Di-.mu.-oxo-(.eta.1:.eta.5-1,2,3,4-tetramethyl-5-methylene-1,3-cyclopentadiene)-bis[(.eta.-pentamethylcyclopentadienyl)titanium], (.mu.-0)2[.mu.-[.eta.1:.eta.5-C5(CH2)(CH3)4]][Ti(.eta.5-C5(CH3)5)]]2, a complex containing a bridging tetramethylmethylenecyclopentadienyl ligand