Beryllium: electrons formation

The 3D contour plots of the electron density changes accompanying the beryllium bond formation in H2Be-OH2 and p2Be-OH2 are reported in Pig. 17.2, as a suitable example. Beryllium bonds for both the complexes are characterized by significant electron density rearrangements all over the whole molecular region of... 

In its general corrosion behaviour, beryllium exhibits characteristics very similar to those of aluminium. Like aluminium, the film-free metal is highly active and readily attacked in many environments. Beryllium oxide, however, like alumina, is, a very stable compound (standard free energy of formation = —579kJ/mol), with a bulk density of 3-025g/cm as compared with 1 -85 g/cm for the pure metal, and with a high electronic resistivity of about 10 flcm at 0°C. In fact, when formed, the oxide confers the same type of spurious nobility on beryllium as is found, for example, with aluminium, titanium and zirconium. 

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 the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

The only way that we can obtain two unpaired electrons for bonding in beryllium is to promote one of the 2s electrons to the 2p level. However, the energy required to carry out this promotion would be sufficiently great to discourage bond formation. It is observed that Be does form reasonably stable bonds with other atoms. Moreover, the two bonds in BeH2 and similar molecules are completely equivalent this would not be the case if the electrons in the two bonds shared Be orbitals of different types. 

Curie and Joliot obtained positron-electron pairs from heavy metals bombarded with high energy (5 MeV) y-rays derived from beryllium mixed with polonium. The average life of the positron is about 10 sec. On colliding with an electron both are annihilated and y-radiation—the annihilation radiation—is emitted. The formation and annihilation of a positron-electron pair is thus represented. 

Beryllium hydride, BeH2, has four valence electrons, two from beryllium and one each from the two hydrogen atoms, all of which appear in its Lewis diagram. In VSEPR theory, the steric number is 2, so the molecule is predicted to be linear, and this prediction is verified by experiment. The electron configuration of the central atom is Be (ls) (2s). There are no unpaired electrons to overlap with H(ls) orbitals, so the VB model fails to predict the formation of BeHi. 

The formation and the shapes of the sp hybrid orbitals and their participation in chemical bonds are shown in Figure 6.41. The first column shows the non-hybridized orbitals on the Be atom, and the second column shows the hybrid orbitals. The amplitude for each hybrid at any point r from the beryllium nucleus is easily visualized as the result of constructive and destructive interference of the 2s and 2p wave functions at that point. Because the sign of the 2s orbital is always positive, whereas that of the 2p orbital is different in the -I- and -z directions, the amplitude of Xi is greatest along -l-z, and that of 2 is greatest along —z. Because the probabilities are the squares of the amplitudes, an electron in xi is much more likely to be found on the left side of the nucleus than on the right the opposite is... 

The uniqueness of the beryllium ion s properties can be attributed to its very small size compared to the sizes of the ions of the other alkaline earths. Because of the small size of a beryllium atom, the valence electrons are held very tightly to the nucleus, effectively preventing the formation of a positive ion. 

Similar behaviour is found with other atoms. Boron in the trivalent state has one s and two p electrons and hybridization leads to the formation of three equivalent hybrid sp2 orbitals lying in the same plane and with a valency angle of 120°. Experimental data6 for B(CH3)3 are in agreement with this prediction. Beryllium and mercury in the excited state necessary for bond formation, have one s and one p electron which form two hybrid sp bonds at an angle of 180° to each other. 

The L shell can accommodate a total of 8 electrons, and as further electrons are added to form the sequence of elements beryllium, boron, carbon, etc., these electrons take their place in the second shell until finally, at the end of the second period, neon (Z = 10) is reached, with both the first and second shells fully occupied and with an electronic structure which can be symbolized as (2, 8). The addition of further electrons to form the sequence of elements sodium, magnesium, etc., of the third period requires the formation of a new shell, and in sodium (Z = 11) a single electron occupies the M shell of principal quantum number 3 again the ionization energies reflect the difference in energy between this electron and those more tightly bound in the L and K shells. 

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