Gold η²-Coordination to Unsaturated and Aromatic Hydrocarbons: The Key Step in Gold-Catalyzed Organic Transformations

For a long time, π-complexes of gold in any of its oxidation states were considered elusive species of uncertain stoichiometry, low stability, and limited relevance. The recent upsurge of research activities in homo- and heterogeneous catalysis of reactions by gold and its compounds has now provided detailed and extensive information on almost all molecular prototypes with Au0, Au+, and Au3+ bound to alkenes, alkynes, and even arenes with low η2-hapticity (or, in special cases, η1 and η3). This extremely important progress in organogold chemistry is reviewed here with an emphasis on the hard experimental evidence and the structure and energy characteristics of gold π-coordination in molecular systems.