Q.Explain, in terms of the ionic vs. covalent character of the two reagents, why $\text{KCN}$ reacts with a haloalkane through carbon (giving an alkyl cyanide) while $\text{AgCN}$ reacts through nitrogen (giving an alkyl isocyanide).

Cyanide is an ambident nucleophile whose negative charge is shared between carbon and nitrogen, and which end reacts is decided by the character of the counter-cation. Potassium, a hard, weakly polarising cation, leaves $\text{CN}^-$ essentially free and ionic in solution, so the reaction proceeds through the softer, more polarisable, more nucleophilic carbon end -- giving the cyanide (nitrile) product. Silver, by contrast, is a soft, strongly polarising cation that binds covalently and preferentially to cyanide's carbon end (silver-carbon bonds of this kind are well known to be strong and covalent), which effectively removes carbon fro …

Compare how strongly/covalently each cation binds cyanide's carbon end, and connect that directly to …

Explaining the regiochemistry by 'silver being a better catalyst' rather than the corre …