Beryllium hydride, bonding

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. 

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. 

Apart from the vast number of compounds containing B-H-B bridge bonds, a good many other three-centre E-H-E links are found, especially where E or E is B, Be or Li. Beryllium hydride BeH2, is a onedimensional polymeric solid (isostructural with BeCl2 and SiS2 see Section 3.3), whose structure can be rationalised in terms of Be-H-Be (3c, 2e) bridge bonds ... 

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. 

Consider how we might explain the bonding in a compound of divalent beryllium, such as beryllium hydride, BeH2. The beryllium atom, with only four electrons, has a configuration of ls22s. 

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. 

Any atom surrounded by only two groups is linear and has a bond angle of 180°. Two examples illustrating this geometry are BeH2 (beryllium hydride) and HC=CH (acetylene). We consider each carbon atom in acetylene separately. Because each C is surrounded by two atoms and no lone pairs, each H-C-C bond angle in acetylene is 180°, and therefore all four atoms are linear. 

Compounds in which hydrogen is bonded to the alkali and alkaline-earth metals except beryllium are prepared by direct synthesis from the metals or amalgams. Beryllium hydride is prepared by pyrolysis or reduction of organic derivatives. 

Beryllium hydride prepared by these methods is an amorphous polymer, and IR spectra indicate long-chain bridge bonding. 

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. 

The bonds in beryllium hydride (BeH2) molecules are polar, and yet the dipole moment of the molecule is zero. Explain. 

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. 

As we saw in Example 4.14, beryllium hydride has two shared electron pairs around the beryllium atom. These electron pairs have minimum repulsion if they are located as far apart as possible while still bonding the hydrogen to the central atom. This condition is met if the electron pairs are located on opposite sides of the molecule, resulting in a Unear structure, 180° apart ... 

Beryllium hydride, chloride, and di-methylberylhum form polymeric bridged species but, whereas the bridging in the chloride is via an electron pair on chlorine atoms and can be regarded as an electron-pair donor bond, the bonding in the hydride and in the methyl compound involves two-electron three-centre bonds. Coordi-... 

PRACTICE PROBLEM 1.26 What do the bond angles of beryllium hydride surest about the hybridization state of... 

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...

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