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 Me 3 SiC 2 SiMe 3 (mono-functional alkynes: C&z.tbd;C) in Cp′ 2 M(η 2 -Me 3 SiC 2 SiMe 3 ) (Cp′ = substituted η 5 -cyclopentadienyl), of α-di-Ph 2 P- (hetero-tri-functional: P, C&z.tbd;C, P) in Cp′ 2 M(η 2 -Ph 2 C 2 PPh 2 ), and of α-mono-SiH- (hetero-bi-functional: C&z.tbd;C, SiH) and α-di-SiH-substituted alkynes (hetero-tri-functional: SiH, C&z.tbd;C, SiH) in Cp′ 2 M(η 2 -Me 3 SiC 2 SiMe 2 H) and Cp′ 2 M(η 2 -HMe 2 SiC 2 SiMe 2 H), respectively, were considered. Additionally, unusual all-C-metallacycles like metallacyclopentynes Cp′ 2 M[η 2 -C(R 2 )-C 2 -C(R 2 )-] (formed by coordination of tri-functional trienes: C&z.dbd;C, C&z.dbd;C, C&z.dbd;C), five-membered metallacycloallenes Cp′ 2 M[η 2 -C(R)&z.dbd;C&z.dbd;C(R)-C(R 2 )-] (formed by coordination of bi-functional enynes: C&z.dbd;C, C&z.tbd;C), metallacyclocumulenes Cp′ 2 M[η 2 -C(R)&z.dbd;C&z.dbd;C&z.dbd;C(R)-] (formed by coordination of bi-functional diynes: C&z.tbd;C, C&z.tbd;C), and seven-membered metallacyclocumulenes Cp′ 2 M[η 2 -C(R)&z.dbd;C(C 2 R)-C(R)&z.dbd;C&z.dbd;C&z.dbd;C(R)-] (formed by coordination of two bi-functional diynes: C&z.tbd;C, C&z.tbd;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. Priority of equilibrium: reactivity of unusual group 4 metallocene complexes is best explained by the equilibrium and only additionally by the mesomerism/valence tautomerism. The equilibrium predominates the empirically found experimental results.