VSEPR Theory - VSEPR and Localized Orbitals

VSEPR and Localized Orbitals

The VSEPR theory places each pair of valence electrons in a bond or a lone pair found in a local region of the molecule. Molecular orbital theory yields a set of orbitals that have the symmetry of the molecule and that are delocalized over several atoms. However these orbitals can be transformed into an equivalent set of localized molecular orbitals.

In the water molecule for example, molecular orbital calculations yield two bonding orbitals each extending over all three atoms, which can be combined into two localized orbitals, one for each bond. However, molecular orbital and natural bond orbital calculations give two nonequivalent lone pairs, one s-rich sp2 hybrid in the plane of the molecule and one pure p orbital perpendicular to this plane. This is one of the limitations of VSEPR theory, in that it treats all lone pairs as equivalent when in fact they are not. Thus, although VSEPR theory provides a good description of localized bonding orbitals, it is inaccurate in predicting the shapes of localized nonbonding orbitals.

Nonetheless, the delocalized and proper localized orbitals provide completely equivalent descriptions of the ground state, since the total wavefunction for all electrons is a Slater determinant, which is unchanged by the transformation of the orbitals.