Valence Bond Concepts Applied to the Molecular Mechanics Description of Molecular Shapes. 2. Applications to Hypervalent Molecules of the P-Block

A fascinating aspect of inorganic chemistry is the occurrence of
complicated and varied molecular shapes.
However, these same features lead to difficulties in developing
molecular mechanics (MM) methods that are suitable
for inorganic molecules. In this paper we demonstrate that simple
valence bond concepts can guide the construction
of a new MM force field for hypervalent molecules of the p-block of the
periodic table. The primary difficulty in
applying valence bond concepts to the MM description of hypervalent
molecular shapes is the occurrence of intrinsically
delocalized bonding arrangements, such as the three-center
four-electron bond of XeF2. The inclusion of
resonating
configurations into the MM method provides a mechanism for surmounting
the difficulties presented by hypervalent
molecules. By making the contributions of the individual
configurations to the total potential energy function
dependent
on the molecular geometry, we find that both equilibrium geometries and
fluxional pathways of hypervalent molecules
can be modeled with impressive accuracy. This model, which we call
HyperValent Valence Bond (HV-VB), is
readily extended to hypervalent molecules containing mixed ligands.
By using the valence bond model to derive the
HV-VB method, the results of our MM computations indirectly become
discriminating tests of the basic concepts of
the model. The ideas that Pauling first presented more than six
decades ago exhibit remarkable robustness.