Double-Dimethylsilyl-Bridged Dicyclopentadienyl Group 4 Metal Complexes. X-Ray Molecular Structures of M[(Me2Si)2(Η5-C5H 3)2]Cl2 (M = Ti, Zr) and (Ticl3)2{Μ-[(Me2Si)2(Η 5-C5H3)2]}