Reactivities of eight phosphate diesters each coordinated to a dinuclear Co(III) complex were investigated ([Co-2(tacn)(2)(OH)(2){O2P(OR)(2)}](3+); tacn = 1,4,7-triazacyclononane). Four of the complexes were coordinated by substituted phenyl methyl phosphates (substituent m-F, F-NO2 (la); p-NO2 (Ib); m-NO2 (Ic); unsubstituted (Id)) and two by substituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsubstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phosphate (4)) were also investigated. Hydrolysis of the phosphate diesters in la to Id takes place by intramolecular oxide attack on the bridging phosphate while hydrolysis of 3 principally takes place by intermolecular hydroxide attack : on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by intramolecular transesterification. Dimethyl phosphate dissociates from 4 without any observable cleavage of the diester. These results can be understood in terms of the More O＇Ferrall-Jencks energy diagram, and they indicate that there will be considerable cooperativity between the oxide activation and leaving group activation.