Beryllium electron-deficient

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. 

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. 

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. 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

The structural data for these systems are collected in Table V along with data for several other beryllium derivatives including the simple hydrides that also form electron-deficient bridged systems of high stability. 

Limited theoretical studies (31, 89) on the electron-deficient beryllium derivatives have been interpreted to imply that extensive Be—Be bonding occurs. If such bonding does occur, the increased bond length observed in the phenylethynyl(methyl)beryllium trimethylamine adduct takes on additional significance since the Be—Be distance in this derivative is increased by almost 0.3 A over that observed in dimethyl-and diethylberyllium. Moreover, if cyclic trimers are formed, then increased metal-metal distances would be likely, thus reducing the probability of stabilization of the bridged system by Be—Be bonding. Additional studies will be required both on structures and of spectroscopic properties of the species to answer these questions. 

A characteristic feature of the structuie of most electron-deficient substances is that the atoms have ligancy that is not only greater than the number of valence electrons but is even greater than the number of stable orbitals.66 Thus most of the boron atoms in the tetragonal form of crystalline boron have ligancy 6. Also, lithium and beryllium, with four stable orbitals and only one and two valence electrons, respectively, have structures in which the atoms have ligancy 8 or 12. All metals can be considered to be electron-deficient substances (Chap. 11). 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

An operational description is that one reactant (the more ionic compound with the more electropositive metal) transfers alkyl anions to the other. Thus the four methyl groups in Li2BeMe4 form a distorted tetrahedron around the beryllium, with longer distances to the lithium ions. However, this description is oversimplified. The low-temperature nuclear magnetic resonance (NMR) spectrum of Li3MgMe5 has three different methyl resonances, suggesting structure (14), related to the MeLi tetramer. Ate complexes with zinc and aluminum compounds also form. Electron-deficient bridge-bonded structures, exemplified by the X-ray structure of... 

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). 

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. 

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

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. 

Only one bimetallic mechanism is presented here, as an example, the one originally proposed by Natta. He felt that chemisorptions of the organometallic compounds to transition metal halides take place during the reactions. Partially reduced forms of the di- and tri-chlorides of strongly electropositive metals with a small ionic radius (aluminum, beryllium, or magnesium) facilitate this. These chemisorptions result in formations of electron-deficient complexes between the two metals. Such complexes contain alkyl bridges similar to those present in dimeric aluminum and beryllium alkyls. The polymeric growth takes place from the aluminum-carbon bond of the bimetallic electron-deficient complexes . ... 

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. 

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