Chemical Society Reviews

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 Pd⁰ 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 Pd⁰ 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.