Beryllium alkyl bridges

As is the case with alkyl bridges between aluminum atoms, these bridges between beryllium and magnesium atoms are relatively weak, and the metal orbitals are put to better use by addition of Lewis bases (L), which cleave the polymer chains, forming MR2L2 monomeric molecules, in which carbon atoms are no longer hypercoordinated [Eq. (2.8)]. In weakly basic solvents dimers (30) that retain alkyl bridges (and so hypercoordinate carbon atoms) may be formed. 

Metals can attain a more stable electronic configuration by accepting electrons from n-donor and n-donor systems. As discussed in Chapter 2, a variety of compounds such as alkylfithiums, aluminum, beryllium, magnesium, and other related systems form stable alkyl bridged species where the metal draws upon the electrons in a bonds in the alkyl substituent to attain electronic stability. 

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 . ... 

The example shown is soluble in benzene, a suitable solvent for recrystallization, and may have a structure similar to those of analogous aluminium compounds (p 108) or the isoelectronic beryllium alkyls (p 43) with alkyl bridges linking adjacent lithium and boron atoms, a structure nevertheless unusual in view of the absence of association in the boron alkyls themselves. The tetra-alkylammonium salt (isoamyl)4NB(isoamyl)4 in which the cation and anion are large synunetrical ions of virtually the same size and presumably similar mobilities has proved a useful reference electrolyte for the evaluation of single ion conductivities in such solvents as MeCN, MeN02 and PhN02. 

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. 

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... 

Lithium alkyls in ether or benzene show a mean degree of association of from three to seven, whereas phenyl- and benzyllithium are dimeric in ether (17, 135). The lower degree of association in ether may stem from etherate formation. The structure of these auto-complexes may be analogous to that of beryllium and aluminum alkyls, or perhaps a lithium atom acts as a Lewis acid that is, Li [Li(C6H6)2]e. Wittig (135) favors this formulation over a phenyl bridging scheme. 

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