Entirely inorganic polyoxometalate cluster-anion ligands are herein used to entrap and impart heat resistance to metastable metal-oxide based NCs. This is demonstrated by trapping 6-line ferrihydrite NCs that form as intermediates in the spontaneous conversion of β-FeOOH to hematite (α-Fe2O3). The results suggest that polyoxometalate ligation may prove to be a general method for stabilizing metastable phases as the cores of entirely inorganic macro-anion like complexes.