The magnetic circular dichroism (MCD) study of [Co-2 (mu-OH)(mu-Ph(4)DBA)(TMEDA)(2)(OTf)], in which Ph(4)DBA is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis (diphenylacetate) and TMEDA is N, N, N',N' -tetramethylethylenediamine, is presented. This complex serves as an excellent spectroscopic model for a number of dicobalt(II) enzymes and proteins that have both the mu-hydroxo, mu-carboxylato bridging and asymmetric 6- and 5-coordination. The low-temperature MCD spectrum of the model complex shows bands at 490, 504, and 934 nm arising from d-d transitions on the 6-coordinate Co-II and bands at 471, 522, 572, 594, and 638 nm arising from d-d transitions on the 5-coordinate Co-II. The most intense MCD bands are at 504 and 572 nm for 6- and 5-coordinate Co-II, respectively, and these two bands are found in the MCD spectra of dicobalt(II)-substituted methionine aminopepticlase from Escherichia coli (CoCoMetAP), g lyce rophosph odieste rase from Enterobacter aerogenes (CoCoGpdQ), aminopepticlase from Aeromonas proteolytica (CoCoAAP), and myohernerythrin from Themiste zostericola (CoCoMyoHry). These dicobalt(II)-substituted proteins are known to have one 5- and one 6-coordinate Co-II bridged by one or two carboxylates and either a water or a hydroxide. The uncertainty of the bridging waters state of protonation is problematic, as this is a likely candidate for the attacking nucleophile in the climetallohydrolases. Analysis of the variable-temperature variable-field (VTVH) MCD data determined that the Co-II ions in the model complex are ferromagnetically coupled with a Jof 3.0 cm-1. A comparison of all dicobalt(II) complexes and dicobalt(II)-substituted protein active sites with the mu-hydroxo/aqua, mu-carboxylato bridging motif reveals that J is either zero or negative (antiferromagnetic) in the mu-aqua systems and positive (ferromagnetic) in the mu-hydroxo systems. It was also determined that the Co-II ions in CoCoAAP and CoCoMyoHry are ferromagnetically coupled, each with a Jof 3.4 cm(-1), which suggests that these ions have a mu-hydroxo bridging ligand.