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: 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.