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) (<b>1</b>), [K(C₂₂₂)][(^FtbsL)Zn₂Ni] (<b>2</b>), and [K(THF)₃][(^FtbsL)Zn₂Ni(NAd)] (<b>3</b>). 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 (<i>f</i>', <i>f</i>″) and their uncertainties. Occupancy studies on all three clusters were conducted in three ways and compared: (i) using the <i>f</i>' values of each metal site refined from diffraction data collected at energies along the Ni and Zn K-edges; (ii) using the <i>f</i>' 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.