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Doubly bridged zirconocene derivatives with C2 symmetrically disposed substituents, ((CH3)2-Si)2 (3,4-dimethyl-l,2-cyclopentadienediyl) 2ZrCl2 (4A) and ((CH3)2Si)2(4,5,6,7-tetrahydro-1,2-indenediyl)2ZrCl2 (4B) were synthesized by reactions of the corresponding ligand dilithium salts with ZrCU in toluene; the racemic isomers were obtained by fractionated crystallization of the diastereomeric product mixtures. X-ray diffraction studies give space group with a = 9.596 (5) A b = 13.160(7) A, c = 16.645 (9) A, a = 96.60 (4)°, ß = 90.33 (4)°, y = 98.66 (4)°, and Z = 4 (two crystallographically independent molecules) for 4A and space group 2 / with a = 9.994 (4) A, b = 15.040(6) A, c = 15.078(8) Á, ß = 95.09(4)°, and Z = 4 for 4B. Both complexes show the expected C2-axial symmetry. In combination with methylalumoxane, 4A and 4B are slowly converted to catalysts for the polymerization of propene, which yield polymers with low and medium isotacticities, respectively. Control experiments indicate that conversion of these complexes to active catalysts involves degradation of their strained ligand frameworks; intact 4A and 4B do not appear to give rise to catalytic activity.

Titanocene and zirconocene complexes with a phenyl or I-naphthyl substituent in one ,B-position of each Cs-ring ligand and with an interannular tetramethylethano-or dimethylsilano-bridge have been prepared. Racemic complex isomers, which are formed together with varying amounts of their meso isomers, were separated from the latter and characterised by lH NMR spectroscopy and X-ray crystallographic studies. * For Part XXII see ref. 15.

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