Reactivities of five phosphonate esters each coordinated to a dinuclear Co(III) complex were investigated ([Co-2(tacn)(2)(OH)(2){O2P(Me)OAr}](3+); tacn = 1,4,7-triazacyclononane; substituent = m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); p-Cl (1d); unsubstituted (1e)). Hydrolysis of the phosphonate esters in 1a to 1e is specific base catalyzed and takes place by intramolecular oxide attack on the bridging phosphonate. These data define a Bronsted beta(Ig) of -1.12, considerably more negative than that of the hydrolysis of the uncomplexed phosphonates (-0.69). For 1b, the kinetic isotope effects in the leaving group are, (18)k(Ig) = 1.0228 and (15)k = 1.0014, at the nonbridging phosphoryl oxygens 18K(nonbridge) = 0.9954, and at the nucleophilic oxygen (18)k(nuc) = 1.0105. The KIEs and the beta(Ig) data point to a transition state for the alkaline hydrolysis of 1b that is similar to that of a phosphate monoester complex with the same leaving group, rather than the isoelectronic diester complex. The data from these model systems parallel the observation that in protein phosphatase-1, which has an active site that resembles the structures of these complexes, the catalyzed hydrolysis of aryl methylphosphonates and aryl phosphates are much more similar to one another than the uncomplexed hydrolysis reactions of the two substrates.