A novel route to metallacyclopropene (η 2 -vinyl) complexes from alkynes: synthesis and X-ray crystal structure of trans-[ReCl{C(CH 2 Ph)CH 2 }(Ph 2 PCH 2 CH 2 PPh 2 ) 2 ][BF 4 ]

Treatment of the allene complex trans-[[graphic omitted]CHPh)(dppe)2](dppe = Ph2PCH2CH2PPh2) with acids, HA (A = BF4 or PF6), in CH2Cl2 or tetrahydrofuran gives the metallacyclopropene (η2-vinyl) complex trans-[[graphic omitted]H2}(dppe)2][A] whose structure (A = BF4) has been authenticated by X-ray crystallography; the site of protonation is in agreement with extended Hückel calculations which show, in the HOMO, a high electronic density on the allene carbon atom which is not directly bonded to the metal.