Covalence in the [Cr(CN)6]3– ion studied by X-ray diffraction from Cs2KCr(CN)6

Abstract

A high-precision X-ray diffraction data set on the compound Cs₂KCr(CN)₆ has been collected at 120 K, and analysed in terms of the charge-density distribution and chemical bonding features. It has been useful to model the density only at the level of cubic symmetry, viz. a regular octahedral [Cr(CN)₆]^3– ion, because small effects, apparently arising from anharmonic motion, perturb the data with high values of h, restricting the resolution along the a axis for charge-density purposes. Molecular form-factors have been derived for the CN^– ion by fitting the theoretical electron density by a multipole analysis, enabling a more critical comparison between experiment and theory than is usual. In the region of the chromium atom and its six bonded carbon neighbours the density may be described by the superposition of a ‘prepared’t_2g³e_g⁰ Cr³⁺ ion and the unco-ordinated cyanide ions, with some σ donation and π back donation resulting in t_2g^2.3(4)e_g^0.6(5). On the nitrogen atoms of the cyanide ions significant charge [0.3(1) e] appears to have been polarised from the π orbitals into the σ‘lone pair,’ probably by the ‘intramolecular’ influences of the cations of the crystal. These cations are themselves perturbed away from simple spherical 1+ charged species by the contacts with the nitrogen atoms.