An application of HSAB theory is the so-called Kornblum's rule (after Nathan Kornblum) which states that in reactions with ambident nucleophiles (nucleophiles that can attack from two or more places), the more electronegative atom reacts when the reaction mechanism is S1 and the less electronegative one in a S2 reaction. This rule (established in 1954) predates HSAB theory but in HSAB terms its explanation is that in a S1 reaction the carbocation (a hard acid) reacts with a hard base (high electronegativity) and that in a S2 reaction tetravalent carbon (a soft acid) reacts with soft bases.
According to findings, electrophilic alkylations at free CN occur preferentially at carbon, regardless of whether the S1 or S2 mechanism is involved and whether hard or soft electrophiles are employed. Preferred N attack, as postulated for hard electrophiles by the HSAB principle, could not be observed with any alkylating agent. Isocyano compounds are only formed with highly reactive electrophiles that react without an activation barrier because the diffusion limit is approached. It is claimed that the knowledge of absolute rate constants and not of the hardness of the reaction partners is needed to predict the outcome of alkylations of the cyanide ion.