For PDF modelling we need to include the carboxylate head group, but PDF is in general not very sensitive to the organic fraction. Detailed information on the presence of uncoordinated ligands or coordination modes is hard to obtain from PDF. We turn now to FTIR, TGA and NMR. In Figure , we show the FTIR spectra of selected oxo clusters (the complete FTIR spectra of all clusters are presented in Figure ). The CH 2 stretches (2800-3000 cm -1 ) gain intensity with increasing chain length. The small, sharp peak around 3450 cm and the broad peak between 3500 and 3000 cm -1 were assigned to the OH moieties onto the cluster, which are involved in hydrogen bonding. While all these samples were extensively purified by multiple precipitation/re-dissolution cycles, it is clear that there is still protonated carboxylic acid in the sample (observed at 1710 cm -1 ), presumably hydrogen bonded to the cluster. Such hydrogen bonding is also observed for the co-crystallized acetic acid in the single-crystal structure of Zr12-acetate, see Figure . Our efforts to purify the clusters further and remove the hydrogen bonded acid were not successful or even compromised the integrity of the cluster core, see supporting information. Since FTIR is not quantitative, we used TGA to quantify the amount of excess carboxylic acid. We find about 1.1 -3.7 excess ligand molecules per Zr6 monomer, see supporting information. There is no clear trend among the different clusters. Only the Zr12-acetate stands out with 5.5 excess ligands per monomer.