Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions. Organopalladium chemistry timeline 1873 - A. N. Zaitsev reports reduction of benzophenone over palladium with hydrogen. 1894 - Phillips reports that palladium(II) chloride reduces to palladium metal by contact with ethylene. 1907 - Autoclave technology introduced by Vladimir Ipatieff makes it possible to carry out high pressure hydrogenation. 1956 - In the Wacker process ethylene and oxygen react to acetaldehyde with catalyst PdCl2/CuCl2 1957 - Tetrakis(triphenylphosphine)palladium(0) is reported by Malatesta and Angoletta. 1972 - The Heck reaction is a coupling reaction of a halogenide with an olefin. Pd(0) intermediates are implicated. 1973 - The Trost asymmetric allylic alkylation is a nucleophilic substitution. 1975 - The Sonogashira coupling is a coupling reaction of terminal alkynes with aryl or vinyl halides. 1994 - The Pd-catalyzed Buchwald-Hartwig amination for C-N bond-forming reactions. Palladium(II) Alkene complexes Unlike Ni(II), but similar to Pt(II), Pd(II) halides form a variety of alkene complexes. The premier example is dichloro(1,5‐cyclooctadiene)palladium. In this complex, the diene is easily displaced, which makes it a favored precursor to catalysts. In the industrially important Wacker process, ethylene is converted to acetaldehyde via nucleophilic attack of hydroxide on a Pd(II)-ethylene intermediate followed by formation of a vinyl alcohol complex. Fullerene ligands also bind with palladium(II). Palladium(II) acetate and related compounds are common reagents because the carboxylates are good leaving groups with basic properties. For example palladium trifluoroacetate has been demonstrated to be effective in aromatic decarboxylation: Allyl complexes The iconic complex in this series is allylpalladium chloride dimer (APC). Allyl compounds with suitable leaving groups react with palladium(II) salts to pi-allyl complexes having hapticity 3. These intermediates too react with nucleophiles for example carbanions derived from malonate esters or with amines in allylic amination as depicted below Allylpalladium intermediates also feature in the Trost asymmetric allylic alkylation and the Carroll rearrangement and an oxo variation in the Saegusa oxidation. Palladium-carbon sigma-bonded complexes Various organic groups can bound to palladium and form stable sigma-bonded complexes. The stability of the bonds in terms of bond dissociation energy follows the trend: Pd-Alkynyl > Pd-Vinyl ≈ Pd-Aryl > Pd-Alkyl and the metal-carbon