Organolanthanoids XX. The synthesis of YbCp 2 Cl(THF) 0 . 5 (Cp = cyclopentadienyl; THF = tetrahydrofuran), YbCp 2 X(THF) (X = Br or I), and (YbCp 2 X) 2 (X = Cl, Br, or I) by oxidation of YbCp 2 with metal and organic halides, and the determination of the X-ray crystal structure of YbCp 2 I(THF)