Why do SN2 reactions of alkyl halides proceed differently with KCN and AgCN?

Question

Alkyl halides react with $\\ce{KCN}$ to form alkyl cyanides as the main product, whereas the use of $\\ce{AgCN}$ leads to isocyanides as the chief product. Why does this happen?

Answer 1

Potassium floats off as a separate ion. Silver cation makes partially covalent bond with negative carbon, leaving the nitrogen to be more nucleophilic.