Experimental observation of charge-shift bond in fluorite CaF₂

On the basis of a multipole refinement of single-crystal X-ray diffraction data collected using an Ag source at 90 K to a resolution of 1.63 Å^-1, a quantitative experimental charge density distribution has been obtained for fluorite (CaF₂). The atoms-in-molecules integrated experimental charges for Ca²⁺ and F^- ions are +1.40 e and -0.70 e, respectively. The derived electron-density distribution, maximum electron-density paths, interaction lines and bond critical points along Ca²⁺...F^- and F^-...F^- contacts revealed the character of these interactions. The Ca²⁺...F^- interaction is clearly a closed shell and ionic in character. However, the F^-...F^- interaction has properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This conclusion is supported by the topology of the electron localization function and analysis of the quantum theory of atoms in molecules and crystals topological parameters. The Ca²⁺...F^- bonded radii - measured as distances from the centre of the ion to the critical point - are 1.21 Å for the Ca²⁺ cation and 1.15 Å for the F^- anion. These values are in a good agreement with the corresponding Shannon ionic radii. The F^-...F^- bond path and bond critical point is also found in the CaF₂ crystal structure. According to the quantum theory of atoms in molecules and crystals, this interaction is attractive in character. This is additionally supported by the topology of non-covalent interactions based on the reduced density gradient.