Why propene is not polymerized by (Cp*2YH)2: reactions of yttrium alkyl complexes with alkenes produce allyl and vinyl yttrium complexes

Yttrium alkyl complexes Cp*2YR react with CH bonds of alkenes to form either yttrium allyl complexes or yttrium vinyl complexes. Less substituted alkenes react faster, consistent with prior alkene coordination. The selectivity of the reaction of Cp*2YR with CH bonds is allylic CH3≫vinyl CH≫allylic CH2. Propene is readily metallated by Cp*2YR giving the η3-allyl complex Cp*2Y(η3-CH2CHCH2) which does not react further with propene. This explains why Cp*2YR (R=alkyl, H) complexes make poor propene polymerization catalysts.