Crystal structure disorders: disorders involving F and O atoms

 Not all structure solutions are straightforward. Some involve disordering of atomic positions, a problem that, for example, made the elucidation of the structure of C₆₀ difficult. Examples of disordered structures occur commonly in oxofluorides because the O and F atoms are similar in size and possess similar electronic properties. Thus, in a crystal containing molecules of an oxofluoride XF_xO_y, a given atomic position might be occupied by O in one molecule and by F in another molecule. The overall result is modelled by fractional occupation of each site by O and F. Fractional occupancies can lead to difficulties in determining true X^_F and X^_O bond lengths and true bond angles. The compound [F₂NO]‏[AsF₆]^- represents a classic example of the problem. Although first prepared and characterized

in 1969, its structure was not reported until 2001. The [F₂NO]‏ ions in crystalline [F₂NO][AsF₆] are disordered such that the fluorine occupancy of each ‘F’ position is 78% and 77% respectively (rather than being 100%), and the fluorine occupancy of the ‘O’ position is 45% (rather than being 0%). The paper cited in the further reading below illustrates how the structural data can be treated so that meaningful N^_O and N^_F bond lengths and F^_N^_F and F^_N^_O bond angles are obtained. Crystalline [F₂NO][AsF₆] is composed of infinite chains of alternating cations and anions. There are close contacts between the N atom of each cation and the F atoms of adjacent [AsF₆]^- ions as shown in the figure.