Beryllium chloride hybrid orbitals

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. 

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. 

Beryllium chloride is a substance of low melting point, is non-conducting when in the molten state, and is soluble in many organic solvents. All these characteristics point to a covalent compound, but it is difficult to see how this is to result from a beryllium atom with a fully filled outer s orbital. X-ray studies have established that the molecule contains two linear Be—Cl bonds, of equal strength. The problem is solved with the introduction of a concept of hybrid orbitals. 

Using this sp-hybridized beryllium, let us construct beryllium chloride. An ritremely important concept emerges here bond angle. For maximum overlap - rween the sp orbitals of beryllium and the p orbitals of the chlorines, the two. "iorine nuclei must lie along the axes of the sp orbitals that is, they must be ocated on exactly opposite sides of the beryllium atom (Fig. 1.6). The angle chlorine bonds must therefore be 180°. 

The sp hybrid orbitals then overlap with 3p orbitals of two chlorine atoms to form beryllium chloride. 

Overlap of orbitals from central and surrounding atoms BeCl2- In beryllium chloride, the Be atom is sp hybridized. Figure 11.2B depicts the hybridization of Be in a vertical orbital box diagram, and part C shows the diagram with shaded contours. Bond formation with Cl is shown in part D. Two empty unhybridized 2p orbitals of Be he perpendicular to each other and to the sp hybrids. The hybrid orbitals overlap the half-filled 3p orbital in each of two Cl atoms. (The 3p and sp hybrid orbitals that are partially colored on the left become fully colored on the right, after each orbital is filled with two electrons.)... 

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