Abstract
Monocationic bis‐allyl complexes [Ln(η^3^‐C~3~H~5~)~2~(thf)~3~]^+^[B(C~6~X~5~)~4~]^−^ (Ln=Y, La, Nd; X=H, F) and dicationic mono‐allyl complexes of yttrium and the early lanthanides [Ln(η^3^‐C~3~H~5~)(thf)~6~]^2+^[BPh~4~]~2~^−^ (Ln=La, Nd) were prepared by protonolysis of the tris‐allyl complexes [Ln(η^3^‐C~3~H~5~)~3~(diox)] (Ln=Y, La, Ce, Pr, Nd, Sm; diox=1,4‐dioxane) isolated as a 1,4‐dioxane‐bridged dimer (Ln=Ce) or THF adducts [Ln(η^3^‐C~3~H~5~)~3~(thf)~2~] (Ln=Ce, Pr). Allyl abstraction from the neutral tris‐allyl complex by a Lewis acid, ER~3~ (Al(CH~2~SiMe~3~)~3~, BPh~3~) gave the ion pair [Ln(η^3^‐C~3~H~5~)~2~(thf)~3~]^+^[ER~3~(η^1^‐CH~2~CHCH~2~)]^−^ (Ln=Y, La; ER~3~=Al(CH~2~SiMe~3~)~3~, BPh~3~). Benzophenone inserts into the LaC~allyl~ bond of [La(η^3^‐C~3~H~5~)~2~(thf)~3~]^+^[BPh~4~]^−^ to form the alkoxy complex [La{OCPh~2~(CH~2~CHCH~2~)}~2~(thf)~3~]^+^[BPh~4~]^−^. The monocationic half‐sandwich complexes [Ln(η^5^‐C~5~Me~4~SiMe~3~)(η^3^‐C~3~H~5~)(thf)~2~]^+^[B(C~6~X~5~)~4~]^−^ (Ln=Y, La; X=H, F) were synthesized from the neutral precursors [Ln(η^5^‐C~5~Me~4~SiMe~3~)(η^3^‐C~3~H~5~)~2~(thf)] by protonolysis. For 1,3‐butadiene polymerization catalysis, the yttrium‐based systems were more active than the corresponding lanthanum or neodymium homologues, giving polybutadiene with approximately 90 % 1,4‐cis stereoselectivity.