Oxides internuclear distance

An examination of the H—F spin-spin coupling of 2-substituted-3-trifluoromethylquinoxalines and their mono- and dk/V-oxides indicated that JH F alters as the H—F internuclear distance varies. This effect is best explained by a through-space interaction.264... 

A slightly different problem arises when we consider the lower oxidation states of metals. We know that CaF2 is stable. Why not CaF as well Assuming that CaF would crystallize in the same geometry as KF and that the internuclear distance would be about the same, we can calculate a lattice energy for CaF. U0 = — 795 kJ mol"1. The terms in the Bom-Haher cycle are... 

Other traditional scales, such as that of Goldschmidt, are more empirically based and rely upon experimental internuclear distances found in oxides and fluorides (which are expected to be particularly ionic) assumed values for the radii of F and O2- are needed. A very complete tabulation, based on a large amount of modern data for oxides and fluorides, both binary and complex, has been published by Shannon and Prewitt using values of 133 pm for F and 140 pm for O2-. One of the merits of this collection is that it recognises the dependence of ionic radii oh the coordination number. For example, in the case of Cd2+ radii are quoted for coordination numbers 4, 5, 6, 7, 8 and 12 as shown below ... 

On the other hand, the degree of localization of the 5/electrons increases in the oxide, especially in UO2. Indeed, the localization increases with the internuclear distance because the overlap of neighbouring 5/ orbitals decreases. However, an absorption jump is always observed in the absorption spectra of oxides the 5/localization is therefore weaker in the light actinides than in the rare earths, except for 5-Pu. 

Goldschmidt and, more recently, Sharmon and Prewitt, concentrated on the analysis of experimental data (mostly fluorides and oxides) with the aim of obtaining a set of ionic radii which, when combined in pairs (equation 5.6), reproduced the observed internuclear distances. In view of the approximate nature of the concept of the ionic radius, no great importance should be attached to small differences in quoted values so long as self-consistency is maintained in any one set of data. Further, some dependence of ionic size on coordination number is expected if we consider the different electrostatic interactions that a particular ion experiences in differing environments in an ionic crystal qon for a given ion increases slightly with an increase in coordination number. 

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