Democracy for electrons?

Spin-coupled theory has been used to investigate the bonding in several hypercoordinate and 'normal octet' compounds of main group elements. It was found that d basis functions play much the same qualitative role in hypercoordinate and normal molecules, acting as polarization functions. There are no obvious demarcations in the energy penalty per bond of excluding such functions. No evidence was found to support the traditional notions of spndm hybridization. The spin-coupled approach, also known as the full GVB model, provides a very clear and simple picture of the bonding in all of the molecules studied. In SF6, for example, the sulfur atom contributes six equivalent, nonorthogonal spx-like hybrids which delocalize onto the fluorine atoms. Each of these two-centre orbitals overlaps with a distorted F(2p) function, with the perfect-pairing spin function dominating. The spin-coupled description of PF5 is entirely analogous, with remarkably little differentiation between axial and equatorial bonds. A key consideration for all of the hypercoordinate species studied is the polarity of the various bonds.

We have suggested that less emphasis than hitherto be placed on the 'octet rule' and that the so-called democracy principle be asserted: any valence electron can participate in chemical bonding if provided with a sufficient energetic incentive. This idea was pursued for phosphorus and sulfur halides, for XeF2, and for the (CH5)-, (SiH5)- and (SiF5)- anions. It is argued that there are no significant qualitative differences between the hypercoordinate nature of first-row, second-row and noble gas atoms in appropriate chemical environments.

Spin-coupled orbitals in SF6


A nonbonding spin-coupled orbital on sulfur in various oxides, fluorides and oxofluorides