Suspensions of CoMo6S24@NU1K crystallites, when irradiated at 390 nm, yield MOF linker excited-states that can readily reduce PTMs and thereby sensitize the reductive catalytic formation of H2 from water. We find, however, that hydrogen evolution is preceded by a fourhour induction period during which the PTMs shed sulfurs and expose metal ions. The loss, which is limited to sulfur, translates to between 3.5 and 6 sulfur anions per PTM after 30 hours. Thus, at least for photochemical conversion of residual water to dihydrogen, the as-synthesized PTM is a pre-catalyst for the catalytically competent species, CoMo6S24-x y-@NU1K, where x is large enough to expose multiple cluster metal ions to reactive substrates. An attractive, albeit speculative, interpretation of the onset of catalytic activity with sulfur loss, is that the loss permits formation of Mo-hydride, which, in turn, can combine with sulfur-bound protons to form H2. Notably, this mechanism, which is reminiscent of that used by hydrogenase, has been shown to be important for MOF-node-grafted, singlemetal(molybdenum)-atom catalysts that feature, in the resting state, a pair of sulfhydryl ligands and a distorted tetrahedral coordination environment that leaves two sites open for substrate binding .31 Further work will be required, however, to establish mechanisms. Given the great variety of extant POMs and the large number of water-stable, extant Zr-MOFs, we suggest that POM isolation and immobilization, based on pore-size matching, will provide synthetic access to a large and varied family of capping-ligand-free PTMs of well-defined metalnuclearity and abundant catalytic activity for bond-forming reduction reactions. If the observed reduction-initiated loss of multiple sulfur anions from CoMo6S24@NU1K is more general phenomenon, then PTMs should be capable of presenting multiple open coordination sites (candidate reactant binding sites) on immediately proximal, redoxactive metal-ions. Further, a PTM as simple as CoMo6S24-x y- @NU1K should, in principle, be capable of mediating the delivery of a dozen or more electrons, based on Mo(IV/II) and Co(II/I/0) redox couples. The simultaneous presence of coordinated sulfide (weak base) and sulfhydryl (weak acid) species as PTM ligands could, in principle, usefully assist with proton management. While the MOF-immobilized PTM arrays described here are satisfyingly active for electrocatalytic and framework-sensitized photocatalytic hydrogen evolution, the real promise of these new clusters may be as metal-nuclearity-defined catalysts, electrocatalysts, and photo-catalysts for more complex reduction reactions.