Dinuclear Pd-I complexes with bridging allyl and related ligands

There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd-0 or Pd-II oxidation states. However, in the last decade, dinuclear Pd-I complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd-0 and Pd-II oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear Pd-I complexes. A significant proportion of the known dinuclear Pd-I complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear Pd-I complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear Pd-I complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.