Charge density in the CoCl42– ion: a comparison with spin density and theoretical calculations

Abstract

The charge density observed by X-ray diffraction in Cs₂CoCl₄ is reported by means of maps, multipole refinements, and constrained valence-orbital refinements. A cobalt atom configuration of 3d-t₂^3.3(3)3d-e^4.0(4)′4p′^1.0(3) was obtained with a cobalt charge of +0.7(2) and a chlorine charge of –0.7(1). Within the CoCl₄^2– ion the bonding is close to ionic with 0.30(5) e donated from each chlorine atom into, mainly, the diffuse cobalt 4′p′ orbitals. The charge density of the CoCl₄^2– ion has a slight trigonal distortion from cubic symmetry whereas the magnetic properties show mm symmetry. The difference is rationalised in terms of the arrangement of the chloride ions and of the surrounding caesium ions. This ‘intermolecular’ effect on the charge density is comparable in size with the covalent redistribution of charge within the ion. Difference density maps and the refinements indicate that close neighbours cause a charge redistribution in the ions from the outermost diffuse orbitals to localised regions at approximately the ionic radius along the interionic vector. This concentration of charge into small regions of space, while an ‘intermolecular’ effect, is reminiscent of overlap effects often seen in ‘true’ chemical bonds. In this case the charge transfer of this ‘intermolecular polarisation’ is comparable in size with those in the Co–Cl bond. The charge density in the CoCl₄^2– ion is compared with the experimental spin-density distribution and with theoretical calculations. The qualitative agreement with the theory is good, suggesting that the poor agreement between theory and the spin-density results arises from the neglect of electron correlation effects, to which the spin density is more sensitive, or, less probably, to spin-sensitive effects from the rest of the crystal, as the theory is for isolated CoCl₄^2– ions.