Half-sandwich Ni(II) complexes [Ni(Cp)(X)(NHC)]: From an underestimated discovery to a new chapter in organonickel chemistry

Complexes [Ni(Cp)(X)(NHC)] and their ionic derivatives [Ni(Cp)(L)(NHC)]+ have attracted significant interest in recent years owing to their simple preparation, stability in the solid state, and versatile reactivity in both stoichiometric and catalytic reactions. In this review, the syntheses of the title complexes are comprehensively presented with respect to the scope of each ligand. The stoichiometric reactions involving [Ni(Cp)(X)(NHC)] as substrates, providing access to novel organometallics or inorganics, are also presented. Detailed discussion of the solid state structures of [Ni(Cp)(X)(NHC)] and [Ni(Cp)(L)(NHC)]+ based on the data deposited in Cambridge Structural Database is presented. It is shown on this basis that Ni2+ should be treated as four-coordinate with square planar coordination polyhedron. The second part of this review deals with the reported to date catalytic applications of [Ni(Cp)(X)(NHC)] in numerous reactions, such as Suzuki-Miyaura cross-coupling, Kumada-Tamao-Corriu cross-coupling, arylamination reactions, or polymerizations of olefins.