Synthetic Co(III)complexes containing N5 donor sets undergo glutathionylation to generate biomimetic species of glutathionylcobalamin (GSCbl), an important form of cobalamin (Cbl)found in nature. For this study, a new Co(III)complex was synthesized derived from the polypyridyl pentadentate N5 ligand N4PyCO 2Me (1). The compound [Co(N4PyCO 2Me)Cl] Cl 2 (3)was characterized by X-ray crystallography, UV-vis, IR, 1H NMR, and 13C NMR spectroscopies and mass spectrometry (HRMS). Reaction of 3 with glutathione (GSH)in H2O generates the biomimetic species [Co(N4PyCO 2Me)(SG)] 2+ (5), which was generated in situ and characterized by UV-vis and 1H NMR spectroscopies and HRMS. 1H NMR and UV-vis spectroscopic data are consistent with ligation of the cysteine thiolate of GSH to the Co(III)center of 5, as occurs in GSCbl. Kinetic analysis indicated that the substitution of chloride by GS-occurs by a second-order process [k1 = (10.1 ± 0.7)× 10 -2 M -1 s -1]. The observed equilibrium constant for formation of 5 (K obs = 870 ± 50 M -1)is about 3 orders of magnitude smaller than for GSCbl. Reaction of the Co(III)complex [Co(Bn-CDPy3)Cl]Cl 2 (4)with GSH generates glutathionylated species [Co(Bn-CDPy3)(GS)] 2+ (6), analogous to 5. Glutathionylation of 4 occurs at a similar rate [k2 = (8.4 ± 0.5)× 10-2 M -1 s -1], and the observed equilibrium constant (Kobs = 740 ± 47 M -1)is slightly smaller than for 5. Glutathionylation showed a significant pH dependence, where rates increased with pH. Taken together, these results suggest that glutathionylation is a general reaction for Co(III)complexes related to Cbl.