A terminal Pd-oxo unit is reported. The unit is encapsulated in a cavity defined by two [A-
-PW
9O
34]
9- units fused together by one [WO(OH
2)]
4+ center and forms from Pd(II) in buffered media in the presence of O
2. Both X-ray diffraction and EXAFS data are consistent with a Pd-oxo bond distance of ca. 1.65 Å.
17O NMR studies confirm that the solid-state structure is maintained in solution.