Beryllium hydride bonding orbitals

According to this simple picture, beryllium hydride should have two different types of H-Be bonds —one as in 1 and the other as in 2. This is intuitively unreasonable for such a simple compound. Furthermore, the H-Be-H bond angle is unspecified by this picture because the 2s Be orbital is spherically symmetrical and could form bonds equally well in any direction. 

Draw the Lewis structure for beryllium hydride, BeH2. Draw the orbitals that overlap in the bonding of BeH2, and label the hybridization of each orbital. Predict the H—Be — H bond angle. 

The figure depicts the electron clouds in a complete beryllium hydride molecule. Notice that the hydrogen Is orbitals are not hybridized here they are still spherical. The two bonds in beryllium hydride are the regions of overlap between the hydrogen 7s and beryllium sp hybrid orbitals. 

Even though the valence would be correct after promotion, the structure still would be wrong. Beryllium hydride would have two different kinds of bonds, and methane would have three identical bonds formed by overlap of H(ls) with the C(2p) orbitals and a different bond formed by H(ls) and C(2s). Pauling proposed that new orbitals with the proper symmetry for bond formation could be formed by hybridization of 2s and 2p orbitals after promotion. The Be(2s) and Be(2pz) orbitals would combine to form two equivalent hybrid orbitals oriented 180° apart. The C(2s) would hybridize with the three C 2p) orbitals to give four equivalent new orbitals in a tetrahedral arrangement around the carbon atom. 

Beryllium hydride is formed in a similar fashion. Here the overlap is between the sp hybrid orbitals of the beryllium atom and the partly filled Is orbital from each of two hydrogen atoms, to give two covalent bonds. 

Figure 1.14. Symbolic representations of the SCF-MO bonding orbitals for beryllium hydride (BeH2) (a) Be 2s and positive Is H contribution, (b) Be 2p and negative Is contributions, (c) beryllium hydride (BeH2) halves shown as two sp orbitals from Be and two Is orbitals...

Consider the molecule beryllium hydride, BeH2. Beryllium has two electrons in the 1 orbital and two electrons in the 2s orbital. Without unpaired electrons, this arrangement does not appear to allow for bonding. However, it takes a relatively small amount of energy... 

In boron and beryllium there are fewer electrons than available orbitals. The outer orbitals can be filled, for example, in dimethyl beryllium, by a mechanism whereby one carbon atom orbital overlaps with an orbital from each of two neighboring beryllium atoms. The resulting high-molecular-weight chain of dimethyl beryllium thus has three center bonds, which leads to an absurd structural formula when valences are represented in the normal way. Boron hydrides and some aluminum compounds have similar structures ... 

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