Science of Synthesis

Negishi, E.-i.; Takahashi, T., Science of Synthesis, (2003) 2, 785.

Bis(η5-cyclopentadienyl)zirconium or -hafnium complexes containing a π-ligand are classified as complexes of oxidation state +2. They are formally described as being complexes of the type [M(Cp)2(η2-X=Y)] (M = Zr, Hf). However, these compounds can be also regarded as three-membered metallacycles; in this case, the formal oxidation state of the metal is considered to be +4. The number of this type of zirconocene or hafnocene derivatives reported has rapidly increased since the 1980s. These complexes are found to be very useful in organic synthesis and their catalytic reactions have been also developed. Bis(η5-cyclopentadienyl)zirconium or -hafnium complexes of oxidation state +2 include those involved in the coupling reactions with unsaturated compounds to afford five-membered zircona- or hafnacycles, respectively. The stoichiometric versions of these reactions are described in Section ‌2.11.5‌. In this section, attention is focused on the methods of preparation of bis(η5-cyclopentadienyl)zirconium or -hafnium complexes with π-compounds, such as bis(η5-cyclopentadienyl)zirconium complexes with alkenes, benzynes, carbonyl compounds, and alkynes, etc., as well as the formation of bis(η5-cyclopentadienyl)zirconium or -hafnium complexes of oxidation state +2 stabilized with ligands such as phosphines. C—C bond formation reactions catalyzed by bis(η5-cyclopentadienyl)zirconium(II) or -hafnium(II) complexes and other related reactions are also described. In comparison with bis(η5-cyclopentadienyl)zirconium(II) complexes, bis(η5-cyclopentadienyl)zirconi­um(III) complexes have not yet been extensively investigated.

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• 8.Meeeee-Meee, (8888) M 88-8, 888.