Bis(Cyclooctatetraene) Derivatives of Zirconium(IV) and Hafnium(IV): Syntheses and Lewis Base Adducts. Crystal Structures of [Zr(eta8-C8H8)(eta4-C8H8)], [Hf{eta8-C8H6(SiMe3)2}{eta4-C8H6(SiMe3)2}], [Zr(eta8-C8H8) (eta4-C8H8) (NH3)] and [Zr(eta8-C8H8) (eta4-C8H8)(CNBut)]

The syntheses of [M(η8-C8H8)(η4-C 8H8)] (M = Zr, 2 or Hf, 3) and [M{η8-C8H6(SiMe3) 2}{η4-C8H6(SiMe 3)2}] (M = Zr, 4 or Hf, 5) are reported. The η8 and η4 bonding modes for C8H8 were established in the solid state by X-ray analyses, while in solution a single C8H8 environment was observed by 1H NMR spectroscopy even at low temperature in accord with fluxional behaviour for these compounds. Complexes 2-5 behave as Lewis acids toward tetrahydrofuran, NH3 and ButNC and the adducts [Zr(η8-C8H8)(η4-C 8H8)(NH3)] 6 and [M(η8-C8H8)(η4-C 8H8)(CNBut)] (M = Zr, 7 or Hf, 8) have been isolated and characterized by X-ray analysis. Crystallographic details: 2, monoclinic, space group P21/c, a = 13.754(1), b = 7.798(1), c = 12.753(1) Å, β = 114.02(1)°, Z = 4 and R = 0.037 for 1267 independent observed reflections; 5, orthorhombic, space group Pca21, a = 14.418(1), b = 11.285(1), c = 18.898(1) Å, Z = 4 and R = 0.036 for 2239 independent observed reflections; 6, orthorhombic, space group P212121, a = 9.815(1), b = 14.129(2), c = 9.561(1) Å, Z = 4 and R = 0.022 for 1556 independent observed reflections; 7, triclinic, space group P1, a = 11.172(1), b = 13.469(1), c = 14.303(1) Å, α = 62.80(1), β = 78.01(1), γ = 77.26(1)°, Z = 4 and R = 0.028 for 2994 independent observed reflections.