It was three years ago that I first blogged on the topic of the Sn2 reaction. Matthias Bickelhaupt had suggested that the Sn2 reaction involving displacement at a carbon atom was an anomaly; the true behaviour was in fact exhibited by the next element down in the series, silicon. The pentacoordinate species shown below (X=Si) is naturally a minimum, and the fact that for carbon (X=C) one gets instead a transition state resulting in a significant thermal barrier (~ 20 kcal/mol) was a manifestation of abnormal behaviour.