The UP>2(3e)-vinyl complex [Mo{=C(Ph)CHPh}{P(OMe)3}2Cp] is oxidized by [FeCp2]+, [CPh3]+, orB(C6F5)3 to form the 17-electron cation [Mo{=C(Ph)CHPh}{P(OMe)3}2Cp]+, which on warming loses H toform the cationic 2(4e)-alkyne complex [Mo(2-PhCCPh){P(OMe)3}2Cp]+. In the case of the borane there isevidence for a competing reaction between the 2-vinylcomplex and the acid (H2O)B(C6F5)3, resulting in theformation of a labile trans-stilbene complex.