Occupancy Determination from Resonant X-ray Diffraction

We analyze the effect of integrated resolution, data scaling, structural refinement, and crystal symmetry on the extracted scattering perturbations and their uncertainties for anomalous (resonant) X-ray diffraction on mixed-metal molecular clusters. We probe the metal constituency, substitutional homogeneity, and positional disorder of both biased and unbiased ligand environments. Anomalous X-ray diffraction studies were conducted on (^FtbsL)Zn₂Ni(py) (1), [K(C₂₂₂)][(^FtbsL)Zn₂Ni] (2), and [K(THF)₃][(^FtbsL)Zn₂Ni(NAd)] (3). Analysis of diffraction data collected at energies along the Ni and Zn K-edges on [Zn₂Ni] clusters reveals data resolution, and the accuracy of the structural model greatly impacts the resonant scattering factors (f', f″) and their uncertainties. Occupancy studies on all three clusters were conducted in three ways and compared: (i) using the f' values of each metal site refined from diffraction data collected at energies along the Ni and Zn K-edges; (ii) using the f' value of each metal site refined from diffraction data collected with in-house, Cu Kα radiation; and (iii) refining each metal site as a Zn/Ni disordered pair and fixing the relative Zn-to-Ni occupancies as 2:1 across the core with supporting spectroscopic evidence from scanning electron microscope energy-dispersive spectroscopy. First, we observe highly ordered structural compositions, even when statistical mixing was anticipated; and second, we discovered that in-house X-ray diffraction collection was as precise in composition determination as synchrotron sources for this study.