Alkyl halides give cyanides with KCN but isocyanides with AgCN as the products. Why?

Explanation

The cyanide ion is a resonance hybrid of the following structures :
`barC-= N hArr barN -= C`
It is clear from the resonating structures that cyanide ion can attack the nucleophilic site through .C. as well as through .N.. Such a nucleophile which is capable of attacking through more than one site is called ambident nucleophile. KCN is predominantly ionic and therefore, both .C. and .N. atoms are free to donate electron pair. Since, C-C bond is relatively stronger than C-N bond, therefore, the attack occurs mostly through the carbon atom of the cyanide group forming alkyl cyanides as the major product. On the other hand, AgCN is predominantly covalent. Therefore, in this case only N atom is free to donate electron pair and the attack mostly occurs through the N-atom of the cyanide group forming alkyl isocyanides as the major product. Thus,
`RX overset(KCN)to underset("Alkyl cyanide")(RCN)` (attack occurs through .C.)
`RX overset(AgCN)to underset("Alkyl isocyanide")(R-NC)`(attack occurs through .N.)