Synthetic and reactivity studies of mono- and dicyclopentadienyl titanium, zirconium and hafnium complexes with the chlorodimethylsilyl-cyclopentadienyl ligand. X-ray molecular structure of Hf{(η5-C5H4)SiMe2OSiMe2(η5-C5H4)}Cl2 and Zr(η5-1,3-tBu2C5H3)(η5-C5H4SiMe2-η-NtBu)Cl