Double-Dimethylsilyl-Bridged Dicyclopentadienyl Group 4 Metal Complexes. X-ray Molecular Structures of M[(Me2Si)2(.eta.5-C5H3)2]Cl2 (M = Ti, Zr) and (TiCl3)2{.mu.-[(Me2Si)2(.eta.5-C5H3)2]}