In an aqueous solution (pH 3) containing active charcoal as a solid phase, reaction of [Co-III(tpa)(CO3)](+)(1) (tpa = tris(2-pyridylmethyl)amine) and 2 equiv. of an active phosphate ester, disodium 4-nitrophenyl phosphate (NPP), at 60 degrees C gave [Co-III(tpa)(PO4)] (2) (8% yield) and [{Co-III(tpa)}(2)(mu-P2O7)](2+) (3) (51% yield) as isolated products. The X-ray crystal structure of main product 3 revealed that an NPP-derived diphosphate bridged to the dinuclear Co(III) centers. The diphosphate formation was not detected without active charcoal or under neutral pH condition.