π-Allylic complexes of ruthenium

Reaction of [Ru(π-C₄H₇)₂C₈H₁₂] with various phosphorus ligands gives soluble crystalline complexes of the type [Ru(π-C₄H₇)₂L₂][L = P(OMe)₃, P(OEt)₃, P(OCH₂)₃CMe, or P(OCH₂)₃CEt; or L₂= Me₂PCH₂CH₂PMe₂]. The ¹H n.m.r. spectra of these complexes showed that the π-allylic groups were asymmetrically bonded to ruthenium. The synthesis of [Ru(π-C₃H₅)₂(CO)₂] is reported and its ¹H n.m.r. spectrum interpreted in terms of two equivalent π-allyl groups asymmetrically bonded to ruthenium. Variable-temperature n.m.r. studies show that, in addition to a syn–anti exchange at high temperatures, there is a rapid intramolecular rearrangement at room temperature, in which the CH₂ groups of the π-allyl groups effectively exchange environments. This is rationalised on the basis of an intramolecular ‘twist’ mechanism.