New mononuclear amidine complexes, fac-[Re(CO)(3)(Me(2)bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me(2)bipy (1), 5,5'-Me(2)bipy (2), and 6,6'-Me(2)bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re-I(CO)(3)(Me(2)bipy)(CH3CN)]BF4 complex with the C-2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B-12 model, (py)Co(DH)(2)Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)(3)(Me(2)bipy)(mu-(HNC(CH3)(pyppz)))Co(DH)(2)Cl]BF4 [(4,4'-Me(2)bipy (4), 5,5'-Me(2)bipy (5), and 6,6'-Me(2)bipy (6)]. H-1 NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H-2/6 signal for 4-6. The wrong-way H-2/6 shift of (4-Xpy)Co(DH)(2)Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the H-1 NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re-I(CO)(3)] core to a vitamin B-12 model could be extended to natural B-12 derivatives. Because B-12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of Tc-99m and Re-186/188 radiopharmaceuticals for targeted tumor imaging and therapy