The hexacarbonyldicobalt complexes of propargylic cations are generated readily, predominantly from alkynedicobalt complexes bearing propargylic leaving groups. These cations have both good stability and reactivity, and enter into reaction with a wide range of carbon‐ and heteroatom‐based nucleophiles in a reliable manner; these are widely known as Nicholas reactions. In conjunction with the ready incorporation and removal of the dicobalt unit, these reactions have been employed extensively in total synthesis.