The hybridization model and two-center molecular orbitals

Each of the two sp hybrid orbitals on the metal atom may be combined with the pz orbital on one of the halogen atoms to form a two-center orbital which accommodates two electrons. See Fig. 10.3. Since bonds are stronger when the overlap between central atom and ligand atomic orbitals is large, the hybridization model, like the VSEPR model, provides an explanation for the linearity of the Be, Mg and Group 12 dihalides. 

In the case of Ba, however, the situation is the opposite the energy required for excitation to a valence shell configuration, 110 kJ mol , is much smaller than the energy required for excitation to an configuration, 151 kJ mol  

Hybridization of the valence shell s and dxz orbitals to form two equivalent sd hybrid orbitals is illustrated in Fig. 10.4 the combination s + dxz has particularly large values in the first and third quadrants, the combination s - dxz particularly large values in the second and fourth. When each of the two sd hybrids on the metal atom is combined with a valence shell p orbital on a halogen atom, the resulting valence angle would be 90°, or perhaps a little bit larger due to repulsion between the halogen atoms. See Fig. 10.4. 

The hybridization model also offers a rationalization of the fact that the formation of the first M-X bond results in a smaller energy release than the second formation of the first bond is accompanied by rehybridization which requires energy. In Fig. 10.5 we plot 

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