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The reaction of allylcyclopentadienyllithium with 0.5 molar equivalents of MC14(thf)z (M = Zr, Hf) furnished the ($-C5H4CH2CH= CH&MC12 complexes 3a (Zr) and 3b (Hf), which cleanly added 9-borabicyclo[3.3.1]nonane (9-BBN) to form J2MCl2 (lOa, b). Treatment of 3 with butadiene-magnesium led to coupling of one Cp-bonded allyl unit with butadiene to give 5; the remaining CpCHzCH = CH2 group underwent clean H[B(C,HI4)] addition when exposed to the 9-BBN hydroboration reagent. (Alkenyl-Cp)2ZrC12 complexes (9 a -d) were prepared starting from fulvenes by deprotonation with the help of LDA and reaction with ZrCl,(thf),. The complexes [q5-CsH4CH= C(CH3)R],ZrC12 9 b (R = CH3) and 9c (R = Ph) added 9-BBN to give [q2-C5H4CH(9-BBN)-CH(CH,)R],ZrCl, 12 (R = CH3, rac-/meso-N 1: 1) and 13 (R = Ph, fhreo-leryfhro-% 1: 1). The mixtures of diastereoisomers were in each case separated by fractional crystallization. The stereochemical assignments of complexes rac-12 and meso-12 were achieved by means of the stereochemistry of polypropylene formed at the lZ/methylalumoxane-derived homogeneous Ziegler-type olefin polymerization catalysts. At -50°C the catalyst from the achiral meso-12 precursor produced isotactic polypropylene only by means of "chain end control" (rs z 0.7), whereas the use of rac-12 led to a pronounced participation ( = 30%) of effective "enantiomorphic site control" (a = 0.95) in the CC-coupling reaction.Metallocene complexes bearing substituents at the C p rings have been of great importance for organometallic chemistry and catalysis2). Such complexes of the late transition metals can often be derived from the unsubstituted parent systems by means of conventional Cp-ring functionalization. The many known electrophilic substitution reactions at the ferrocene cyclopentadienyls are typical examples'). The high reactivity of the bent metallocenes of the oxophilic early transition elements is often prohibitive for a similar Cp-substitution chemistry at the organometallic stage. Therefore, Cp-substituted or -functionalized metallocene halides of e.g. the group 4d-elements have usually been prepared differently, i.e., mostly by reaction of suitably substituted R-Cp anion equivalents with the metal halides ' 1.The hydroboration reaction has been used for functionalizing Cp-alkenyl-substituted ferrocenes ' ). We thought that the introduction of the electrophilic BR2 functionality might be less critical and easier to be compatible with the general reactivity patterns of zirconocene and hafnocene halides than e.g. the exposure of such complexes to the rather harsh conditions typically employed in Friedel-Crafts-type chemistry. Indeed, we found that hydroboration of many (alkenylCp),MC12 complexes (M = Zr, Hf) proceeded cleanly. We here describe some typical examples of forming alkyl borane Cp-substituted zirconocene dichlorides from their respective (alkenyl-Cp),ZrCl, precursors by means of the hydroboration reaction. Two of the selected examples show interesting * ) New address: Organisch-Chemischcs Tnstitut der Universitiit M...

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