I’m thinking about hypervalence a lot these days. I think I may use it as a case study in my dissertation. It’s an interesting story, anyway: Since the 1970s, chemists have offered apparently competing explanations for why hypervalency occurs in some kinds of molecules and not others, and the debate continues to the present day. One type of explanation for hypervalency involves reference to the strictly chemical concept of molecular orbitals, and the other refers only to physical features of the participating atoms, such as atomic size and the number of protons in a nucleus.
So, I want to look at the merits and drawbacks of each explanation and survey the current state of the debate. I think right now the physical explanation has more wind in its sails. And I think context-sensitive explanatory goals can permit both explanations to play useful roles in chemical practice. But the current trend in favor of the physical explanation is interesting, and I wonder if it isn’t a reflection of the fact that many chemists value accommodating experimental data more highly than producing a unified theoretical basis for explanatory claims about chemical bonds.
More to come on hypervalence as papers materialize.