Addition of various RNH2 to fac-[Re(CO)(3)(5,5'-Me(2)bipy)(CH3CN)]BF4 (1) converts the acetonitrile ligand to the amidine ligand (a superbase) in fac[Re(CO)3(5,5'-Me(2)bipy)(HNC(CH3)NHR)]BF4 products. Each complex has four conceivable isomers (E, E', Z, and Z') because the amidine CN bonds have double-bond character, and the two remote NHR group substituents are different. The reaction of 1 in acetonitrile is complete in 6 to 96 h (25 degrees C) and forms fac[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)NHR)]BF4 E' and Z isomers. Only the E' isomer formed crystals (R = methyl, isopropyl, isobutyl, tert-butyl, and benzyl). Upon dissolution of such crystals in acetonitrile-d(3), NMR spectra with highly dominant E' signals gradually changed (similar to 15 mm at room temperature) to spectra with signals for an equilibrium mixture of E' and Z isomers. Such slow E'-to-Zisomer interchange is also indicated by 2D ROESY NMR data used primarily to assign solution structure. Equilibrium ratios (E':Z) of similar to 65:35 for R = methyl, isopropyl, and isobutyl and 83:17 for R = tert-butyl demonstrate that increasing the remote NHR group steric bulk above a threshold size favors the E isomer. Consistent with this trend, fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)NH2)BF4, with a remote NH2 (low bulk) group, favors the Z isomer. In contrast, although the remote NH(benzyl) group in fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)NH(CH2C6H5)BF4 has only moderate bulk, the E isomer has high abundance as a result of favorable 5,5'-Me(2)bipy/benzyl stacking, evidence for which is present in both solid and solution states. The fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)NHR)]BF4 E isomer can be detected in solvents of low polarity. However, the Z isomer was not observed, undoubtedly because unfavorable remote-group clashes with the equatorial ligands destabilize this isomer. Challenge studies with a 5-fold excess of 4-dimethylaminopyridine in acetonitrile-d(3) establish that fac[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)NHCH(CH3)(2)]BF4 is robust because the isopropylamidine ligand was not displaced, consistent with the superbase character of amidine ligands.