Reaction of the amido complex (eta(5)-C5H5)Re(NO)(PPh(3))(NH2) (2) and hexafluoroacetone gives the methyleneamido complex (eta(5)-C5H5)Re(NO)(PPh(3))(N=C(CF3)(2)) (3, 58%). Addition of TfOH to 3 yields the sigma-imine complex [(eta(5)-C5H5)Re(NO)(PPh(3))(eta(1)-N(H)=C(CF3)(2))](+) TfO(-) (4, 96%). Similar reactions of 2 with trifluoroacetaldehyde and then TfOH give the sigma-imine complex [(eta(5)-C5H5)Re(NO)(PPh(3))(eta(1)-N(H)=C(CF3)H)](+) TfO(-) (5, 78%) and sometimes small amounts of the corresponding pi-trifluoroacetaldehyde complex. Reaction of 5 and t-BuO(-)K(+) gives the methyleneamido complex (eta(5)-C5H5)Re(NO)(PPh(3))(N=C(CF3)H) (6,82 %). The IR and NMR properties of 3-6 are studied in detail. The C-13 NMR spectra show C=N signals (157-142 ppm) diagnostic of sigma-binding modes. No evidence is observed for pi isomers of 4 or 5. Analogous O=C(CF3)X complexes give exclusively pi isomers, and rationales are discussed. Reactions of 3 or 6 with MeOTf and heteroatom electrophiles are also described.