Among the metals of the first transition series, cobalt (Co) is next to iron (Fe) and exhibits three oxidation states (+1, +2, and +3) much like its congener iron (+2, +3, and +4). However, unlike iron, it is present in a relatively small number of enzymes and cofactors (Kobayashi and Shimizu 1999). In addition, it does not participate in oxygen activation, a process that iron frequently takes part in. In the +3 oxidation state, cobalt enjoys high crystal field stabilization energy (CFSE) in several coordination geometries. Significantly, high stability arises when Co3+ exists in spin-paired (low-spin) electronic configuration within an octahedral coordination environment of strong ligands. Such stability often makes the Co3+center resistant to substitution processes such as exchange of a bound water molecule for the substrate. Such a center however can serve as a...