Beryllium, dimers

P.J. Bruna et al., Beryllium-beryllium bonding. 1. Energetics of protonation and hydrogenation of beryllium dimer and its ions. J. Phys. Chem. 96, 6269-6278 (1992)... 

CiftH3gPftSi3, 2,2,5,5,7,7-Hexamethyl-3,6-di-t-butyl-phospha-2,5,7-trisilanobornane, 44B, 589 CiftHftoBe2NftSift, (Bis(dimethylsilyImethylamido)methy1)beryllium dimer, 45B, 702... 

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... 

Four-membered heterocycles exist in the structures of dimeric di-/-butylarsino zinc complex 32 and in the related gallium compound 33.191,192 In contrast, the di-/-butylarsino beryllium complex BuSAs-BcCp 34 is monomeric.193... 

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. ... 

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. 

In reactivity studies of monomeric Be(NPT2)2- it has been shown that CO2 and CS2 insert into the Be—N bonds to give [Be(O2CNPr 2)2] or Be(S2C. Pr 2)21 whereas the dimeric beryllium dialkylamides can afford more complicated structures.The cubanoid complex [ Be(Cl)(p3-NPEt3) 4] was obtained by the thermal Me3SiCl elimination from [BeCl2(Me3SiNPEt3)] ... 

This compound, which exists as a dimer with tricordinate beryllium, demonstrates explosive combustion. 

The structure of beryllium(II) chloride in its various phases has been described, together with the relevant thermochemistry, in Section 3.3. The dimer Be2Cl4 is an example of scheme (2) bonding, while the chain structure of BeCl2(s) represents scheme (3). Scheme (2) is rather unusual, and in compounds stable at room temperature it is mainly restricted to some organometallic compounds of Be. The example given in Table 6.3 has the structure ... 

To our knowledge there is no experimental evidence for n bonding between a BeR2 unit and a simple alkene or alkyne ligand. However a n interaction has been found in the X-ray crystal structure of the dimeric dipropynylberyllium trimethylamine adduct (48). The unit cell of this molecule contains two independent centrosymmetric dimers in which the alkynyl groups exhibit different types of interaction with the beryllium atoms, one of them forming an effectively electron-precise dimer unit by n interaction as portrayed in XX. 

Dialkyl-beryllium und Trialkyl-aluminium bilden in der Haupt-sache dimere Assoziate. Trialkylaluminium mit voluminosen Resten, z. B. A1(CH2 CHRR)3 (Iso-butyl-Typ), ist dagegen praktisch uberhaupt nicht assoziiert 16). 

In the Group 2 derivatives, less diversity is found the beryllium complex is monomeric, probably for steric reasons, but the Mg-Ba compounds are dimers. Although the M—Si... 

The first clue to the formation of a heterotrimetallic alkoxide was, however, detected in 1985 in the case of beryllium. Its stability was initially ascribed to the small size of beryllium, which hindered the disproportionation of the species. The heterobimetallic isopropoxide (Pr-f-0)Be(/i-0-/-Pr)2Al(0-z-Pr)2 was found on the basis of NMR studies to dimerize on ageing (8) into a product that could be reported as [(0-f-Pr)2Al( -0-/-Pr)2Al(0-i -Pr)2]. This observation led to the feasibility of reactions of the following types, which resulted for the first time in 1985 in the synthesis and characterization of heterotrimetallic isopro-poxides. 

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