Building on our use of polyoxometalate (POM) cluster-anions as ligands for gold-nanoparticle catalysts and building blocks [1], we show how POMs can serve as covalently attached ligands for reactive metal-oxide nanocrystals, providing soluble and isolable assemblies uniquely positioned between molecular macroanions and traditional colloids. For example, covalently coordinated redox-active POMs, serving as ligands for individual anatase titanium-dioxide nanocrystals [2], control hydrogen formation by rationally tuning rates of visible-light driven electron injection into the TiO2 cores, while soluble POM complexes of alpha-Fe2O3 (see Figure) serve as oxidatively and hydrolytically stable catalysts for visible-light driven water oxidation [3]. We now report the synthesis and characterization of POM complexed 3-nm manganese oxide nanocrystals that serve as building blocks for the assembly of water-soluble 200-nm cubic nanostructures. Notably, the hierarchical assembly of these cubic structures, each comprised of more than 50,000 of the manganese-oxide-core POM complexes, induces a shift of UV-vis absorbance into the visible. This results in enhanced visible-light semiconductor properties, whose utility is demonstrated through photoelectrocatalytic water-oxidation. [1] (a) Zhang, M.; Hao, J.; Neyman, A; Wang, Y.; Weinstock, I. A. Inorg. Chem., 2017, 56, 2400-2408 (Front cover); (b) Wang, Y.; Raula, M.; Wang, Y.; Zeiri, O.; Chakraborty, S.; Gan-Or, G.; Gadot, E.; Weinstock, I. A. Ang. Chem. Int. Ed. 2017,56, 7083-7087; (c) Wang, Y.; Zeiri, O.; Raula, M.; Le Ouay, B.; Stellacci, F.; Weinstock, I. A. Nature Nanotech. 2017, 12, 170-176. [2] M. Raula, G. Gan-Or, M. Saganovich, O. Zeiri, Y. Wang, M. R. Chierotti, R. Gobetto, I. A. Weinstock, Angew. Chem. Int. Ed. 2015, 54, 12416 –12421 (“Hot paper”). [3] Chakraborty, B.; Gan-Or, G.; Raula, M.; Gadot, E.; Weinstock, I. A., Nature Comm., 2018, 9, 4896. Figure 1