Beryllium valency states

Divalent beryllium honds through two equivalent sp, or (Sgonal, hybrids. The appropriate ionization energy therefore is not that of ground state beryllium, ls2 2. hut an average of those energies necessary to remove electrons from the promoted, valence state ... 

Boron Trichloride, BCI3. Here the molecule is planar, with the boron atom at the centre of an equilateral triangle of chlorine atoms (Fig. 46). The valence state must be described in terms of three similar hybrid AO s pointing towards the comers of the triangle. Such orbitals can be formed by mixing 2s and two 2p AO s, 2p and 2p say they lie in the plane of the latter and are precisely equivalent (Fig. 47). If the so-called trigonal hybrids are denoted by h, hg and hg, the appropriate boron valence state must be B(ls2 h h2 hg ). The hybrid AO s overlap chlorine 3p AO s, directed towards the boron atom, to form localised MO s similar to those in beryllium chloride. 

Fig. 4.3 Scheme to show the formation of the sp hybridized valence state of a beryllium atom from its ground state. This is a formalism and is not a real observation, e.g. the valence state cannot be observed by spectroscopic techniques. The choice of using the 2p,. orbital for hybridization is arbitrary. 

Therefore we should expect in the gaseous state to find molecules such as BeH2 and BeF2. These molecules have been detected. On the other hand, beryllium has the trouble boron has, only in a double dose. It has two vacant valence orbitals. As a result, BeH2 and BeF2 molecules, as such, are obtained only at extremely high temperatures (say, above 1000°K). At lower temperatures these vacant valence orbitals cause a condensation to a solid in which these orbitals can participate in bonding. We shall discuss these solids in the next chapter. 

Beryllium is normally divalent in its compounds and, because of its high ionic potential, has a tendency to form covalent bonds. In free BeX2 molecules, the Be atom is promoted to a state in which the valence electrons occupy two equivalent sp hybrid orbitals and so a linear X—Be—X system is found. However, such a system is coordinatively unsaturated and there is a strong tendency for the Be to attain its maximum coordination of four. This may be done through polymerization, as in solid BeCk, via bridging chloride ligands, or by the Be acting as an acceptor for suitable donor molecules. The concept of coordinative saturation can be applied to the other M"+ cations, and attempts to achieve it have led to attempts to deliberately synthesize new compounds. 

Now let us consider the states of the atom of beryllium in which one of the valence electrons occupies a 2p orbital and the other occupies a 3p orbital. The two electron spins can combine, as shown in Figure... 

All beryllium nuclei contain four protons and therefore +4 electronic units, so that four electrons orbit the nucleus of the neutral atom. Its electronic configuration is is2 2s2. This can be abbreviated as an inner core of inert helium (a noble gas) plus two s-wave electrons in the second radial s state (He)2S2. This locates Be at the top of Group IIA (Mg, Ca, Sr, Ba) of the periodic table. Beryllium therefore has valence +2. 

The main dijfference between a linear H3 system and beryllium hydride, BeH2, which is a linear molecule, is that the central atom now contributes two valence electrons and four atomic orbitals the 2s orbital, which is doubly occupied in an isolated ground state Be atom (configuration Is, 2s ), and the three 2p orbitals which are empty in Be but lie not too far above 2s in energy. Graphically, we have ... 

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. 

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