A mixed phosphine sulfide/selenide structure as an instructional example for how to evaluate the quality of a model

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(di­phenyl­phosphan­yl)ethyl­ene, namely, 1,2-(ethene-1,2-di­yl)bis­(di­phenyl­phoshpine sulfide/selenide), C(26)H(22)P(2)S(1.13)Se(0.87). 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 inter­atomic 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.