Treatment of two precursors,fac-[Re(CO)(3)(L)-(CH3CN)]BF4 [L = 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me(2)bipy) (1) and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me(2)bipy) (2)], with five C-2-symmetrical saturated heterocyclic amines yielded 10 new amidine complexes, fac-[Re(CO)(3)(L)(HNC(CH3)N(CH2CH2)(2)Y)]BF4 [Y = CH2, (CH2)(2), (CH2)(3), NH, or O]. All 10 complexes possess the novel feature of having only one isomer (amidine E configuration), as established by crystallographic and H-1 NMR spectroscopic methods. We are confident that NMR signals of the other possible isomer (amidine Z configuration) would have been detected, if it were present. Isomers are readily detected :in closely related amidine complexes because the double-bond character of the amidine C-N-3 bond (N-3 is bound to Re) leads to slow E to Z isomer interchange. The new fac-[Re(CO)(3)(L)(HNC(CH3)N(CH2CH2)(2)Y)]BF4 complexes have C-N-3 bonds with essentially identical double-bond character. However, the reason that the Z isomer is so unstable as to be undetectable in the new complexes is undoubtedly because of unfavorable clashes between the equatorial ligands and the bulky N(CH2CH2)(2)Y ring moiety of the axial amidine ligand. The amidine formation reactions in acetonitrile (25 degrees C) proceeded more easily with 2 than with 1, indicating that the distortion in 6,6'-Me(2)bipy resulting from the proximity of the methyl substituents to the inner coordination sphere enhanced the reactivity of the coordinated CH3CN. Reaction times for 1 and 2 exhibited a similar dependence on the basicity and ring size of the heterocyclic amine reactants. Moreover, when the product of the reaction of 1 with piperidine, fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH3)N(CH2CH2)(2)CH2)]BF4, was challenged in acetonitrile-d(3) or CDCl3 with a 5-fold excess of the strong 4-dimethylaminopyridine ligand, there was no evidence for replacement of the amidine ligand after two months, thus establishing that the piperidinylamidine ligand is a robust ligand. This chemistry offers promise as a suitable means for preparing isomerically pure conjugated fac[Tc-99m(CO)(3)L](n+/-) imaging agents, including conjugates with known bioactive heterocyclic amines.