Hydrolysis of four substituted phenyl phosphate monoesters, each coordinated to a dinuclear Cocm, complex, was studied ([Co-2(tacn)(2)(OH)(2){O3P(OAr)}](2+); tacn = 1,4,7-triazacyclononane; substituent m-F, p-NO2 (5a); p-NO2 (5b); m-NO2 (5c); unsubstituted (5d)). Crystallographic data reveal that 5b is an excellent structural model of the active sites of several phosphatases: protein phosphatase-l, kidney bean purple acid phosphatase, and calcineurin-alpha. All of these structures consist of two octahedral metal complexes connected by two oxygen bridges, forming a four-membered-ring diamond core. The pH-rate profile and the O-18 labeling experiment for the hydrolysis of 5b indicates that the oxide bridging the two metal centers in the diamond core is acting as an intramolecular nucleophile for cleaving the coordinated phosphate monoester. The phosphate monoesters in this model system are hydrolyzed more rapidly than those in previously reported model systems. Hence, the dinuclear cobalt complexes 5 appear to be excellent structural and functional models of the above-mentioned phosphatases. The rate of hydrolysis of 5 is highly sensitive to the basicity of the leaving group (beta(1g) 1.10). Detailed analysis of the leaving group dependence for the hydrolysis of 5 indicates only a partial negative charge on the leaving group oxygen at the transition state, further supporting the nucleophilic mechanism.