Beryllium electron-deficient molecules

Electron-Deficient Molecules Gaseous molecules containing either beryllium or boron as the central atom are often electron deficient that is, they have/ewer... 

To illustrate electron-deficient molecules, consider in Scheme 2.9 the molecules that can be constructed from beryllium (Be) and boron (B) with as many hydrogen... 

In this molecule. Be is surrounded by only four electrons / two electron pairs. As we shall learn in section 3.2, chapter 3 that beryllium does not reach the state of Nirvana and forms an electron-deficient molecule. 

In the beryllium chloride molecule (BeCl2(g)), the beryllium atom has only four electrons in its valence shell (Figure 4-31). The molecule is described as electron deficient. The boron trichloride molecule is also electron deficient the central boron atom has only six electrons in its valence shell (Figure 4-32). A related example of an electron-deficient molecule is aluminium trichloride, AICI3. The aluminium atom has only six electrons in its valence shell. All these molecules have incomplete octets. 

Typical examples of this class of Lewis acids are electron-deficient molecules such as the halides of boron, beryllium and aluminium, for example, BCI3, BeCl2 and AICI3. 

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. 

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. 

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 ... 

Note that in the product H3NBF3, which is very stable, boron has an octet of electrons. It is also characteristic of beryllium to form molecules where the beryllium atom is electron-deficient. 

In solid Bep2, a complex network is formed with a Be atom coordination number of 4 (see Figure 3.7). BeCl2 dimerizes to a 3-coordinate structure in the vapor phase, but the linear monomer is formed at high temperatures. This monomeric structure is unstable due to the electronic deficiency at Be in the dimer and the network formed in the solid-state, the halogen atoms share lone pairs with the Be atom in an attempt to fill beryllium s valence shell. The monomer is still frequently drawn as a singly bonded structure, with only four electrons around the beryllium and the ability to accept lone pairs of other molecules to relieve its electronic deficiency (Lewis acid behavior, discussed in Chapter 6). 

It is also characteristic of beryllium to form molecules where the beryllium atom is electron-deficient. 

Draw dot-and-cross diagrams for the following covalently bonded molecules. Show only the outer electron shells. Note that in part d the beryllium atom is electron deficient and in part e the phosphorus atom has an expanded octet, a Tetrachloromethane, CCl b Phosphorus(lll) chloride c Bromine, Brj d Beryllium chloride, BeClj e Phosphorus(V) chloride, PCI5... 

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