Reductive Coupling of Fulvenes with Calcium for C2-Symmetric ansa-Metallocenes: Syntheses and Molecular Structures of trans-Ph2C2H2(η5-C5H4)2Ca(THF)2 and trans-Ph2C2H2(η5-C5H4)2ZrCl2

Reductive coupling of dimethylfulvene with alkaline earth metals yields inseparable mixtures of the tetramethylethylene-bridged complex Me 4 C 2 (C 5 H 4 ) 2 M and the unbridged metallocene ( i- PrCp) 2 M (M = Mg, Ca, Sr, Ba). In contrast, the reductive coupling of phenylfulvene with activated calcium in THF gives nearly quantitative yields of a mixture of cis - and trans -diphenylethanediyl-bridged ansa -calcocenes. The more soluble cis isomer can be removed by recrystallization, yielding the C 2 -symmetric trans -Ph 2 C 2 H 2 (η 5 -C 5 H 4 ) 2 Ca(THF) 2 ( trans - 2 ). The molecular structure of this ansa -calcocene shows the cyclopentadienyl rings to be bound in an η 5 fashion to the calcium center. This versatile reagent gives high yields of C 2 -symmetric transition metal ansa -metallocene complexes via the facile reaction of trans - 2 with metal chlorides (e.g., FeCl 2 and ZrCl 4 ). The molecular structure of the corresponding ansa -zirconocene dichloride complex is described.