A SEARCH FOR PI SIGMA-EQUILIBRIA IN CHIRAL RHENIUM IMINE COMPLEXES OFTHE FORMULA ETA(5)-C5H5)RE(NO)(PPH(3))(N(H)=C(CF3)X)](+)TFO(-) - INVESTIGATION OF ELECTRONIC EFFECTS UPON THE BINDING MODE/

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 sigm a-imine complex (5)-C5H5)Re(NO)(PPh(3))(eta(1)-N(H)=C(CF3)(2))](+) TfO (-) (4, 96%). Similar reactions of 2 with trifluoroacetaldehyde and th en TfOH give the sigma-imine complex ta(5)-C5H5)Re(NO)(PPh(3))(eta(1)- N(H)=C(CF3)H)](+) TfO(-) (5, 78%) and sometimes small amounts of the c orresponding 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 de tail. The C-13 NMR spectra show C=N signals (157-142 ppm) diagnostic o f 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 rati onales are discussed. Reactions of 3 or 6 with MeOTf and heteroatom el ectrophiles are also described.