The controversy over the thermal stability of bis(cyclopentadienyl)zirconium(III) halides. Synthesis of Cp'2ZrX (Cp' = Cp, C5H4Me; X = Cl, Br, I) via photolysis of Cp'2Zr(CH2CHMe2)X. The x-ray structures of [(C5H4Me)2Zr(.mu.-I)]2 and [.eta.5:.eta.5-C10H8] [CpZr(.mu.-I)]2