Hydride abstraction from C5Me5(CO)(2)Re(eta(2)-PhCequivalent toCCH(2)Ph) (1) gave a 3:1 mixture of eta(3)-propargyl complex [C5Me5(CO)(2)Re(eta(3)-PhCH-Cequivalent toCPh)][BF4] (5) and eta(2)-1 -metalla(methylene)cyclopropene complex [C5Me5(CO)(2)Re(eta(2)-PhC-C=CHPh)][BF4] (6). Observation of the eta(2)-isomer requires 1,3-diaryl substitution and is favored by electron-donating substituents on the C-3-aryl ring. Interconversion of eta(3)-propargyl and eta(2)-1-metalla(methylene)cyclopropene complexes is very rapid and results in coalescence of Cp-* H-1 NMR resonances at about -50 degreesC. Protonation of the alkynyl carbene complex C5Me5(CO)(2)Re=C(Ph)Cequivalent toCPh (22) gave a third isomer, the eta(3)-benzyl complex {C5Me5(CO)(2)Re[eta(3)(alpha,1,2)-endo,syn-C6H5CH(Cequivalent toCC(6)H(5))]}-[BF4] (23) along with small amounts of the isomeric complexes 5 and 6. While 5 and 6 are in rapid equilibrium, there is no equilibration of the eta(3)-benzyl isomer 23 with 5 and 6.