Hypercoordinate Bonding to Main Group Elements The Spin-coupled Point of View

Hypercoordinate bonding to main group elements the spin-coupled point of view 

David L. Coopera, Joseph Gerrattb and Mario Raimondi0 

We suggest that the octet rule be demoted in favour of the democracy principle almost all valence electrons can participate in chemical bonding if provided with sufficient energetic incentives. Simple concepts of atomic size and of electronegativity differences prove to be of particular utility in qualitative descriptions. We find no evidence for the utilization of d functions as valence orbitals, or to support notions of p -d back-bonding. 

In spite of all the theoretical evidence, accumulated over many years, it is still commonplace for students to be taught that the existence of hypercoordinate molecules such as SF and PFri relies on the utilization of d orbitals to expand the octet . Indeed, even models based on d sp , dsp2 and dsp3 hybrid orbitals or pn-dn back-bonding are still in use to describe hypercoordinate bonding to second-row elements. Of course, the consensus view that has emerged from most of the 

There are many texts that make the point very clearly that the bonding in a molecule such as SFfi has very little to do with the availability of d atomic orbitals, but this is normally done in the context of MO theory, whereas the general ideas of utilizing d orbitals are much more closely allied with the ideas of classical valence bond theory. This, perhaps, is one of the reasons for the continued survival of such models. The purpose of this Chapter is to describe various calculations which have been performed using modern valence bond theory, in its spin-coupled form, resulting in a useful aide memoire which we term the democracy principle. We argue that there are no significant qualitative differences between the hypercoordinate nature of first-row, second-row and noble gas atoms in appropriate chemical environments. 

---