Protonation of the rhenium eta(2)-alkyne complex C5Me5(CO)(2)Re(eta(2)-MeC equivalent to CMe) (4) with HBF4 at room temperature produced the eta(3)-allyl complex C5Me5(CO)(2)Re(eta(3)-exo,anti-MeHC-CH-CH2)+BF4- (5). The protonation of 4 at -78 degrees C occurred at rhenium to give the rhenium alkyne hydride complex C5Me5(CO)(2)ReH(eta(2)-MeC equivalent to CMe)+BF4- (6). At -16 degrees C, net proton migration from rhenium to the alkyne ligand of 6 occurred to produce the 1-metallacyclopropene complex C5Me5(CO)(2)Re(eta(2)-CMeCHMe)+BF4- (7), which then rearranged to form the eta(3)-allyl complex 5. The degenerate rearrangement of 7 by hydride migration between the two metallacyclopropene carbons was demonstrated by deuterium labeling. Protonation of the rhenium eta(2)-alkyne complex C5Me5(CO)(2)Re(eta(2)-PhC equivalent to CPh) (10) with HBF4 at -78 degrees C initially produced the rhenium alkyne hydride complex C5Me5(CO)(2)ReH(eta(2)-PhC equivalent to CPh)+BF4-(11), which was observed spectroscopically. Upon warming to room temperature, 11 was converted to the stable 1-metallacyclopropene complex C5Me5(CO)(2)Re(eta(2)-CPhCHPh)+BF4-(12), which was characterized by X-ray crystallography, Hybrid density functional theory calculations and natural bond orbital analysis were performed on the 1-metallacyclopropene cation [C5H5(CO)(2)Re(eta(2)-MeCCHMe)](+) to compare eta(2)-vinyl vs 1-metallacyclopropene formulations.