11.4 Palladium Complexes Containing Pd(I), Pd(III), or Pd(IV)

The chemistry of palladium inunusual oxidation states is summarised from the perspective of their potential roles in organic synthesis and catalysis. Oxidation state +4 is well established as a key oxidation state in the organometallic chemistry of palladium, but there are few reports of Pd(I) chemistry except for polyhalogenophenyl derivatives, and little evidence for Pd(III) chemistry. Catalysis involving palladium complexes is dominated by Pd(O) and Pd(II) species, with evidence for Pd(I) involvement in a few systems. There are several catalysis and organic synthesis procedures for which proposals of Pd(IV) intermediates are supported by studies of model reactions, and isolated or spectroscopically detectable complexes, in particular those involving reactions in the presence of strong inorganic oxidants and organosilicon chemistry. Organic syntheses involving alkyl halide oxidative addition to Pd(II) appear to be feasible and, in addition, there are a range of reactions for which Pd(IV) intermediacy is enticing but as yet not supported by model reactions.