New syntheses of ansa-metallocenes or unbridged substituted metallocenes by the respective reductive dimerization of fulvenes with Group 4 metal divalent halides or with Group 4 metal dichloride dihydrides

Two unprecedented, high-yielding syntheses of Group 4 metallocenes have been discovered: (1) reaction of MCl2 with 6-substituted fulvenes to produce substituted ethylene-bis(cyclopentadienyl)metal dichlorides; and (2) reaction of Bun2MCl2 with 6-substituted fulvenes by transfer-hydrometallation to yield bis(substituted cyclopentadienyl)metal dichlorides. Reaction mechanistic models based on the reductive dimerization of ketones or imines are invoked in explaining the stereochemistry of ansa-metallocene formation.