bond length changes in the opposite direction: Pd-Alkynyl < Pd-Vinyl ≈ Pd-Aryl < Pd-Alkyl. Palladium(0) compounds Zerovalent Pd(0) compounds include tris(dibenzylideneacetone)dipalladium(0) and tetrakis(triphenylphosphine)palladium(0). These complexes react with halocarbon R-X in oxidative addition to R-Pd-X intermediates with covalent Pd-C bonds. This chemistry forms the basis of a large class of organic reactions called coupling reactions (see palladium-catalyzed coupling reactions). An example is the Sonogashira reaction: Organopalladium(IV) The first organopalladium(IV) compound was described in 1986. This complex is Me3Pd(IV)(I)bpy (bpy = bidentate 2,2'-bipyridine ligand) It was synthesized by oxidative addition of methyl iodide to Me2Pd(II)bpy. Palladium compounds owe their reactivity to the ease of interconversion between Pd(0) and palladium(II) intermediates. There is no conclusive evidence however for the involvement of Pd(II) to Pd(IV) conversions in palladium mediated organometallic reactions. One reaction invoking such mechanism was described in 2000 and concerned a Heck reaction. This reaction was accompanied by a 1,5-hydrogen shift in the presence of amines: The hydride shift was envisaged as taking place through a Pd(IV) metallacycle: In related work the intermediate associated with the hydride shift remains Pd(II): and in other work (a novel synthesis of indoles with two Pd migrations) equilibria are postulated between different palladacycles: and in certain intramolecular couplings synthetic value was demonstrated regardless of oxidation state: References