Molecular bonds/orbitals ionic

The bonding molecular orbital is symmetrical in a non-polar covalent bond, but asymmetrical in polar covalent and ionic bonds. 

However, it should not be concluded that the purely covalent crystals are necessarily harder than crystals that are partially ionic in character. In fact, the facility with which atoms can be displaced also depends on the facility with which the bonding molecular orbital can be deformed by external perturbation. Although it does not exactly correspond to the same phenomenon, the facility of mechanical deformation of the framework can be thought to be related to the electrical polarizability of bonds, because in quantum mechanics the operators corresponding to interactions between two systems only introduce electrostatic potentials. It is now known that bonds between atoms belonging to the same row have comparable polarizabilities, and that the polarizability increases considerably when these atoms are replaced by one or two atoms belonging to the next row ... 

The additional terms in the molecular orbital wavefunction correspond to states with the two electrons in the same orbital, which endows ionic character to the bond The... 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

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