There are many attractive forces holding the defect together, including the dilated bonds which allow tensile strain relief, and large local polarisation of the lattice due to the electronegative Ni and Oi atoms. The Ni draws charge away from the central Si core, leaving it overall positively charged, despite the electron in the p- orbital. This attracts the oxygen atoms and forms a weak dative bond, inadvertently causing local electrostatic repulsion between the single electron on the Si p- orbital and the oxygen atoms. The drawing of charge to Ni from its Si neighbours also acts as an extra initial attraction for Oi attack, in addition to the dilated bonds.
Figure 8.3 shows the pseudo-wavefunction of the partially filled shallow level of the neutral Ni-O2i defect. The occupation of this orbital is anti-bonding in nature, suggesting that there is repulsion from the oxygen lone-pairs, which will also contribute to the elevation of the Si dangling bond state to a shallow level. There must then be a bonding orbital lying below this state and this shows that the O atoms are to some extent overcoordinated. Therefore the transformation of the deep to shallow level occurs through a combination of electrostatic repulsion together with some dative bonding of the Si radical with the oxygen atoms.