Monocyclopentadienyl-type titanium complexes with the [.eta.5-.eta.5-(C5H4)2SiMe2]2- ligand. X-ray crystal structure of [(TiCl)2(.mu.2-O){.mu.2-.eta.5-.eta.5-(C5H4)2SiMe2}]2(.mu.2-O)2. The first example of a nonplanar titanium oxide ["Ti4O4"] core