What is "hydrocyanation"

n. (context organic chemistry English) Any reaction in which the elements of hydrogen cyanide are added to a molecule, especially across a double bond

Hydrocyanation is, most fundamentally, the process whereby H and CN ions are added to a molecular substrate. Usually the substrate is an alkene and the product is a nitrile. When CN is a ligand in a transition metal complex, its basicity makes it difficult to dislodge, so, in this respect, hydrocyanation is remarkable. Since cyanide is both a good σ–donor and π–acceptor its presence accelerates the rate of substitution of ligands trans from itself, the trans effect. A key step in hydrocyanation is the oxidative addition of hydrogen cyanide to low–valent metal complexes. In hydrocyanation of unsaturated carbonyls addition over the alkene competes with addition over the carbonyl group.