The short-range C-C distances (at 1.5 and 2.6 Å) are not fitted in the refinement. However, the sinusoidal function seems to describe the long range distances quite well, presumably because the ligand chain is disordered. This hypothesis is confirmed by the increase in the relative amplitude of the sine wave from Zr12-acetate to Zr12-oleate (Table ). The improvement in Rw upon including the sinusoid is small for acetate (0.03) while large for oleate (0.09) (see Figure ). A more effective way of dealing with the long ligands is measuring the free ligand explicitly as a background and subtracting its scattering from the data in reciprocal space. For Zr12-oleate, this leads to a significant improvement of the fit. The short range C-C distances disappeared from the PDF (Figure ) and the sine wave is completely obsolete (Table ). The goodness of fit is now comparable to the case of the clusters with shorter chains. Note that the presence of longs ligands reduces the signal-to-noise ratio during the data collection (due to a lower fraction of oxo clusters in the X-ray beam). We conclude that, after initial training on the acetate clusters, our PDF analysis is able to confirm the structure of these previously unreported clusters. Our synthetic procedures indeed lead to the formation of soluble Zr6 oxo clusters, forming monomers or dimers depending on the sterical hindrance of the ligand. We further emphasize that good refinements can only be achieved by including the binding group of the ligand explicitly in the model (even after background subtraction). This is much more important for the small clusters discussed here, than for larger clusters or nanocrystals. For the latter two, there is a larger core of strongly scattering atoms, and relatively fewer, weakly scattering ligands. However, small nanocrystals have typically worse goodness of fits compared to larger ones and it might be worth to consider including ligands explicitly to obtain better refinements. Figure : PDF refinement for Zr12-butanoate and Zr12-octanoate using the Zr12-acetate structure model. PDF refinement of Zr6-methylbutanoate using the Zr6-acetate structure model. Finally, the PDF refinement of Zr12-oleate using the Zr12-propionate structure model, with or without background correction in reciprocal space. If applied in the refinement, the exponentially dampening sine wave is shown (orange dotted lines). The refined parameters are given in Table .