Why does KCN give -CN nucleophile while AgCN gives isocyanide?

Nucleophilic substitution by ##"KCN"## gives an alkyl cyanide, while ##"AgCN"## gives an alkyl isocyanide because ##"KCN"## is ionic and ##"AgCN"## is covalent.

Alkyl halides react with alcoholic ##"KCN"## to form alkyl cyanides

##"KCN"## is an ionic compound. The nucleophile is the cyanide ion

##[:stackrel(-1) ("C")"≡N:"]^-##

The carbon atom is the nucleophile and the reaction is

##underbrace("RX")_color(red)("alkyl halide") + :stackrel("-")("C")"≡N:" stackrel("Δ"color(white)(m)) (→)underbrace("RCN")_color(red)("alkyl cyanide") + "X"^"-"##

Alkyl halides react with alcoholic silver cyanide to form alkyl isocyanides.

The electronegativity difference between ##"Ag"## and ##"C"## is so small that the ##"Ag-C"## bond is covalent.

The nucleophile is ##"Ag-C≡N:"##.

The carbon atom is no longer available to act as a nucleophile, but the N atom can still attack the alkyl halide.

##underbrace("RX")_color(red)("alkyl halide") + "AgCN:" stackrel("Δ"color(white)(m)) (→)underbrace("RNC")_color(red)("alkyl isocyanide") + "AgX"##