December 2010 – Chemical Crystallography

Acta Cryst. (2011), A67, 21-34. [ doi:10.1107/S010876731004287X ]

The practical use of the average and difference intensities of Friedel opposites at different stages of structure analysis has been investigated. It is shown how these values may be properly and practically used at the stage of space-group determination. At the stage of least-squares refinement, it is shown that increasing the weight of the difference intensities does not improve their fit to the model. The correct form of the coefficients for a difference electron-density calculation is given. In the process of structure validation, it is further shown that plots of the observed and model difference intensities provide an objective method to evaluate the fit of the data to the model and to reveal insufficiencies in the intensity measurements. As a further tool for the validation of structure determinations, the use of the Patterson functions of the average and difference intensities has been investigated and their clear advantage demonstrated.

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IUCr

Presented by: Dr. Adrian B. Chaplin
Research Leader: Prof. Andrew S. Weller
Published: Journal of the American Chemical Society

Making and breaking C–C bonds in the solid state: The structure of [Ir(BINOR‑S)(PiPr₃)][BAr^F₄] over the temperature range 100–250 K reveals a dynamic equilibrium between Ir(III) C–C agostic and Ir(V) bis-alkyl tautomers in the solid-state. The solid-state dynamic behaviour is shown in the reaction scheme (a), with populations determined using X-ray diffraction (b) and the Van’t Hoff plot for this process (c, R2(fit) = 0.999).The disordered model at 150 K containing (d) Ir(III) and (e) Ir(V). The combined model and thermal ellipsoid plot of the structure refined without cation disorder modelling are also shown (f & g respectively).

Structure of the Month – December 2010