Revisiting the crystal structure of [Co(NH3)5Cl]Cl2 using synchrotron powder diffraction: to what extent single-crystal diffraction from 1960s got it right?

Abstract

The crystal structure of the purpureo salt, [Co(NH3)5Cl]Cl2, first reported in 1963 and later revised in 1968 (in both cases from single-crystal diffraction) in the space group Pnma (No. 62), has been recently re-examined from synchrotron X-ray powder diffraction using direct methods and the software EXPO2013. The comparison of the Rietveld analysis results using the two published models and the atomic coordinates obtained from powders leads to an improved crystal structure description in the lower symmetry space group Pn21a (No. 33). As a result, the overall atom connectivity and crystal packing remain similar; however, the symmetry and internal geometry of the coordination complex are changed. The distortions from an idealized geometry in Pnma (No. 62) are likely because of energetically favorable hydrogen-bonding motifs in the crystal. The three models are compared, and their validity and limitations are discussed.