Oxygen atom displacements

Figure 5.16 Willis clusters in UOj+y Open circles, oxygens small filled circles, uranium atoms in the UO2 structure. A 2 2 2 defect cluster consists of two interstitial oxygen atoms (large filled circles) and two normal oxygen atoms displaced from their ideal positions (open squares) to new interstitial positions (hatched circles). A 2 1 2 defect cluster would consist of one interstitial oxygen and two normal oxygens displaced from their ideal positions. (After Anderson, 19726.)...

In these rearrangements, there are two consecutive internal SN2 type displacement processes a) an electron pair of the oxygen atom displaces the electron pair of a C —C bond and b) the electron pair of a C—C bond displaces the leaving group. It is therefore pertinent to find out if these processes follow the stereoelectronic principle of the SNj reaction. 

The MX2+X phases contain interstitial anions. As with the anion-deficient phases, these interstitials are not random point defects, but ordered or clustered. The earliest cluster geometry to be postulated was the [2 2 2] cluster in UO2+J, the prototype anion-excess flnorite phase. The cluster is composed of 2 interstitial oxygen atoms displaced along (110), two interstitial oxygen atoms displaced along (111) in UO2+J (Figure 5). Other cluster geometries have also been proposed in this oxide, and the defect structure of this well studied phase is still not completely resolved. 

For oxygen, the oxidation number in compounds is -2 except in peroxides in which it is -1. In the odd example OF2 it is h-2 since fluorine is more electronegative than oxygen and hence, it is observed that nucleophiles attack the oxygen atom displacing fluoride ion. For example, OF2 reacts with water to produce FIF and O2, and with ammonia to produce N2O andNH4F. 

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