This paper compares variations on a structure model derived from an X-ray diffraction data set from a solid solution of chalcogenide derivatives of cis-1,2-bis-(di-phenyl-phosphan-yl)ethyl-ene, namely, 1,2-(ethene-1,2-di-yl)bis-(di-phenyl-phoshpine sulfide/selenide), C26H22P2S1.13Se0.87. A sequence of processes are presented to ascertain the composition of the crystal, along with strategies for which aspects of the model to inspect to ensure a chemically and crystallographically realistic structure. Criteria include mis-matches between F obs 2 and F calc 2, plots of |F obs| vs |F calc|, residual electron density, checkCIF alerts, pitfalls of the OMIT command used to suppress ill-fitting data, comparative size of displacement ellipsoids, and critical inspection of inter-atomic distances. Since the structure is quite small, solves easily, and presents a number of readily expressible refinement concepts, we feel that it would make a straightforward and concise instructional piece for students learning how to determine if their model provides the best fit for the data and show students how to critically assess their structures.
Keywords: F2obsvs F2calc; OMIT command; checkCIF alerts; crystal structure; disordered electron density; displacement ellipsoids; outliers; solid solution; standard interatomic distances.
© Parkin et al. 2023.