There have been various explanations posited for the [$\alpha$-effect](http://en.wikipedia.org/wiki/Alpha_effect). It is currently in the [list of unsolved problems in chemistry](http://en.wikipedia.org/wiki/List_of_unsolved_problems_in_chemistry) on Wikipedia, but, due to a lack of references to that effect, I'm not entirely convinced it really should be listed there. Here's the summary of my research into the topic thus far:

  * I read Ren, Y. & Yamataka, H.<sup>1</sup>, "The alpha-effect in gas-phase SN2 reactions revisited." In it, they claim that explanations based on ground-state destabilization (presumably due to repulsion between the electrons of the nucleophilic atom and the $\alpha$-electrons) are not correct. Their reasoning is that this would result in a difference in the $\Delta G$ between reactants and products, leading to thermodynamic equilibrium effects. They argue that a correct explanation should be one exclusively involving stabilization of the transition state (i.e., minimization of $\Delta G^{\ddagger}$), and go on to offer some explanation for how this may occur (along with experimental data). Intuitively, their conclusion seems reasonable to me, and it also (at least to my naive comprehension) seems eminently testable. I don't know whether equilibrium effects consistent with ground-state destabilization have actually been observed or not; however, if they haven't, shouldn't that put the nail in the coffin of that theory? Or is it simply that the authors are searching for a purely kinetic $\alpha$-effect, so that a distinction between a thermodynamic one needs to be made?
  * Fleming devotes a section to the effect in his book, _Molecular Orbitals and Organic Chemical Reactions_. He notes that the presence of the $\alpha$-lone pair should raise the energy of the HOMO of the nucleophile, but also points that experimental results don't correlate sufficiently well with the HOMO energies of various $\alpha$-nucleophiles. In particular, certain soft electrophiles (per HSAB theory), such as alkyl halides, apparently show anomalously low preference for $\alpha$-nucleophiles. In the context of SET mechanisms, Fleming says that the higher energy of the HOMO and the availability of $\alpha$-electrons (which can stabilize a radical intermediate) ought to have a highly favorable effect on the rate of reaction, and notes that experimental results have borne this out. My interpretation of this is that, while the picture is perhaps murky for anionic mechanisms, transition-state stabilization seems clearly to be operative in SET mechanisms.

I've also read the original 1962 paper by Pearson and Edwards<sup>2</sup>, which also largely argued for transition-state stabilization as the primary explanatory mechanism.

Overall, from my reading thus far, it seems that transition-state stabilization has been most consistently invoked and has the largest wealth of evidence and the most plausible arguments supporting it. What I'd like to ask is, **(a)** are there flaws in my reasoning or understanding of the material, and **(b)** is this truly a fundamentally unsolved problem, or is there actually some emerging consensus among experts?

**References**

1. DOI: 10.1021/ol0526930
2. DOI: 10.1021/ja00860a005