Allenylidenes: Their multifaceted chemistry at rhodium

Rhodium allenylidenes, which represent a class of 'flat' metallabutatrienes, are useful tools for the generation of metal-bound species that are hardly accessible (sometimes not accessible at all) by other synthetic routes. In contrast to various octahedral allenylidene metal complexes, e.g. of Cr-O, Mn-I or Ru-II, the square-planar rhodium compounds trans-[RhX(=C=C=CRR')(PPr3i )(2)] are attacked by both nucleophiles and electrophiles. Moreover, non-polar substrates such as H-2 or Cl-2 react cleanly with the title complexes to give products in which the allenylidene unit is preserved as part of a newly formed ligand. Attempts to prepare a square planar cationic metallaheptahexaene, trans-[Rh(=C=C=CRR')(2)(PPr3i)(2)]+, unexpectedly led to the formation of two isomeric hexapentaenerhodium(I) complexes of which the isomer having the polyene coordinated in an unsymmetrical fashion is the thermodynamically more stable. C-C coupling reactions also occur on treatment of trans-[RhCl(=C=C=CPh2)(PPr3i)(2) with vinyl Grignard reagents or phenylacetylene leading to highly unsaturated RhC5 and RhC5P frameworks from which vinylallenes and phosphacumuleneylides are obtained.