We have synthesized fatty acid capped zirconium and hafnium oxo clusters with a library of linear and branched carboxylic acids. To prove the structure of the oxo clusters, we developed a method based on X-ray PDF analysis. In contrast to traditional PDF analysis of larger clusters and nanocrystals, we showed that the structure models need to include the carboxylate binding groups to obtain excellent refinements. PDF analysis is then able to distinguish Zr4 from Zr6 clusters, and cluster monomers from cluster dimers. Given the larger scattering power of hafnium, the refinements were even more straightforward for the hafnium oxo clusters. We were thus able to prove the structure of our fatty acid capped oxo clusters, despite the fact that they cannot be crystallized. Given the lack of crystallization, we developed purification protocols akin to the ones for colloidal nanocrystals, reinforcing the notion that these fatty acid capped oxo clusters are atomically precise nanocrystal prototypes, of the smallest conceivable scale. We further comprehensively characterized the oxo clusters by FTIR, NMR, TGA and ESI-HR-MS. We find that the cationic cluster cores are charge balanced by chelating and bridging carboxylate ligands and that a small excess of carboxylic acid is hydrogen bonded to the cluster core. Using our characterization tools, we studied carboxylate for carboxylate ligand exchange and found that the Zr6/Hf6 monomer and the Zr12/Hf12 dimer structure can be converted into one another, depending on the structure of the final ligand. Regarding the fatty acid capped metal oxo clusters as smallest conceivable nanocrystal prototypes, we compared their performance as catalyst with larger oxide nanocrystals. We found that, for the same amount of zirconium added, the oxo clusters are much more catalytically active than oxide nanocrystals, due to their higher surface area.