Equilibria and mesomerism/valence tautomerism of group 4 metallocene complexes

Abstract

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: CC) in Cp′₂M(η²-Me₃SiC₂SiMe₃) (Cp′ = substituted η⁵-cyclopentadienyl), of α-di-Ph₂P- (hetero-tri-functional: P, CC, P) in Cp′₂M(η²-Ph₂C₂PPh₂), and of α-mono-SiH- (hetero-bi-functional: CC, SiH) and α-di-SiH-substituted alkynes (hetero-tri-functional: SiH, CC, 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: CC, CC, CC), five-membered metallacycloallenes Cp′₂M[η²-C(R)CC(R)–C(R₂)–] (formed by coordination of bi-functional enynes: CC, CC), metallacyclocumulenes Cp′₂M[η²-C(R)CCC(R)–] (formed by coordination of bi-functional diynes: CC, CC), and seven-membered metallacyclocumulenes Cp′₂M[η²-C(R)C(C₂R)–C(R)CCC(R)–] (formed by coordination of two bi-functional diynes: CC, CC), 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.