical calculations (90) are available to confirm or refute this possibility. 

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. 

Apart from the vast number of compounds containing B-H-B bridge bonds, a good many other three-centre E-H-E links are found, especially where E or E is B, Be or Li. Beryllium hydride BeH2, is a onedimensional polymeric solid (isostructural with BeCl2 and SiS2 see Section 3.3), whose structure can be rationalised in terms of Be-H-Be (3c, 2e) bridge bonds ... 

As with lithium, many properties of beryllium stand distinctly apart from those of its congeners. Again, the tiny size of the beryllium ion is responsible for the peculiarities. If we arbitrarily call the volume of the magnesium ion 1.0 unit, the volumes of the calcium, strontium, and barium ions become, respectively, about 3, 5, and 8 units however, the volume of the beryllium ion is, on the same scale, only 1/8 unit. Since the oxide is amphoteric, one would expect many of the salts of Be+2 to be extensively hydrolyzed in water (as is the case with the salts of Al8+, Zn2+, and Cr8+). The acidity of beryllium-containing solutions is also increased by polymerization of the beryllium-containing ions ... 

Beryllium and magnesium hydrides, BeH2 and MgH2, appear to be polymeric covalent hydrides rather than ionic hydrides as are those formed by the other Group II metals. 

---