The reaction of ferrocenyl acetylene with [RuCp(PR3)(CH3CN)2]PF6 (R=Me, Ph, Cy). Formation of the first allenyl carbene complexes

The reaction of [RuCp(PR3)(CH3CN)2]PF6, where R=Ph and Cy, with ferrocenyl acetylene (HCCFc) affords the first η2-allenyl carbene complexes [CpRu(C(Fc)-η2-CHCCH(Fc))(PR3)]PF6. The new type of CC bond formation is affected by the strong electronic coupling of the ferrocenyl moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the RuC double bond. The vinylidene intermediate appears to occur only in the presence of bulky substituents of the phosphine co-ligand. For small R=Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual η3-allyl carbene complex [CpRu(C(Fc)-η3-CHC(Fc)CHPMe3)]PF6. The X-ray structure of an η2-allenyl carbene complex is presented.