The Zr atoms are arranged as an octahedron and are connected through µ 3 -oxygen bridges, with the largest Zr -Zr distance being 0.5 nm. All zirconium atoms are eight-coordinate and the zirconium-oxygen arrangement is almost identical to bulk, cubic zirconia. Therefore, these clusters can be regarded as the lower limit of zirconia nanocrystals. In contrast to larger nanocrystals, oxo clusters are atomically precise (polydispersity = 0), and their ligand shell is less densely packed. (ligand density = 2.4 ligands/nm 2 , compared to 3-4 ligands/nm 2 for nanocrystals). When capped by ligands with low sterical hindrance, the Zr6 cluster dimerizes to form a Zr12 cluster that has two Zr6 subunits, connected by four inter-cluster bridging carboxylate ligands, 2 see Figure . Apart from seven intra-cluster bridging ligands, there are also three chelating ligands per Zr6 subunit. There exist two types of Zr6 monomor structures, either with all bridging ligands or with three chelating and nine bridging ligands, see Figure . Due to their rigid ligands, the typical oxo clusters have a low solubility, rendering them poorly suitable for subsequent processing. Ligands typically used for providing colloidal solubility to nanocrystals (e.g, oleic acid), have not been explored on clusters. In general, the surface chemistry of oxo clusters has been less studied than that of oxide nanocrystals. Only carboxylate for carboxylate ligand exchange has been explored. At room temperature, the Zr6 and Zr12 clusters do not interconvert and the bridging ligands appear inaccessible for exchange.