Reductive coupling of fulvenes with calcium for C-2-symmetric ansa-metallocenes: Syntheses and molecular structures of trans-Ph2C2H2(eta(5)-C5H4)(2)Ca(THF)(2) and trans-Ph2C2H2(eta(5)-C5H4)(2)ZrCl2

Reductive coupling of dimethylfulvene with alkaline earth metals yields inseparable mixtures of the tetramethylethylene-bridged complex Me4C2(C5H4)(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-Ph2C2H2(eta(5)-C5H4)(2)Ca(THF)(2)(trans-2). The molecular structure of this ansa-calcocene shows the cyclopentadienyl rings to be bound in an eta(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,, FeCl2 and ZrCl4). The molecular structure of the corresponding ansa-zirconocene dichloride complex is described.