Kornblum s rule

With alkali cyanides, a reaction via a SN2-mechanism takes place the alkyl halide is attacked by cyanide with the more nucleophilic carbon center rather than the nitrogen center, and the alkylnitrile is formed. In contrast, with silver cyanide the reaction proceeds by a SnI-mechanism, and an isonitrile is formed, since the carbenium intermediate reacts preferentially with the more electronegative center of the cyanide—i.e. the nitrogen (Kornblum s rule, HSAB concept). ... 

This principle, sometimes called Kornblum s rule, was first stated by Kornblum Smiley Blackwood Iffland J. Am. Chem. Soc. 1955, 77. 6269. 

Kornblum s rule As the nature of a reaction changes from SN1 to SN2, an ambident nucleophile becomes more likely to attack with its less electronegative atom. 

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