Equilibria and mesomerism/valence tautomerism of group 4 metallocene complexes.

In this account, examples of the influence of equilibrium and mesomerism/valence tautomerism on reactions of selected group 4 metallocene complexes will be discussed. Complexes of bis(trimethylsilyl)acetylene Me3SiC2SiMe3 (mono-functional alkynes: C[triple bond, length as m-dash]C) in Cp'2M(η2-Me3SiC2SiMe3) (Cp' = substituted η5-cyclopentadienyl), of α-di-Ph2P- (hetero-tri-functional: P, C[triple bond, length as m-dash]C, P) in Cp'2M(η2-Ph2C2PPh2), and of α-mono-SiH- (hetero-bi-functional: C[triple bond, length as m-dash]C, SiH) and α-di-SiH-substituted alkynes (hetero-tri-functional: SiH, C[triple bond, length as m-dash]C, SiH) in Cp'2M(η2-Me3SiC2SiMe2H) and Cp'2M(η2-HMe2SiC2SiMe2H), respectively, were considered. Additionally, unusual all-C-metallacycles like metallacyclopentynes Cp'2M[η2-C(R2)-C2-C(R2)-] (formed by coordination of tri-functional trienes: C[double bond, length as m-dash]C, C[double bond, length as m-dash]C, C[double bond, length as m-dash]C), five-membered metallacycloallenes Cp'2M[η2-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-C(R2)-] (formed by coordination of bi-functional enynes: C[double bond, length as m-dash]C, C[triple bond, length as m-dash]C), metallacyclocumulenes Cp'2M[η2-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-] (formed by coordination of bi-functional diynes: C[triple bond, length as m-dash]C, C[triple bond, length as m-dash]C), and seven-membered metallacyclocumulenes Cp'2M[η2-C(R)[double bond, length as m-dash]C(C2R)-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-] (formed by coordination of two bi-functional diynes: C[triple bond, length as m-dash]C, C[triple bond, length as m-dash]C), with M = Ti, Zr, and Hf were selected as examples for considerations of equilibrium and mesomerism/valence tautomerism. On the basis of spectroscopic investigations, their molecular structures and selected calculations, it is discussed how the different interpretations of the equilibrium and mesomerism/valence tautomerism are in line with observed reactivity patterns.