The dicobalt form of the metallohydrolase methionine aminopeptidase from Escherichia coli (CoCoEcMetAP) has an active site with one 5-coordinate Co-II and a more weakly bound 6-coordinate Co-II. These metal ions are bridged by two carboxylate amino acid side chains and water or hydroxide, potentially enabling magnetic exchange coupling between the metals. We used variable-temperature, variable-field magnetic circular dichroism to determine whether such coupling occurs. CoCoEcMetAP's MCD spectrum shows distinct d-d transitions at 495 and 567 nm caused by 6- and 5-coordinate Co-II, respectively. The magnetization curves for 5- and 6-coordinate Co-II are very different, indicating that their electronic ground states vary considerably, ruling out any coupling. When the fungal metabolite fumagillin binds to the CoCoEcMetAP, the qualitative MCD spectrum is unchanged; however, VTVH MCD data show that 5- and 6-coordinate Co-II ions have similarly shaped magnetization curves, indicating that the Co-II ions now share the same electronic ground state. Fitting the VTVH MCD data to a model in which dimer wave functions are calculated using a spin Hamiltonian with zero-field splitting showed the Co-II ions to be weakly ferromagnetically coupled, with J = 2.9 cm(-1). Ferromagnetic coupling is unusual for dinuclear Co-II; therefore, to support the CoCoEcMetAP/fumagillin complex results, we also analyzed VTVH MCD data from a matched pair of dinuclear cobalt complexes, 1 and 2. Complex 1 shares the carboxylate and hydroxide-bridged dicobalt(II) structural motif with the active site of CoCoEcMetAP. Complex 2 contains a nearly isostructural Co-II ion, but the Co-III is diamagnetic, so any magnetic coupling is switched off, while the spectral features of the Co-II ion remain. Magnetization data for 1, fitted to the dimer model, showed that the Co-II ions were weakly ferromagnetically coupled, with J = 1.7 cm(-1). Magnetization data for Co-II ions in 2, however, reflect loss of magnetic exchange coupling.