Evidence for Internal Ion Pair Formation upon Insertion of Reactive Alkene in the Zirconium–Carbon Bond of the cp 2 Zr(μ‐C 4 h 6 )b(c 6 f 5 ) 3 Metallocene‐Boron‐Betaine Ziegler Catalyst System

Treatment of the (butadiene)ML 2 complexes 1 [ML 2 = Cp 2 Zr ( a ), Cp 2 Hf ( b ), and (.‐C 5 H 4 CH 3 ) 2 Zr ( c )] with B(C 6 F 5 ) 3 gives the 1:1 addition products (CH 2 CHCHCH 2 –B(C 6 F 5 _ 3 )ML 2 ( 3a – c ). At –40°C the betaine complex 3a inserts one equivalent of methylenecyclopropane to give the regioisomeric insertion products 5a and 6a in a 60:40 ratio. These products exhibit the cyclopropylidene moiety in the α‐ and β‐positions, respectively, relative to zirconium. The corresponding hafnocene complexes 5b and 6b are obtained in a 70:30 ratio starting from 3b . The reaction of 3 ( a – c ) with allene gives a single insertion product ( 7a – c ) in each case where the exo ‐methylene group is in the α‐position to the metal center ([2,1]‐insertion). The complexes 5 – 7 are chiral. They all exhibit a pronounced ·‐interaction of the internal –C 4 H=C 5 H − double bond of the s̀‐ligand chain with the metal center in addition to a metallocene/–C 6 H 2 –[B] ion pair interaction. The relative contributions of the cationic metallocene end of the dipolar complexes 5 – 7 are quite dependent on the steric and electronic properties of the respective metallocene units involved. This is revealed by a comparison to typical 13 C‐NMR parameters of the complexes 5 – 7 with a pair of suitable model complexes, namely the ethylene insertion product 4 into the betaine system 3a and its THF adduct 4 .THF.