Linear beryllium compounds

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. 

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 ... 

Metal complexes have a variety of stractures. Silver complexes are often linear beryllium complexes are usually tetrahedral iron forms a carbonyl compound that has a trigonal bipyramidal structure cobalt(lll) complexes are octahedral and tantalum forms an eight-coordinated fluoride complex (Figure 3.1). Although a variety of coordination numbers and structures have been observed in metal complexes, the only common coordination numbers are four and six the common structures corresponding to these coordination numbers are tetrahedral and square planar, and octahedral, respectively. In studying metal complexes, it soon becomes clear that the octahedral structure is by far the most common of these configurations. 

Table it.1-99 Linear thermal expansion coefBcient a of beryllium compounds... 

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. 

Beryllium chloride is a substance of low melting point, is non-conducting when in the molten state, and is soluble in many organic solvents. All these characteristics point to a covalent compound, but it is difficult to see how this is to result from a beryllium atom with a fully filled outer s orbital. X-ray studies have established that the molecule contains two linear Be—Cl bonds, of equal strength. The problem is solved with the introduction of a concept of hybrid orbitals. 

When the central atom of a small compound has a steric number of 2, the central atom will be sp hybridi2ed. As an example, consider the structure of BeH2- Beryllium has two valence electrons, each of which is used to form a bond. The result is two bonds and no lone pairs, giving a steric number of 2. The central beryllium atom therefore requires only two orbitals and must be sp hybridized. Recall that yr-hybridized orbitals achieve maximal separation when they are linear (Figure 1.42). 

One conld propose bond formation in BeH2 by overlap of the Be 2s orbital with the li orbital of one H and the Be 2p orbital with the second H (Figure 1-15). This schane predicts two different bonds of unequal length, probably at an angle. However, electron repulsion predicts that compounds such as BeH2 should have linear structures (Section 1-3). Experiments on related compounds confirm this prediction and also show that the bonds to beryllium are of equal length. ... 

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